"Andy Champ" wrote in message
. uk...
Bruce wrote:


I'm getting very poor Freeview reception. I expect I need a better
aerial, but I would like to mount it in the loft for ease of access.

Is it possible to get better reception with a very good loft aerial
than with a crappy old small aerial mounted on the chimney stack? I
know that a very good aerial on the chimney stack would be better
still, but I would like to do it myself so only the loft would be
practicable.


Depends where you are. If you're just over the road from Crystal Palace a
bent paper clip will probably work. If a deep valley in the wilds of the
Highlands, nothing will help!

A passive reflector at the top will probably help or freesat/sky.

"Andy Luckman (AJL Electronics)" wrote in message
. ..
In article , dennis@home
wrote:

If anything you can use a cheaper poorer quality aerial for digital TV.

More totally misleading bull from Foggy Dewhurst.

Misleading as in true?
There is no need for an expensive digital aerial and even less need after
the digital switchover.

"Harry Bloomfield" wrote in message
k...
dennis@home laid this down on his screen :
If anything you can use a cheaper poorer quality aerial for digital TV.

That might well be true once analogue is turned off and the power of
digital is increased, but for the moment digital muxes are quite weak. As
distance from both signal sources is increased, digital is always the
first to become completely unusable.

I know, that means you might need a better aerial, but not a digital one.

Digital aerials are just an excuse to rip people off.

On Mon, 06 Oct 2008 22:05:32 +0100, Bruce wrote:

I'm getting very poor Freeview reception. I expect I need a better
aerial, but I would like to mount it in the loft for ease of access.

Is it possible to get better reception with a very good loft aerial
than with a crappy old small aerial mounted on the chimney stack? I
know that a very good aerial on the chimney stack would be better
still, but I would like to do it myself so only the loft would be
practicable.

Much will depend on the roof through which the ærial receives its
signal. The roof will certainly absorb some signal dependant upon its
material - is it slate, clay tile, concrete tile, man-made (plastic)
tile?
Absorbed or lying rain, snow or whatever will affect your signal's
attenuation - indeed, it could possibly slightly affect ghosting
(multipath reception).
Having spent much of a working lifetime (it seems!) trying to resolve
TV reception problems, I would really advise an outside ærial in
virtually every problem case, even though it might involve borrowing a
ladder.
With Freeview you have to consider the frequency range of your local
transmitters when selecting the appropriate ærial. ISTR that there
are three or four multiplexes per area so the appropriate ærial has to
be chosen. A wideband might be the most convenient, but you may get
slightly better gain from the 'correct' ærial for your locality.

Personally I don't have a TV, on account of the poor quality of the
vast majority of most programming.

--
Frank Erskine

On Mon, 06 Oct 2008 22:44:57 +0100 someone who may be Andy Champ
wrote this:-

Depends where you are. If you're just over the road from Crystal Palace
a bent paper clip will probably work. If a deep valley in the wilds of
the Highlands, nothing will help!

The situation is more complicated than that. Local geography is
everything, including trees. One house may get good reception while
the next may need a tall pole to get the aerial over an obstruction.
Moving an aerial by a metre or less may make all the difference.
Television signals are not as good at going through/round things as
FM radio signals are and getting good reception may be a matter of
fiddling.

The deep valley may have a convenient repeater, near a transmitter
it may be necessary to reduce signals to avoid swamping the
television input by fitting an attenuator.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Bruce wrote:
"Andy Luckman (AJL Electronics)" wrote:

In article , Alan
wrote:

There is no such thing as a digital aerial.

Yes and no...


Any CAI certified aerial will have a balun. It will also have been proven to
be able to pass the digital signals without significant data corruption.

I think the reason there are "Digital" aerials is to differentiate between
certified and non certified types. Bear in mind that because analogue
degrades gracefully, you can often get away with any old contractor crap and
still get acceptable signals. Not the case with a digital service.


I'm getting very poor Freeview reception. I expect I need a better
aerial, but I would like to mount it in the loft for ease of access.

Is it possible to get better reception with a very good loft aerial
than with a crappy old small aerial mounted on the chimney stack? I
know that a very good aerial on the chimney stack would be better
still, but I would like to do it myself so only the loft would be
practicable.

Yes, it is, but is never as good as one out in the breeze up high and
with a clear line of sight to the horizon.

Graham. wrote:
"The Natural Philosopher" wrote in message
...
George wrote:
Hi-gain,outdoor in the region of £20/25

Yes I know a normal analogue aerial on a freeview box will work when the
switchover applies but the sister doesn't want a FV box hooked up to the
TV.

Cheers.
That does not compute,Dr spock.

No, you are getting confused by the child shrink.
The Science Officer on the Enterprise was plain Mister.

Actually, it fits rather better.

Given the complete inanity of the original post.

Anyway, I have probably only ever seen half a dozen episodes of
Start-Wreck, by accident, through the years.

dennis@home expressed precisely :
I know, that means you might need a better aerial, but not a digital one.

Digital aerials are just an excuse to rip people off.

Absolutely!

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

tony sayer wrote:

A balun is just a balanced to unbalanced conversion device I can't quite
see how it will corrupt the bits..

.... Or reduce corruption of the bits, hopefully. We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.

The relevance of this to DTT reception is that the house end of the
feeder is likely to run near to mains wiring, and usually ends up
connected to mains powered appliances (the DTT box and other TV
equipment). There's likely to be significant RF coupling between
mains-borne impulsive interference - from all those sparking switch and
thermostat contacts, etc. - and the outer of the feeder. The balun-less
aerial connection allows this interference into the signal path, as do
any other 'leaks' in the integrity of the feeder system - notably old
unscreened and isolated outlet plates and poorly screened receiver flyleads.

DVB-T is alas rather susceptible to impulsive interference. It causes
those momentary drop-outs ('blocking') and sound clicks. The problem
isn't so bad as in the early DTT days, but it's certainly still there.
Good RF practice wrt screening and baluns to keep the coax cold is the
best way of reducing it. The change to 8K FFT length that will come
with ASO/DSO will help further, and DVB-T2 shouldn't suffer in this way
since it includes a long time-interleave option (like DAB) to smear out
impulse events in the time domain.

In the meantime, with the present interim transmission arrangements...

I think the reason there are "Digital" aerials is to differentiate between
certified and non certified types. Bear in mind that because analogue
degrades gracefully, you can often get away with any old contractor crap and
still get acceptable signals. Not the case with a digital service.

