In article p87Ud.18186$uc.17012@trnddc09, Jerry Martes says...

How does transmitting-antenna theory differ from receiving-antenna theory?

The insulators are bigger!

:^)

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
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"Jerry Martes" wrote in message
news:%WOTd.62179$wc.45982@trnddc07...

Andrew

All of my experience with commercial Amplified Antennas indicated that
they are usually not able to satisfactorily improve the system
performance.
The concept of including an amplifier between the receiver and the
antenna is quite a good idea where transmission line loss is
"unavoidable". There is value in designing the receiver's input circuit
so it can be adjusted to accomodate whatever antenna impedance gets
connected to it. But, that is seldom done these days.

For your consideration - Since a car can get decent AM recption in your
neighborhood, there is adequet AM signal strength. I'd suggest that if
you have two antenna terminals in the back of a reasonably good household
AM receiver, you'll be able to construct a fairly simple antenna that
works well for all stations.

Can you locate the receiver close to where a long wire can be made
vertical and as long as practical? Then, can a wire be connected to a
ground (water pipe).

Dont be confused by thinking that the car antenna is just a short
telescoping element. The telescoping (short) mast is actually a probe
that couples to the car itself. The car is the antenna. The coax feed
line in the car is only a necesary component for minimizing induced noise.
Dont include any in the house radio antenna for AM.

For FM in remote locations, and in valleys, I have had some gtreat
results with simple home made Yagis. There are alot of web sites on Yagi
antennas. I am available for comments on any aspect of FM Yagi design. It
seems so easy to design and build FM Yagis, I'd encourage you to build a
Yagi for your FM needs. I would expect that you could design and build a
Yagi that performs better than the ones I've built. But, all those I've
built have have worked "good enough".

Jerry
Jerry,

Thanks for the info I'll look around for a "yagi" plan for the FM and go
from there. As far as AM is concerned I can easily run 35' of wire on the
exterior wall and access the receivers with a short horizontal run. Can I
connect more then one receiver to the antenna? or do I need several runs,
and if so could I run say twisted pair or speaker wire and use each
conductor as 1 antenna?

Thanks again

Andrew

"Jerry Martes" wrote in message
news87Ud.18186$uc.17012@trnddc09...

"Ed Huntress" wrote in message
...
"Robert Swinney" wrote in message
...
Ed,
You're on it like a cheap suit! A ground plane must have an area at
least
1/4 lambda ^2 to be effective. The top of a car doesn't have
sufficient
area to work as ground plane at broadcast freqs.

Yeah, it would take one hell of a car. And an effective radiator would
be
tall enough to wipe out the power lines wherever you drove, while the
ground
plane would wipe out the utility poles on both sides of the road. g

Essentially, ground plane
radials provide an artificial ground that is elevated to the effective
height of the antenna wherever it is above earth. It is generally
understood when we speak of "ground plane" re. communications antennas
we
are referring to 50 ohm antennas.

Thanks, Bob. It's good to hear that antenna theory didn't invert itself
since I studied for my 1st Class Phone license.

I'm not following this thread very closely but it sounds to me that some
people are mixing up transmitting-antenna theory with receiving-antenna
theory. A car antenna is just a conductor stuck up there to suck up as
much
electromagnetic radiation as possible. Some of them are loaded at the
base,
but I always assumed that was for FM. 'Don't know for sure.

--
Ed Huntress

Ed

How does transmitting-antenna theory differ from receiving-antenna
theory?

For one thing, it's a matter of coherent radiation. You need it in
transmitting. It may be useful (in the inverse) in receiving, but in
receiving, what you need more than anything is a useable signal. So long
antennas that may be picking up from numerous directions can be useful in
receiving, especially if they pick up more signal strength.

You don't have to worry about harmonic radiation in receiving. You don't
have to worry about power reflected to the transmitter. So impedance
coupling is less of an issue.

There are other things but it's been a long, long time since I've looked.

--
Ed Huntress

"jim rozen" wrote in message
...
In article , Robert Swinney says...

Ed,
You're on it like a cheap suit! A ground plane must have an area at
least
1/4 lambda ^2 to be effective.

Umm. It won't be a resonant antenna. But the car's body
will improve the signal seen at the input to the radio.