Not strictly scientific is that ?. More clever marketing for the aerial
industry and a chance to make a few quid..

.... there's little doubt that DTT is considerably more demanding of
aerial performance than analogue, if you want acceptable results. Part
of the problem is that (some) people will tolerate appallingly poor
analogue reception without complaint, but will take the DTT box back to
the shop if they get impulse interference break-up. DTT reception has
to be pretty 'solid' to be acceptable. Analogue degrades gradually, if
not gracefully, and with a modicum of technical knowledge you can see
what the problem is: noise, multipath, temporary tropospheric
interference, etc. are all quite easy to identify. With digital the
margin gets invisibly used up and then you fall off the cliff not
knowing why you fell. With a marginal aerial system this will happen
more frequently.

Is there such a thing as a 'digital aerial'? Obviously a passive aerial
doesn't care about the details of the signals' coding and modulation (at
least for signals with similar RF bandwidths) - although an active one
with a built-in preamp might. Isn't it equally valid to say that
there's no such thing as an 'analogue aerial' either? More to the point
though, some aerials are certainly more suitable than others for DTT
reception and to that extent the term "digital aerial" does have some
meaning, IMHO.

Wideband or grouped? (Wideband is Group W, so maybe that's grouped
too!) An argument in favour of using wideband aerials where not
initially necessary is future-proofing. Once the 6-multiplex DSO plan
is complete (3-multiplex for most relay sites) there's a fair chance
that we'll see additional DTT services coming on in what Ofcom call
'interleaved spectrum' and these are quite likely to be out-of-group
relative to the original local analogue plan. It remains to be seen
whether the TXs will be co-sited, but if they are then wideband aerials
will be advantageous. Yes, the log-periodic (Benchmark standard 4) is a
good solution.

Further reading:
http://www.cai.org.uk/downloads/Guid...0 Aerials.pdf

--
Andy

Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

In article , Graham.
scribeth thus


"tony sayer" wrote in message
...
In article , David Hansen
scribeth thus
On Mon, 06 Oct 2008 14:41:42 +0100 someone who may be "Dave Plowman
(News)" wrote this:-

Thought digital was more robust to some kinds of signal problems?

The difference is that analogue degrades gracefully. Digital tends
to either work or not work, there is little between other than some
popping noises on the audio and some picture freezing between the
two states.

There is also the question of analogue having far more bandwidth
which means it can cope with certain types of shot well, while the
sort of digital systems we have in mass use cannot do so.


Can you explain that please .. "shot well"?....




He said "it can cope with certain types of shot well", which I took
to mean camera shot. Ie. low bitrate giving rise to motion artefacts etc.
on fast moving subjects, flowing water, etc. etc.

LOL!... Should've seen that one and theres me thinking he meant "shot
noise"!.....

Course the analogue we see these days invariably started life as a
digital signal in the studio and distribution systems but then it was
put through the "mincer" with the gory results that we see all too
often;(...
--
Tony Sayer

In article , Andy Wade spambucket@ma
xwell.myzen.co.uk scribeth thus
tony sayer wrote:

A balun is just a balanced to unbalanced conversion device I can't quite
see how it will corrupt the bits..

... Or reduce corruption of the bits, hopefully. We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.

Don't tell I, tell 'ee!.. As they say in Dorset..

The relevance of this to DTT reception is that the house end of the
feeder is likely to run near to mains wiring, and usually ends up
connected to mains powered appliances (the DTT box and other TV
equipment). There's likely to be significant RF coupling between
mains-borne impulsive interference - from all those sparking switch and
thermostat contacts, etc. - and the outer of the feeder. The balun-less
aerial connection allows this interference into the signal path, as do
any other 'leaks' in the integrity of the feeder system - notably old
unscreened and isolated outlet plates and poorly screened receiver flyleads.

Indeed as its been doing for all these years..


DVB-T is alas rather susceptible to impulsive interference.

Yep such a wondrous system;!..

It causes
those momentary drop-outs ('blocking') and sound clicks. The problem
isn't so bad as in the early DTT days, but it's certainly still there.
Good RF practice wrt screening and baluns to keep the coax cold is the
best way of reducing it.

A simple signal to noise ratio problem..

The change to 8K FFT length that will come
with ASO/DSO will help further, and DVB-T2 shouldn't suffer in this way
since it includes a long time-interleave option (like DAB) to smear out
impulse events in the time domain.

In the meantime, with the present interim transmission arrangements...

Indeed, the cowboys have never had it so good;!..


I think the reason there are "Digital" aerials is to differentiate between
certified and non certified types.

Umm.. I've just bought a couple of Triax group A's recently I don't
recall any certificates stuck thereon?..

Bear in mind that because analogue
degrades gracefully, you can often get away with any old contractor crap and
still get acceptable signals. Not the case with a digital service.

Not strictly scientific is that ?. More clever marketing for the aerial
industry and a chance to make a few quid..

... there's little doubt that DTT is considerably more demanding of
aerial performance than analogue, if you want acceptable results.


Well seeing that the ERP's are so very relatively low then not too
surprising really. Still come the allowed increases....

Part
of the problem is that (some) people will tolerate appallingly poor
analogue reception without complaint, but will take the DTT box back to
the shop if they get impulse interference break-up.
DTT reception has
to be pretty 'solid' to be acceptable. Analogue degrades gradually, if
not gracefully, and with a modicum of technical knowledge you can see
what the problem is: noise, multipath, temporary tropospheric
interference, etc. are all quite easy to identify. With digital the
margin gets invisibly used up and then you fall off the cliff not
knowing why you fell. With a marginal aerial system this will happen
more frequently.

Is there such a thing as a 'digital aerial'? Obviously a passive aerial
doesn't care about the details of the signals' coding and modulation (at
least for signals with similar RF bandwidths) - although an active one
with a built-in preamp might. Isn't it equally valid to say that
there's no such thing as an 'analogue aerial' either? More to the point
though, some aerials are certainly more suitable than others for DTT
reception and to that extent the term "digital aerial" does have some
meaning, IMHO.

Yes I would have thought that the OP would have known that;!..


Wideband or grouped? (Wideband is Group W, so maybe that's grouped
too!) An argument in favour of using wideband aerials where not
initially necessary is future-proofing. Once the 6-multiplex DSO plan
is complete (3-multiplex for most relay sites) there's a fair chance
that we'll see additional DTT services coming on in what Ofcom call
'interleaved spectrum' and these are quite likely to be out-of-group
relative to the original local analogue plan. It remains to be seen
whether the TXs will be co-sited, but if they are then wideband aerials
will be advantageous. Yes, the log-periodic (Benchmark standard 4) is a
good solution.


Indeed. A very good choice if existing levels will permit the usage
which sometimes in the current intermediate climate they just don't have
enough gain whereas a grouped Yagi will...

Further reading:
http://www.cai.org.uk/downloads/Guid...20Benchmarked%
20Aerials.pdf

And then fail to benchmark some of their members and what they get up to
or throw up;!..
--
Tony Sayer

In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....
--
Tony Sayer

In article ,
The Natural Philosopher wrote:
Andy Wade wrote:

We've done this one often enough Tony, both here and on
uk.tech.digital-tv. If you connect a coax feeder to a symmetrical
aerial (such as the centre-fed dipole of a typical TV aerial) without
using any form of balun then the outer of the feeder becomes part of
the aerial. If you're transmitting, the outer of the coax will be
'RF-hot' and will radiate, quite possibly causing EMC problems to
low-level parts of the transmitting equipment as well as safety
concerns if high RF power is involved. When receiving, unwanted
signals picked up on this hot feeder will find their way into the
receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

From the mists of my memory I recall a folded dipole has a characteristic
impedance of 300 ohms - and an aerial balun changes this to the UK 75.

--
*Succeed, in spite of management *

Dave Plowman London SW
To e-mail, change noise into sound.

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.
I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....

No, thats not the point.

The point is 'balanced with respect to what?'

I.e. a 'balanced' aerial feeding an 'unbalanced' feeder works just as well.

At the mathematical level, the balanced bit merely applies to the
voltage difference between the antenna feed points. You can 'earth' one,
the other, or neither and it changes nothing really.

As for putting a balun between the antenna and the cable, unless there
is impedance matching to be done, its a waste of signal. The coax simply
is a piece of coax terminated (hopefully) at 75ohms at each end. What it
picks up as interference doesn't change between it having a
balun/antenna or just an antenna stuck on it. The only advantage I can
see is a slight reduction in likelihood of damage under high EMF
situations, like lightning strikes nearby.


I did just enough antenna theory at college to realise I didn't want to
do any more, and to realise that the equations governing it were
independent of the actual reference voltage of any part.

I have done more than enough RF engineering to understand that at RF,
there is no such thing as 'ground' or 'earth' either.

I spent enough time with a few people who really DID understand antenna
theory to realize that in reality, you can probably count them on the
fingers of one hand.

The reality is that there are some standard designs, that have been
dreamed up by a few people years ago, and everything is a simple
reinvention of the yagi, dipole, quarter wave whip, log periodic etc.etc
etc.

Putting a few turns of wire round a ferrite lump sounds like pure brand
differentiation to me.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Andy Wade wrote:

We've done this one often enough Tony, both here and on
uk.tech.digital-tv. If you connect a coax feeder to a symmetrical
aerial (such as the centre-fed dipole of a typical TV aerial) without
using any form of balun then the outer of the feeder becomes part of
the aerial. If you're transmitting, the outer of the coax will be
'RF-hot' and will radiate, quite possibly causing EMC problems to
low-level parts of the transmitting equipment as well as safety
concerns if high RF power is involved. When receiving, unwanted
signals picked up on this hot feeder will find their way into the
receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

From the mists of my memory I recall a folded dipole has a characteristic
impedance of 300 ohms - and an aerial balun changes this to the UK 75.


Thats a different issue: there a balun has validity - as impedance matching.

I don't refute that, merely the ******** abut what is stuck on the end
of a piece of coax (as long as its a 75 ohm load at the frequencies in
question) being of any relevance to the interference pickup of the cable
itself.

Cables only act as antennae when they are mismatched. If the antenna is
75ohms, then it matters not one jot if its fed via a balun, or not.

The Natural Philosopher wrote:
Andy Wade wrote:

We've done this one often enough Tony, both here and on
uk.tech.digital-tv. If you connect a coax feeder to a symmetrical
aerial (such as the centre-fed dipole of a typical TV aerial) without
using any form of balun then the outer of the feeder becomes part of
the aerial. If you're transmitting, the outer of the coax will be
'RF-hot' and will radiate, quite possibly causing EMC problems to
low-level parts of the transmitting equipment as well as safety
concerns if high RF power is involved. When receiving, unwanted
signals picked up on this hot feeder will find their way into the
receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it
cares about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

Sorry, but your perceptions about this are almost completely skewed.
Andy has it just right.

There is no kind of "RF ground" or "RF earth" up there at the aerial -
it's fallacy that invariably leads to the wrong conclusions. The only
things that exist up there are potential *differences* and some concepts
of "balance".

(Yes, you did mention potential differences, but immediately let the
"earth" fallacy creep back in).

The stuff about one limb of a dipole being "a 'reflector' boosting the
signal into the other half" is also complete nons\\\\\\\\ a completely
unhelpful way of thinking about it.

Likewise the final paragraph about baluns being generally associated
with twisted pair. A balun makes a transition between a "balanced" (ie
physically and electrically symmetrical) system, and an "unbalanced"
system like coax. Andy is talking about the transition between the
dipole (balanced) and the top of the coax feedline (unbalanced), which
is a classic application for a balun.

A balun may also include an impedance transformation, but that is
categorically *not* its primary function.


(PS: Just read your other two postings, and the same comments apply.)



--
Ian White

Ian White wrote:
The Natural Philosopher wrote:
Andy Wade wrote:

We've done this one often enough Tony, both here and on
uk.tech.digital-tv. If you connect a coax feeder to a symmetrical
aerial (such as the centre-fed dipole of a typical TV aerial)
without using any form of balun then the outer of the feeder becomes
part of the aerial. If you're transmitting, the outer of the coax
will be 'RF-hot' and will radiate, quite possibly causing EMC
problems to low-level parts of the transmitting equipment as well as
safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way
into the receiver, however perfect the screening of the coax itself.

I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb
of the dipole at its feed end, and it becomes a 'reflector' boosting
the signal into the other half. Ie. the 'balanced' nature of the
antenna is a myth. It doesn't care what the potential is of any limb:
what it cares about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

Sorry, but your perceptions about this are almost completely skewed.
Andy has it just right.