Maybe, maybe not. You may be defeating signal strength, since the "ground
plane" doesn't really function as a ground plane.


It doesn't *have* to be a resonant system for the antenna
to work.

True enough, but the effect of a small ground plane may or may not be
helpful.

--
Ed Huntress

"Eric R Snow" wrote in message
...
On Sat, 26 Feb 2005 17:33:20 -0500, "Ed Huntress"

wrote:

If I understand it correctly, (which I doubt ), since the car doesn't
act as the ground plane, the ground must. If this so, how come a car
radio that worked very well at recieving faint stations, hardly picks
up anything when I tried to use on the bench with a regular car
antenna? The antenna was connected properly, with the cable plugged
into the radio and the antenna mounting screw connected to the ground
screw on the radio.
Thanks,
Eric

'Don't know, Eric. I'd have to go study it again to have any idea.

--
Ed Huntress

"jim rozen" wrote in message
...
In article wY5Ud.56428$uc.36861@trnddc04, Jerry Martes says...


You probably got a mixture of messages while reading these posts.
But, I
would put myself amoungst thoes who wouldnt find value in using the term
"ground plane" when working with a conductor thats shorter than a few
electrical degrees at the frequency of interest.

It's a semanitic issue, I know. For the sake of argument, one
could say that the body of a car is the 'other part of a non-resonant
dipole antenna.'

Because it's a mostly flat part and mostly planar in shape, the
temptation to call it a ground plane is obvious.

If it were a 2 meter whip antanna stuck on the roof of the car,
it would be a very good description. For 1 MHz radiation the description
does leave a bit to be desired as you say.

Circuit board designers call the continuous conductor on their
board a "ground plane" even though that could be a few inches
square.

It is a ground plane in that case, in terms of DC, and in terms of
preventing certain radiation from penetrating it. What it is NOT is a ground
plane in the sense the term is used in antenna theory: as a plane of ground
potential that serves as a sort of clamp against which the radiative energy
emitted from the radiator reflects.

For me the term "ground plane" does not have to have any
particular wavelength of interest to be applied. I like the
term 'ground plane' even for a car body, for 1 MHz am because
it's descriptive of the *shape* of the conductor more than
anything else. That, and the input coil of the receiver is
stuck across that coax feedline, and the shield of the coax
is bonded in most cases to the chassis of the radio, and to
the car body at the other end.

The idea being that there is some rf voltage developed between
the bottom of the vertical and the body of the car, by virtue
of it (the vertical) being immersed in the local rf field.

This voltage is larger than the voltage that would be there, if the
car body were absent; that is if the coax shield simply stopped and
the whip were tagged on the end, out in the middle of nowhere.

Granted not a lot bigger, about a factor of two or three
probably. Remember that while the car is sitting on top
of rubber tires, there's a large capacitence between the car
body, and the actual ground.

No, no. the interelectrode distance is much too great for there to be much
capacitance. The thickness of a tire reduces the capacitance to a value so
low you'd be hard-put to measure it. Do the calculation; it's 'way down
there, despite the area of the car. If it were 1/4 inch or so, it would be
significant -- if the car was sitting on a sheet of metal that was grounded,
which it is not.

My guess is that the whole system is functioning as an unbalanced dipole.
I'll see if I can find anything on it later tonight. I don't recall any
discussions about it in the various ARRL handbooks.

--
Ed Huntress

"Andrew V" wrote in message
...

"Jerry Martes" wrote in message
news:%WOTd.62179$wc.45982@trnddc07...

Andrew

All of my experience with commercial Amplified Antennas indicated that
they are usually not able to satisfactorily improve the system
performance.
The concept of including an amplifier between the receiver and the
antenna is quite a good idea where transmission line loss is
"unavoidable". There is value in designing the receiver's input circuit
so it can be adjusted to accomodate whatever antenna impedance gets
connected to it. But, that is seldom done these days.

For your consideration - Since a car can get decent AM recption in your
neighborhood, there is adequet AM signal strength. I'd suggest that if
you have two antenna terminals in the back of a reasonably good household
AM receiver, you'll be able to construct a fairly simple antenna that
works well for all stations.

Can you locate the receiver close to where a long wire can be made
vertical and as long as practical? Then, can a wire be connected to a
ground (water pipe).