There is no kind of "RF ground" or "RF earth" up there at the aerial -
it's fallacy that invariably leads to the wrong conclusions. The only
things that exist up there are potential *differences* and some concepts
of "balance".


Exactly waht I said.


(Yes, you did mention potential differences, but immediately let the
"earth" fallacy creep back in).

The stuff about one limb of a dipole being "a 'reflector' boosting the
signal into the other half" is also complete nons\\\\\\\\ a completely
unhelpful way of thinking about it.


Well it is and it isn't. I was merely trying to make the point that
connecting one side of a presumably screened cable to one side of a
dipole does not 'unbalance' it.


Likewise the final paragraph about baluns being generally associated
with twisted pair. A balun makes a transition between a "balanced" (ie
physically and electrically symmetrical) system, and an "unbalanced"
system like coax. Andy is talking about the transition between the
dipole (balanced) and the top of the coax feedline (unbalanced), which
is a classic application for a balun.

No, you have just refuted your own argument.

The cable can ot be considered unbalanced just because one side of it is
earthed at one end, that is exactly what you said, and I agree.. and it
make bugger all difference if you actually have no cable at all, and
simply earth one side of the dipole - in which case its a quarter wave
whip and a ground plane.



A balun may also include an impedance transformation, but that is
categorically *not* its primary function.



No, and yet in his case its its ONLY valid function.


(PS: Just read your other two postings, and the same comments apply.)




I suggest you go and actually study the theory then.

your two statements are contradictory.

If you want the ultimate in RF screening - or audio, you use a balun in
a TWO core cable that is shielded.


A piece of coax terminate with a 75om load, does no change its
interference reaction whether or not its that 75ohms is a balun and
antenna, or indeed simply a 75ohm antenna.

At best, all you have done is DC isolated (and to an extent out of band
RF isolated) the ANTENNA from the CABLE. But in terms of actual RF
interference pickup, that is generally very small beer, and in theory,
it won't make a hoot of difference. For exactly the reasons you
described. The antenna doesn't care whether its floating, 'earthed' at
one end, or 'earthed' in the middle. The transfer of energy from the EM
waves is not with respect to any particular volatage on any particular
part of it. It appears as simply a 75ohm impedance generated EMF across
the two output terminals.

All that is in question here is where isolating that from the unbalance
cable has any effect wghatsoever. I can see no mechanism other than
dimly grasped and misunderstood 'don't connect balanced to unbalanced'
to account for your view. And yet you have already admitted, nay
seemingly CORRECTED me, to say that you cant realy consider a dipole to
be balanced or unbalanced!

Let's just repalace that dipole with something that it really
represents. A signal generator, that is completelty unearthed in any
sense at all frequencies. Generating e.g. TV signals. Now, why would
anyone introduce a lossy balun into its feed if not for impedance
matching or marketing?

The Natural Philosopher wrote:

[Ian White had written}
Likewise the final paragraph about baluns being generally associated
with twisted pair. A balun makes a transition between a "balanced"
(ie physically and electrically symmetrical) system, and an
"unbalanced" system like coax. Andy is talking about the transition
between the dipole (balanced) and the top of the coax feedline
(unbalanced), which is a classic application for a balun.

No, you have just refuted your own argument.

What I just gave is the normal textbook description of what a balun is
for. If you find that in any way self-contradictory, the problem is all
yours.

The cable can ot be considered unbalanced just because one side of it
is earthed at one end, that is exactly what you said, and I agree.. and
it make bugger all difference if you actually have no cable at all, and
simply earth one side of the dipole - in which case its a quarter wave
whip and a ground plane.

In standard engineering usage, coaxial cable is called "unbalanced"
because it doesn't have the same left-right symmetry that a "balanced"
dipole does.


A balun may also include an impedance transformation, but that is
categorically *not* its primary function.


No, and yet in his case its its ONLY valid function.

Again, it's only wrong according to your own misunderstanding of the
subject.


(PS: Just read your other two postings, and the same comments apply.)


I suggest you go and actually study the theory then.

Oh I have, I have... and also designed baluns, built them, measured
them, used them, written about them and had more discussions about them
than you could possibly imagine.

That's why I can say with confidence that Andy and I are right in the
mainstream on this subject, and you are in a minority of one.



--
Ian White

Ian White wrote:
The Natural Philosopher wrote:

[Ian White had written}
Likewise the final paragraph about baluns being generally associated
with twisted pair. A balun makes a transition between a "balanced"
(ie physically and electrically symmetrical) system, and an
"unbalanced" system like coax. Andy is talking about the transition
between the dipole (balanced) and the top of the coax feedline
(unbalanced), which is a classic application for a balun.

No, you have just refuted your own argument.

What I just gave is the normal textbook description of what a balun is
for. If you find that in any way self-contradictory, the problem is all
yours.

The cable can ot be considered unbalanced just because one side of it
is earthed at one end, that is exactly what you said, and I agree..
and it make bugger all difference if you actually have no cable at
all, and simply earth one side of the dipole - in which case its a
quarter wave whip and a ground plane.

In standard engineering usage, coaxial cable is called "unbalanced"
because it doesn't have the same left-right symmetry that a "balanced"
dipole does.


A balun may also include an impedance transformation, but that is
categorically *not* its primary function.


No, and yet in his case its its ONLY valid function.

Again, it's only wrong according to your own misunderstanding of the
subject.


(PS: Just read your other two postings, and the same comments apply.)


I suggest you go and actually study the theory then.

Oh I have, I have... and also designed baluns, built them, measured
them, used them, written about them and had more discussions about them
than you could possibly imagine.

That's why I can say with confidence that Andy and I are right in the
mainstream on this subject, and you are in a minority of one.



We will have to agree to disagree.

You gave repeatedly stated that the concept of balancing cant apply to
an RF dipole, which I agree with, and yet you persist in saying that you
need a balun to properly match it to an unbalanced coax, which by your
definitions, is also a relatively meaningless term, since at RF the
actual voltages with respect to ground at the far end are fairly
indterminate.

You are just parroting two statements, probably culled from simplistic
text books, without actually thinking what they actually mean when put
together.

If you cant see the logical contradictions in your arguments, I cant
really have anything more to say.

In article ,
The Natural Philosopher wrote:
You gave repeatedly stated that the concept of balancing cant apply to
an RF dipole, which I agree with,

How come many countries use balanced feeders, then?

--
*If God had wanted me to touch my toes, he would have put them on my knees

Dave Plowman London SW
To e-mail, change noise into sound.

In message , tony sayer
wrote

Further reading:
http://www.cai.org.uk/downloads/Guid...use%20of%20Ben
chmarked%
20Aerials.pdf

And then fail to benchmark some of their members and what they get up to
or throw up;!..

They're called Cowboy Aerial Installers for obvious reasons.
--
Alan
news2006 {at} amac {dot} f2s {dot} com

The Natural Philosopher wrote:
Ian White wrote:
[...]

You gave repeatedly stated that the concept of balancing cant apply to
an RF dipole,

I never said any such thing! If you can read that into anything I did
write, it's no wonder you've got yourself into a twist.

which I agree with, and yet you persist in saying that you need a balun
to properly match it to an unbalanced coax, which by your definitions,
is also a relatively meaningless term, since at RF the actual voltages
with respect to ground at the far end are fairly indterminate.

No, you still don't understand...

You are just parroting two statements, probably culled from simplistic
text books, without actually thinking what they actually mean when put
together.

If you cant see the logical contradictions in your arguments, I cant
really have anything more to say.

Funny, I'd been thinking of saying exactly those same things to you.


For everyone else, the bottom line is that Andy Wade was exactly right
in saying:
" If you connect a coax feeder to a symmetrical aerial (such as the
centre-fed dipole of a typical TV aerial) without using any form of
balun then the outer of the feeder becomes part of the aerial. If
you're transmitting, the outer of the coax will be 'RF-hot' and will
radiate, quite possibly causing EMC problems to low-level parts of the
transmitting equipment as well as safety concerns if high RF power is
involved. When receiving, unwanted signals picked up on this hot feeder
will find their way into the receiver, however perfect the screening of
the coax itself."

Hence the need for a balun, to prevent all that.


--
Ian White

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
You gave repeatedly stated that the concept of balancing cant apply to
an RF dipole, which I agree with,

How come many countries use balanced feeders, then?

thats a feeder innit ;-)

The Natural Philosopher wrote:
Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
You gave repeatedly stated that the concept of balancing cant apply
to an RF dipole, which I agree with,

How come many countries use balanced feeders, then?

thats a feeder innit ;-)


Just in case that was a serious comment, heres teh serious asnser is I
see it.

There are two basic forms of antenna that present a purely resistive
impedance the basic quarter wave, which presents at 75 ohms, and the
half wave that presents at typically 300 ohms.

Its so happens also that a parallel pair of wires, or a coaxial cable,
represents over a broad range of frequencies, a non reactive
transmission line.

It also happens that a coaxial cable at 300 ohms characterisic impedance
is almst impossible to arrange. The materials available simply don't
allow it, so most coax is around the 50-75ohm range, and therefore suit
cable as feeders for low impedance quarter wave antennae.

If you use a half wave style antenna, you need to either couple it to
300ohm transmission line - two strands of copper at the edges of a flat
ribbon of insulator - or use an impedance matching transformer, which
will probably erroneously be called a balun, to connect it to a 75ohm
coaxial cable.

i suspect that the whole thing springs from people referring to 300ohm
line as 'balanced' and coax as 'unbalanced' at which point a 'balun' is
declared to be the thing that connects them. Neither of the two
transmission lines are balanced or unbalanced, and the function of the
blob of ferrite and wires that connects them is actually to perform an
impedance match.


Note that in either case, the system is neither balanced nor unbalanced
electrically, as it has no real reference voltage about which it may be
said to be balanced. You can earth one side or in fact the other, of any
of the systems without affecting operation.

Balance operation only really has meaning when its - typically -
transmitting between two systems which share a common third path. It is
in effect a way of eliminating 'ground loops' since the signal current
does not share the ground parts of the circuit, and hence the
possibility of ground current interfering with the signal simply does
not exist.

At RF th concept is almost meaningless: wire inductance over any length
is so high as to render the concept of a common ground of low impedance
almost meaningless. If you want an RF grund, the best you can do is a
big sheet of conductor, and if you need to move signals about, a piece
of wire or a track is a very bad way to do it. All RF circuitry that
takes signal over anything but the smallest distances (less than 1/8th
wavelength typically) is done with a transmission line of some sort or
another. Even if that's simply a copper track on one side of a board
with a ground plane on the other.

Balun merely means balanced to unbalamced: it has absolutely no real
meaning at RF. Other than as an impedance matching device, one side of
which has one limb connected to a local ground plane.

Or just an impedance matching device, or possibly a way to DC isolate
two circuits.

If an antenna has a transformer on it, its because its built to a
different impedance to the line its designed to feed, or because someone
decided that it would be a way to con people that its somehow better.

Obviously a 300ohm antenna feeding 75 ohm coax, with no impedance
matching transformer, is going to be an appalling mismatch, and the
whole coax will indeed act as an antenna, but thats not because its
unbalanced, its because its not been matched impedance wise, and is no
longer acting as a transmission line should.

Ian White wrote:
The Natural Philosopher wrote:
Ian White wrote:
[...]

You gave repeatedly stated that the concept of balancing cant apply to
an RF dipole,

I never said any such thing! If you can read that into anything I did
write, it's no wonder you've got yourself into a twist.

which I agree with, and yet you persist in saying that you need a
balun to properly match it to an unbalanced coax, which by your
definitions, is also a relatively meaningless term, since at RF the
actual voltages with respect to ground at the far end are fairly
indterminate.

No, you still don't understand...

You are just parroting two statements, probably culled from simplistic
text books, without actually thinking what they actually mean when put
together.

If you cant see the logical contradictions in your arguments, I cant
really have anything more to say.

Funny, I'd been thinking of saying exactly those same things to you.


For everyone else, the bottom line is that Andy Wade was exactly right
in saying:
" If you connect a coax feeder to a symmetrical aerial (such as the
centre-fed dipole of a typical TV aerial) without using any form of
balun then the outer of the feeder becomes part of the aerial.

This is compleletly wrong.

as LONG as the impedance presented by the antenna is 75ohms, the cable
has no effect on reception whatsoever other than presenting some loss..

Its perfectly true if its a 300ohm antenna, *designed* to work with a an
*impedance matching transformer*, but not because its unbalanced, its
beacuse the coax is no longer terminated.