Dont be confused by thinking that the car antenna is just a short
telescoping element. The telescoping (short) mast is actually a probe
that couples to the car itself. The car is the antenna. The coax feed
line in the car is only a necesary component for minimizing induced
noise. Dont include any in the house radio antenna for AM.

For FM in remote locations, and in valleys, I have had some gtreat
results with simple home made Yagis. There are alot of web sites on Yagi
antennas. I am available for comments on any aspect of FM Yagi design.
It seems so easy to design and build FM Yagis, I'd encourage you to build
a Yagi for your FM needs. I would expect that you could design and
build a Yagi that performs better than the ones I've built. But, all
those I've built have have worked "good enough".

Jerry
Jerry,

Thanks for the info I'll look around for a "yagi" plan for the FM and go
from there. As far as AM is concerned I can easily run 35' of wire on the
exterior wall and access the receivers with a short horizontal run. Can I
connect more then one receiver to the antenna? or do I need several runs,
and if so could I run say twisted pair or speaker wire and use each
conductor as 1 antenna?

Thanks again

Andrew

Andrew

I have never considered connecting two receivers to one antenna at AM.
I'd avoid doing that just because I'm chicken. There are guys around here,
like Don Foreman who could comment on the hazards of connecting two
receivers together at their antenna terminals.

For FM, do you have a specific station you want to listen to? It gets
pretty easy to build a Yagi from old scraps of wire and PVC if you dont have
to rotate the antenna and listen to stations from various azimuth angles.
I'd be happy to scan some of the sketches i used to build the 6 element FM
yagi I made last month. My sister lives about 40 miles South of Palm
springs and there is a 8,000 foot mountain in the middle of the path to her
house. The signals were very unreliable with the "regular FM antenna" at
her house. With the 6 element Yagi the radio was so clear, night and day,
that I'd try a 3 element if I had it to do again.

Jerry

Ed

You are way ahead of me with antenna theory. I got thrown off as soon as
I read "coherent radiation". Thats a term I never heard of as related to
antenna theory.
I probably need to get into reading more books. I've been under the
impression that antennas behave the same for transmit as for receive.

Jerry


"Ed Huntress" wrote in message
...
"Jerry Martes" wrote in message
news87Ud.18186$uc.17012@trnddc09...

"Ed Huntress" wrote in message
...
"Robert Swinney" wrote in message
...
Ed,
You're on it like a cheap suit! A ground plane must have an area at
least
1/4 lambda ^2 to be effective. The top of a car doesn't have
sufficient
area to work as ground plane at broadcast freqs.

Yeah, it would take one hell of a car. And an effective radiator would
be
tall enough to wipe out the power lines wherever you drove, while the
ground
plane would wipe out the utility poles on both sides of the road. g

Essentially, ground plane
radials provide an artificial ground that is elevated to the effective
height of the antenna wherever it is above earth. It is generally
understood when we speak of "ground plane" re. communications antennas
we
are referring to 50 ohm antennas.

Thanks, Bob. It's good to hear that antenna theory didn't invert itself
since I studied for my 1st Class Phone license.

I'm not following this thread very closely but it sounds to me that
some
people are mixing up transmitting-antenna theory with receiving-antenna
theory. A car antenna is just a conductor stuck up there to suck up as
much
electromagnetic radiation as possible. Some of them are loaded at the
base,
but I always assumed that was for FM. 'Don't know for sure.

--
Ed Huntress

Ed

How does transmitting-antenna theory differ from receiving-antenna
theory?

For one thing, it's a matter of coherent radiation. You need it in
transmitting. It may be useful (in the inverse) in receiving, but in
receiving, what you need more than anything is a useable signal. So long
antennas that may be picking up from numerous directions can be useful in
receiving, especially if they pick up more signal strength.

You don't have to worry about harmonic radiation in receiving. You don't
have to worry about power reflected to the transmitter. So impedance
coupling is less of an issue.

There are other things but it's been a long, long time since I've looked.

--
Ed Huntress

"Jerry Martes" wrote in message
news:Cf8Ud.49111$uc.35358@trnddc03...

I have never considered connecting two receivers to one antenna at AM.
I'd avoid doing that just because I'm chicken. There are guys around
here,
like Don Foreman who could comment on the hazards of connecting two
receivers together at their antenna terminals.

Any good info on yagis will tell you that the characteristic impedence of a
yagi is either 300 ohms (loop dipole active element) or 75 ohms (plain
dipole active element). These are more-or-less figures. A specific yagi may
turn out to be 150 or 50 ohms, depending on element spacing. Any good design
you find on the web should specify the impedance.

For the first, you use TV twin-lead. For the second, you have a bit of a
problem because, although coax will give you the right impedence, coax is
unbalanced and the antenna is balanced. This matters less in receiving than
in transmitting but it will attenuate your signal.

All of which is to say, a home made yagi makes a great FM- or single-channel
TV antenna (I've made them, using oak flooring planks to hold the elements),
but you really have to watch the way you feed the signal into the receiver,
or you'll lose so much signal strength that it can all be a waste of time.

A loop-element yagi feeding the receiver with 300-ohm TV twin lead is your
best bet. If you want to feed multiple receivers, you need an antenna
transformer/splitter. These are sometimes called "baluns," which is a
misnomer unless you're going from balanced (twin lead) to unbalanced (coax).
They may be very hard to find in all-300-ohm form. A common form is a
300-ohm-in, 75-ohm-out balun (in this case, the term is correctly used). You
can find them easily at TV shops or maybe Radio Shack, and you can then use
the 300-ohm twin-lead from antenna to balun, and 75-ohm coax from balun to
your receiver.

None of this info should be hard to find on the web. None of it is hard to
build or buy. The antennas are a snap to make for FM frequencies, especially
if you don't have to rotate them for different stations. The baluns are very
cheap, and the wire also is pretty cheap.

--
Ed Huntress

"Jerry Martes" wrote in message
news:dm8Ud.49138$uc.28761@trnddc03...
Ed

You are way ahead of me with antenna theory. I got thrown off as soon
as
I read "coherent radiation". Thats a term I never heard of as related to
antenna theory.
I probably need to get into reading more books. I've been under the
impression that antennas behave the same for transmit as for receive.

Well, that's not a bad place to start. A more accurate way to think of it is
that a good transmitting antenna will make a good receiving antenna. But a
good receiving antenna may make a miserable transmitting antenna, which will
burn up your transmitter in a second.

There is more info about on transmitting antennas than on receiving types.
Receiving antennas can be much simpler and work very well at the same time.

--
Ed Huntress

Ed Huntress wrote:


For one thing, it's a matter of coherent radiation.

Say what?


Ed, antennas are reciprocal devices. One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

Kevin Gallimore


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
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Ed

I must have misread your previous comment that ANTENNA THEORY is different
for transmit antennas than for receive antennas. I'm pretty sure the
Antenna Theory is the same for both Transmit and Receive.

There hasnt been a question that any given antenna might be difficult to
use as either a receive antenna or a transmit antenna. I'm convinced that
it would very unusual to find an antenna who's radiation pattern or its
terminal impedance is different for transmit than for receive. Where have I
gone wrong?

Jerry





"Ed Huntress" wrote in message
...
"Jerry Martes" wrote in message
news:dm8Ud.49138$uc.28761@trnddc03...
Ed

You are way ahead of me with antenna theory. I got thrown off as soon
as
I read "coherent radiation". Thats a term I never heard of as related to
antenna theory.
I probably need to get into reading more books. I've been under the
impression that antennas behave the same for transmit as for receive.

Well, that's not a bad place to start. A more accurate way to think of it
is
that a good transmitting antenna will make a good receiving antenna. But a
good receiving antenna may make a miserable transmitting antenna, which
will
burn up your transmitter in a second.

There is more info about on transmitting antennas than on receiving types.
Receiving antennas can be much simpler and work very well at the same
time.

--
Ed Huntress

"Jerry Martes" wrote in message
news:5v7Ud.45587$uc.4010@trnddc08...

"Eric R Snow" wrote in message
...
On Sat, 26 Feb 2005 17:33:20 -0500, "Ed Huntress"

wrote:

If I understand it correctly, (which I doubt ), since the car doesn't
act as the ground plane, the ground must. If this so, how come a car
radio that worked very well at recieving faint stations, hardly picks
up anything when I tried to use on the bench with a regular car
antenna? The antenna was connected properly, with the cable plugged
into the radio and the antenna mounting screw connected to the ground
screw on the radio.
Thanks,
Eric


Eric

I'd submit that your experience with transferring the car's "antenna" to
the bench (without the car) gives credence to my theory that the car is
the antenna, and what we call a car antenna is a probe that senses the
currents induced in the car by the radio wave as it passes (at AM
frequencies).