If
you're transmitting, the outer of the coax will be 'RF-hot' and will
radiate, quite possibly causing EMC problems to low-level parts of the
transmitting equipment as well as safety concerns if high RF power is
involved. When receiving, unwanted signals picked up on this hot feeder
will find their way into the receiver, however perfect the screening of
the coax itself."

Hence the need for a balun, to prevent all that.



We are not transmitting.

The Natural Philosopher wrote:

This is compleletly wrong.

You are completely wrong, and are now introducing red herrings.

as LONG as the impedance presented by the antenna is 75ohms, the cable
has no effect on reception whatsoever other than presenting some loss..

Oh dear, since you don't want to give up, I'll have one more go.

Think in terms of symmetry rather than balance, and think about where RF
current is able to flow rather than voltages to some arbitrary reference
point. The aim of a feeder is connect an antenna structure to a
receiver or transmitter as transparently as possible; in particular we
don't want the feeder to become part of the aerial and hence to radiate
or receive. The effects of such a 'hot' feeder are almost always
undesirable (which is where we came in).

Unfortunately the feeder, made of metal wires, doesn't know this and
given half a chance it will become part of the aerial. From the
receiving viewpoint the feeder, being 'metal in the sky,' _will_ receive
signals and interference and we must try to arrange things so that these
unwanted signals don't reach the receiver's input. IOW what we want to
prevent is common-mode to differential-mode coupling in the feed
arrangements. The wanted signals are clearly in differential mode
(currents in opposite directions on the legs of a 2-wire feeder) whereas
the interference received on the feeder itself is in common-mode.

Now consider a dipole antenna, mounted in the clear and centre-fed with
a symmetrical 2-wire feeder (300 ohm ribbon, or similar). We'll
transmit from a balanced or floating source. This is a perfectly
symmetrical arrangement and each leg of the feeder will carry equal and
opposite currents and deliver them to the two halves of the dipole.
This is just what we want; there's no common-mode current on the feeder
and no far-field radiation from it, assuming the wire spacing is small
compared to the signal wavelength. So far, so good I hope. By
reciprocity what applies for transmission also applies in reverse for
reception, and common-mode interference received on the feeder doesn't
get in to the receiver.

Now replace the symmetrical feeder with a coax cable and connect it to
the dipole terminals with no balun. This arrangement is not symmetrical
- one wire of the feeder is wrapped inside the other.

We need to digress for a moment and think about differential- and
common-mode currents for a coaxial cable. Diff-mode is fairly obvious -
equal & opposite currents on the inner and outer conductors. Provided
the frequency is high enough (higher than a few hundred kHz for the sort
of coax we're talking about) the skin effect ensures that the current in
the outer conductor flows on its inside surface. There's negligible
current on the outer surface and hence no external field - and that's
the great advantage of coax. Any common-mode RF current, OTOH, has to
flow entirely on the outer surface of the outer conductor and leads to
radiation. It's this fact, that any common-mode current flows in one of
the two conductors only, that makes coax 'unbalanced.'

Going back to the centre-fed antenna, the dipole leg connected to the
centre conductor sees only the differential-mode current, whereas the
other leg sees the opposite diff-mode current /plus/ any common mode
current. We now have a situation where the current in the two halves of
the dipole can be unequal, with the difference flowing in common-mode on
the outside of the feeder. Just how the current divides between the
driven element connection and the outer of the feeder will depend on the
relative impedances and radiation resistances looking into the element
and the feeder's common mode path. Since the latter is likely to be
several wavelengths long and routed past all manner of obstacles, this
will generally be unpredictable.

Another way of looking at the situation is to separate, conceptually,
the two feeder modes - IOW we have a dipole fed by a nice symmetrical
feeder (diff-mode) but it has a random long length of wire (the outer of
the coax) connected to one leg. Looked at that way, the balun-less feed
suddenly seems quite horrendous (which it is).

Tonight's further reading is an excellent introduction to the subject of
baluns:
http://www.bcba15324.pwp.blueyonder....y/Paper-44.pdf

--
Andy

Andy Wade wrote:
The Natural Philosopher wrote:

This is compleletly wrong.

You are completely wrong, and are now introducing red herrings.

as LONG as the impedance presented by the antenna is 75ohms, the cable
has no effect on reception whatsoever other than presenting some loss..

Oh dear, since you don't want to give up, I'll have one more go.


Dont bother. Look up the Wiki and you will see that apart from issue
with powerful transmitting antennae, where circulating currents in the
coax outer make it significant, the balun is an impedance matching item
solely.


http://en.wikipedia.org/wiki/Dipole_antenna


Specifically

"When a dipole is used both to *transmit* and to receive, the
characteristics of the feedline become much more important.
Specifically, the antenna must be balanced with the feedline. Failure to
do this causes the feedline, in addition to the antenna itself, to
radiate. RF can be induced into other electronic equipment near the
radiating feedline, causing RF interference. Furthermore, the antenna is
not as efficient as it could be because it is radiating closer to the
ground and its radiation (and reception) pattern may be distorted
asymmetrically. At higher frequencies, where the length of the dipole
becomes significantly shorter than the diameter of the feeder coax, this
becomes a more significant problem. One solution to this problem is to
use a balun"

Note that this applies SOLELY to high power use as a transmitting
antenna. IT HAS NOTHING TO DO WITH A RECEIVING ANTENNA.


A little knowledge, it appears, in your case, is a dangerous thing.

WE are taking TV reception aerials, Not kilowatt transmitters.

The Natural Philosopher wrote:

Cables only act as antennae when they are mismatched. If the antenna is
75ohms, then it matters not one jot if its fed via a balun, or not.

In principle a coaxial cable doesn't act as an antenna (receiving or
transmitting) simply because of mismatch. Mismatch /per se/ doesn't
cause current to flow on the outer surface of the outer conductor.

OTOH mismatch increases the attenuation in the cable and increases the
peak current and voltage at points along the feeder in accordance with
the standing wave pattern, effectively reducing its power rating. Also
the presence of reflections may cause deterioration of a signal in the
time domain - ghosting, intersymbol interference, etc., particularly on
long feeders.

In reality imperfect screening ('leakage') leading to feeder pick-up or
radiation may be worsened by the presence of a standing wave but this is
usually in the realm of second order effects compared to the
consequences of omitting a balun where one is required.

--
Andy

In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:
The Natural Philosopher wrote:

This is compleletly wrong.

You are completely wrong, and are now introducing red herrings.

as LONG as the impedance presented by the antenna is 75ohms, the cable
has no effect on reception whatsoever other than presenting some loss..

Oh dear, since you don't want to give up, I'll have one more go.


Dont bother. Look up the Wiki and you will see that apart from issue
with powerful transmitting antennae, where circulating currents in the
coax outer make it significant, the balun is an impedance matching item
solely.


Dear O dear!, a Balun can be an impedance match as well as a balanced to
unbalanced converter!...

http://en.wikipedia.org/wiki/Dipole_antenna


Specifically

"When a dipole is used both to *transmit* and to receive, the
characteristics of the feedline become much more important.
Specifically, the antenna must be balanced with the feedline. Failure to
do this causes the feedline, in addition to the antenna itself, to
radiate. RF can be induced into other electronic equipment near the
radiating feedline, causing RF interference. Furthermore, the antenna is
not as efficient as it could be because it is radiating closer to the
ground and its radiation (and reception) pattern may be distorted
asymmetrically. At higher frequencies, where the length of the dipole
becomes significantly shorter than the diameter of the feeder coax, this
becomes a more significant problem. One solution to this problem is to
use a balun"

Thats not terribly well written..as is quite a bit on wikipedia..


Note that this applies SOLELY to high power use as a transmitting
antenna. IT HAS NOTHING TO DO WITH A RECEIVING ANTENNA.

With all due respect there is no difference in an aerial system you use
to transmit and to receive there is nothing that sets them apart in
theory at all.

In practice there is in that you can get way with impedance mismatches
in say FM or TV aerials, all you do is loose some signal, set up
reflections, alter the polar pattern etc.

You don't really want to do this in transmission as RF power is
expensive and you simply don't want to waste it, nor do you want to have
currents about the place in the wrong places..


A little knowledge, it appears, in your case, is a dangerous thing.

With all respect, learned old sage, I think you demean Mr Wade who's
spent years in TV and Radio aerial design and as well as running
companies that make TV and distribution equipment!..

WE are taking TV reception aerials, Not kilowatt transmitters.

As above the theory is the same..

Suggest you read up the article thats been attached to the previous
post. Look up the difference betwixt Marconi quarter waves, open and
folded dipoles, differing types of Baluns and matching systems etc,
etc..
--
Tony Sayer

In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.
I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.

I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....

No, thats not the point.

It is the point as subsequent posts will show. The only inaccuracy in
that statement is where you are driving a Marconi quarter wave vertical
where it is an unbalanced system!...
--
Tony Sayer

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:
The Natural Philosopher wrote:

This is compleletly wrong.
You are completely wrong, and are now introducing red herrings.

as LONG as the impedance presented by the antenna is 75ohms, the cable
has no effect on reception whatsoever other than presenting some loss..
Oh dear, since you don't want to give up, I'll have one more go.

Dont bother. Look up the Wiki and you will see that apart from issue
with powerful transmitting antennae, where circulating currents in the
coax outer make it significant, the balun is an impedance matching item
solely.


Dear O dear!, a Balun can be an impedance match as well as a balanced to
unbalanced converter!...
http://en.wikipedia.org/wiki/Dipole_antenna


Specifically

"When a dipole is used both to *transmit* and to receive, the
characteristics of the feedline become much more important.
Specifically, the antenna must be balanced with the feedline. Failure to
do this causes the feedline, in addition to the antenna itself, to
radiate. RF can be induced into other electronic equipment near the
radiating feedline, causing RF interference. Furthermore, the antenna is
not as efficient as it could be because it is radiating closer to the
ground and its radiation (and reception) pattern may be distorted
asymmetrically. At higher frequencies, where the length of the dipole
becomes significantly shorter than the diameter of the feeder coax, this
becomes a more significant problem. One solution to this problem is to
use a balun"

Thats not terribly well written..as is quite a bit on wikipedia..

Note that this applies SOLELY to high power use as a transmitting
antenna. IT HAS NOTHING TO DO WITH A RECEIVING ANTENNA.

With all due respect there is no difference in an aerial system you use
to transmit and to receive there is nothing that sets them apart in
theory at all.

As I said, a little knowledge is a dangerous thing.

I guess you are the sort of person who runs a 20hp electric motor off
bell wire..cos a piece of wire is a piece of wire, irrespective of the
current its carrying..





In practice there is in that you can get way with impedance mismatches

we are not talking imepdanace mismatches. W are talking about
'balancing' a dipole on a caoax feed 'top reduce interference'



in say FM or TV aerials, all you do is loose some signal, set up
reflections, alter the polar pattern etc.

And the result of THOSE *IS* actually to end up with the coax acting as
an antenna.




You don't really want to do this in transmission as RF power is
expensive and you simply don't want to waste it, nor do you want to have
currents about the place in the wrong places..

A little knowledge, it appears, in your case, is a dangerous thing.

With all respect, learned old sage, I think you demean Mr Wade who's
spent years in TV and Radio aerial design and as well as running
companies that make TV and distribution equipment!..


Well so did I, once. And its not necessary to have full understanding of
anything to be commercially successful either.


WE are taking TV reception aerials, Not kilowatt transmitters.

As above the theory is the same..

It is not. The currents in te cable are a few hundreds of orders greater.

The wiki article specifically uses the application of baluns to
transmitters *only* in respect of 'balancing' the dipole and isolating
it from the feeder.




Suggest you read up the article thats been attached to the previous
post. Look up the difference betwixt Marconi quarter waves, open and
folded dipoles, differing types of Baluns and matching systems etc,
etc..

Baluns as impedance matchers, which shouldn't be called baluns, I have
no problem with. Any half wave system that is feeding 75 ohm coax needs one.

Baluns as a means of isolating a *receiving* antenna of 75ohms impedance
from a cable of the same, are completely useless in a *receiving*
environment, and do sod all to reduce interference. Their use in a
transmitting environment is to reduce the fractional power *off* the
cable..a pretty small fraction, but measurable.

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.
I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.
I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....
No, thats not the point.

It is the point as subsequent posts will show. The only inaccuracy in
that statement is where you are driving a Marconi quarter wave vertical
where it is an unbalanced system!...

I am not talking driving anything.

I am talking about receive only.