Jerry

Jerry,

You are correct. The car, although not an efficient ground plane at AM
freqs., does in fact, act as part of the antenna. The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its radiation
below the whip (probe). That type of antenna is "tuned" by varying it's
length until it presents 50 ohms to the transmission line. A whip shorter
than 1/4 wave length, AM broadcast on a car for example, can't be tuned to
exhibit 50 ohms - i.e., it can't be a 1/4 wave dipole because there is
simply not enough room. Such an antenna presents a much higher capacitive
impedance. The cable feeding such an antenna is not a transmission line in
the truest sense; it is more like a driven conductor inside a shield.

Bob Swinney

--
Ed Huntress
(remove "3" from email address for email reply)
"axolotl" wrote in message
...
Ed Huntress wrote:


For one thing, it's a matter of coherent radiation.

Say what?


Ed, antennas are reciprocal devices. One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

Kevin Gallimore


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption
=----

Jim sez: It won't be a resonant antenna. But the car's body
will improve the signal seen at the input to the radio.

Tha's what Jerry has been trying to tell you. If by resonant you mean an
antenna that presents a transmission line with its characteristic
impedance -- then a whip, even though it is too short to be a 1/4 wave
dipole, will still be a better antenna if it has an approximation to a
ground plane under it. Jerry's car is part of the antenna!

Bob Swinney

"jim rozen" wrote in message
...
In article , Robert Swinney says...

Ed,
You're on it like a cheap suit! A ground plane must have an area at least
1/4 lambda ^2 to be effective.


It doesn't *have* to be a resonant system for the antenna
to work.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

Bob

I wonder if all this is actually helping Andrew V. I think we've offered
him more info than he really wanted.
I had convinced myself I was (am) pretty well informed on antenna design
and made the mistake of taking offense to being corrected about ground
planes. I suspect Andrew would have been better served if I had *not*
commented about my use of the term "ground plane".

Andrew does have an interesting project if he is a long distance from AM
and FM broadcast stations. But, that is exactly the situation that
interests me. I like antennas. I like VHF DX

Jerry


"Robert Swinney" wrote in message
...

"Jerry Martes" wrote in message
news:5v7Ud.45587$uc.4010@trnddc08...

"Eric R Snow" wrote in message
...
On Sat, 26 Feb 2005 17:33:20 -0500, "Ed Huntress"

wrote:

If I understand it correctly, (which I doubt ), since the car doesn't
act as the ground plane, the ground must. If this so, how come a car
radio that worked very well at recieving faint stations, hardly picks
up anything when I tried to use on the bench with a regular car
antenna? The antenna was connected properly, with the cable plugged
into the radio and the antenna mounting screw connected to the ground
screw on the radio.
Thanks,
Eric


Eric

I'd submit that your experience with transferring the car's "antenna" to
the bench (without the car) gives credence to my theory that the car is
the antenna, and what we call a car antenna is a probe that senses the
currents induced in the car by the radio wave as it passes (at AM
frequencies).

Jerry

Jerry,

You are correct. The car, although not an efficient ground plane at AM
freqs., does in fact, act as part of the antenna. The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its
radiation below the whip (probe). That type of antenna is "tuned" by
varying it's length until it presents 50 ohms to the transmission line. A
whip shorter than 1/4 wave length, AM broadcast on a car for example,
can't be tuned to exhibit 50 ohms - i.e., it can't be a 1/4 wave dipole
because there is simply not enough room. Such an antenna presents a much
higher capacitive impedance. The cable feeding such an antenna is not a
transmission line in the truest sense; it is more like a driven conductor
inside a shield.

Bob Swinney

"axolotl" wrote in message
...
Ed Huntress wrote:


For one thing, it's a matter of coherent radiation.

Say what?

That refers to the fact that you don't want to transmit harmonics if you can
avoid it. You want to transmit a single frequency. In receiving, it doesn't
usually matter. In fact, antennas made strictly for receiving aren't usually
tuned at all.