In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.
I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.
I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....
No, thats not the point.

It is the point as subsequent posts will show. The only inaccuracy in
that statement is where you are driving a Marconi quarter wave vertical
where it is an unbalanced system!...

I am not talking driving anything.

I am talking about receive only.


You just don't get the fact the whether transmitting or receiving the
theory and operation are identical!...
--
Tony Sayer

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Wade wrote:

We've done this one
often enough Tony, both here and on uk.tech.digital-tv. If you connect
a coax feeder to a symmetrical aerial (such as the centre-fed dipole of
a typical TV aerial) without using any form of balun then the outer of
the feeder becomes part of the aerial. If you're transmitting, the
outer of the coax will be 'RF-hot' and will radiate, quite possibly
causing EMC problems to low-level parts of the transmitting equipment as
well as safety concerns if high RF power is involved. When receiving,
unwanted signals picked up on this hot feeder will find their way into
the receiver, however perfect the screening of the coax itself.
I don't think that is actually correct.
It is as equally valid to say that the coax outer 'grounds' one limb of
the dipole at its feed end, and it becomes a 'reflector' boosting the
signal into the other half. Ie. the 'balanced' nature of the antenna is
a myth. It doesn't care what the potential is of any limb: what it cares
about is the difference. It can be 'earthed' at any pint.

Baluns are generally valid on unscreened unearthed transmission lines.
Like twisted pair.
I think that Andy's explanation, and these not a lot you can tell 'im
about TV aerials and associated equipment's, is quite adequate for the
layman...

In Transmission you just don't in practice have unbalanced aerials made,
just -not the done thing-;!.....
No, thats not the point.
It is the point as subsequent posts will show. The only inaccuracy in
that statement is where you are driving a Marconi quarter wave vertical
where it is an unbalanced system!...
I am not talking driving anything.

I am talking about receive only.


You just don't get the fact the whether transmitting or receiving the
theory and operation are identical!...


Not so.

It's like saying that wiring up a motor is no different from wiring up a
microphone.


And in fact its perfectly sensible to run a microphone earth down the
same line as that returning current from a large electric motors.

On Thu, 09 Oct 2008 12:06:20 +0100 someone who may be The Natural
Philosopher wrote this:-

You just don't get the fact the whether transmitting or receiving the
theory and operation are identical!...

Not so.

It's like saying that wiring up a motor is no different from wiring up a
microphone.

A better analogy would be saying that wiring up a motor is no
different to wiring up an alternator. That is the case, as
demonstrated by the motors of trams and an increasing number of
trains being used as alternators in regenerative braking. There are
some differences to do with how the stator magnetic fields are
produced (I am not talking about permanent magnet motors), but these
are ancillary and minor as they are in radio/television systems.

Whether transmitting or receiving the aerial is part of a system and
the theory and operation are identical. If they were not identical
it is difficult to see how a signal could be transmitted between the
two.

Of course television transmitting aerials don't generally look like
television receiving aerials. There are several good reasons for
this, which can be simplified to the fact that the transmissions are
generally in many directions while the reception is (generally) in
one direction.

An exception to this are some of the smaller relays. For example
Aberbeeg http://tx.mb21.co.uk/gallery/aberbeeg.php receives
signals on a log periodic aerial and transmits them on log periodic
aerials.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus

I am talking about receive only.


You just don't get the fact the whether transmitting or receiving the
theory and operation are identical!...

Well not identical, but "reciprocal": whatever affects the transmit
situation will affect receiving in a similar manner.

There can be a communication problem here because RF engineers tend to
take this "reciprocity principle" for granted. Even when thinking about
receiving aerials, they often switch to talking about the transmitting
situation because it is easier to describe. That is perfectly valid
between professionals, but it can leave sceptical outsiders such as TNP
believing they aren't keeping to the subject; when actually they are.

What TNP also hasn't grasped is that the outside surface of the entire
length of coax can act as a long-wire aerial. This is completely
independent of its normal function of "piping" signals down the inside
[1]. In this long-wire mode, the coax can pick up interference from
whatever sources it happens to run close to - notably of course the
mains wiring, which is directly connected to many generators of
impulsive and other noise.

All of these interference signals are picked up on the outside surface
of the coax, but they normally cannot get in through the shield [1]. If
the shielding is good they can only get inside the coax at the top,
where the TV aerial is connected. But then they compete with the TV
signal itself, and Andy explained why this can be a particular problem
for digital signals.

The aim of the balun on a TV aerial is simply to block the entry point
at the top of the coax, for interference signals that have been picked
up by the coax in its unwanted long-wire mode.


[1] Google for "skin effect", which makes the inside and outside of the
coax shield behave as two completely separate conductors.


--
Ian White

In article ,
The Natural Philosopher wrote:
And in fact its perfectly sensible to run a microphone earth down the
same line as that returning current from a large electric motors.

You obviously have little idea about best audio practice. That is having a
completely separate ground from other non audio equipment - which in this
case would be your motor.

Besides, most decent mics are balanced and have no earth connection...

--
*Marathon runners with bad footwear suffer the agony of defeat*

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
And in fact its perfectly sensible to run a microphone earth down the
same line as that returning current from a large electric motors.

You obviously have little idea about best audio practice. That is having a
completely separate ground from other non audio equipment - which in this
case would be your motor.

Besides, most decent mics are balanced and have no earth connection...

I think you missed the note of utter sarcasm that was meant to be
obvious in my post, Dave


I.e. that a common earth with large circulating currents is a disaster,
but there is nothing ultimately wrong with a large unbalanced coax
feeder (as long as its not manipuleated) for a low impedance microphone.

If you want more, sticking a transformer on the end don't cut the
mustard, but using TWIN balanced feed in a seperate shield does,

However what is under discussion is nothing more or less than a length
of 75ohm coax, which it is claimed 'will pick up signals in the sheath'

well so it will, but thats why its earthed and a coax. So that all that
signal is applied equally to the 'cold' side of your RF input stage.
How you drive it isn't going to effects its properties as an aerial.

The Natural Philosopher wrote:
However what is under discussion is nothing more or less than a length
of 75ohm coax, which it is claimed 'will pick up signals in the sheath'

Not "in" - ON the outer surface of the sheath. Completely different
signals can flow on the inside and outside of a coaxial cable. The balun
is to stop interference entering the coax at the open end where it
connects to the aerial.



--
Ian White