That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600 kHz)
without tuning. If you tried that with a transmitting antenna, your
impedance at the feedline end would be all over the map, and the reflected
power would, at some point, probably blow up your output stage.



Ed, antennas are reciprocal devices.

A transmitting antenna will be capable of transmitting and receiving. A
receiving antenna usually is not capable of it, unless it's also designed
for transmitting. Car antennas are not.

One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

Again, receiving antennas for the AM band usually work better the longer
they are. Most receiving antennas are low-Q designs so impedance matching is
less of an issue. Still, they can have a gain advantage due to length
(longwire effect), and the gain may outweigh the loss due to mismatch.

In any case, the point is that, in practice, you can get good performance in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used for
transmitting.

--
Ed Huntress

Jerry sez:

"I like antennas. I like VHF DX"

Yeah! Better'n cell phone DX.

Bob Swinney


"Jerry Martes" wrote in message
news:Ja9Ud.62446$wc.35089@trnddc07...
Bob

I wonder if all this is actually helping Andrew V. I think we've
offered him more info than he really wanted.
I had convinced myself I was (am) pretty well informed on antenna design
and made the mistake of taking offense to being corrected about ground
planes. I suspect Andrew would have been better served if I had *not*
commented about my use of the term "ground plane".

Andrew does have an interesting project if he is a long distance from AM
and FM broadcast stations. But, that is exactly the situation that
interests me. Jerry


"Robert Swinney" wrote in message
...

"Jerry Martes" wrote in message
news:5v7Ud.45587$uc.4010@trnddc08...

"Eric R Snow" wrote in message
...
On Sat, 26 Feb 2005 17:33:20 -0500, "Ed Huntress"

wrote:

If I understand it correctly, (which I doubt ), since the car doesn't
act as the ground plane, the ground must. If this so, how come a car
radio that worked very well at recieving faint stations, hardly picks
up anything when I tried to use on the bench with a regular car
antenna? The antenna was connected properly, with the cable plugged
into the radio and the antenna mounting screw connected to the ground
screw on the radio.
Thanks,
Eric


Eric

I'd submit that your experience with transferring the car's "antenna"
to the bench (without the car) gives credence to my theory that the car
is the antenna, and what we call a car antenna is a probe that senses
the currents induced in the car by the radio wave as it passes (at AM
frequencies).

Jerry

Jerry,

You are correct. The car, although not an efficient ground plane at AM
freqs., does in fact, act as part of the antenna. The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its
radiation below the whip (probe). That type of antenna is "tuned" by
varying it's length until it presents 50 ohms to the transmission line.
A whip shorter than 1/4 wave length, AM broadcast on a car for example,
can't be tuned to exhibit 50 ohms - i.e., it can't be a 1/4 wave dipole
because there is simply not enough room. Such an antenna presents a much
higher capacitive impedance. The cable feeding such an antenna is not a
transmission line in the truest sense; it is more like a driven conductor
inside a shield.

Bob Swinney

Robert Swinney wrote:

The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its radiation
below the whip (probe).

Are you confusing a dipole with a monopole?

Kevin Gallimore

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"Jerry Martes" wrote in message
news:gQ8Ud.31503$uc.15851@trnddc01...
Ed

I must have misread your previous comment that ANTENNA THEORY is
different
for transmit antennas than for receive antennas. I'm pretty sure the
Antenna Theory is the same for both Transmit and Receive.

There hasnt been a question that any given antenna might be difficult to
use as either a receive antenna or a transmit antenna. I'm convinced that
it would very unusual to find an antenna who's radiation pattern or its
terminal impedance is different for transmit than for receive. Where have
I
gone wrong?

Oh, boy, now I have to shake 30 years of cobwebs out. g

The *basic* theory is the same. In practice, there are both theoretical and
practical differences.

In receiving, harmonics don't matter as much, if at all. So you aren't
worried about the impedance of the receiving antenna at harmonics to the
frequency you're interested in.

Another theoretical matter is that you may experience different polarization
in receiving, and a cockeyed wire may actually do better than a highly
polarized and tuned antenna. For example, in the daytime, AM broadcast
waves, transmitted by ground wave, are vertical-polarized. At night, when
the waves are skipped off the ionosphere, the polarization varies in an
(apparently) random way. A low-Q, non-directional antenna may be better for
receiving in such circumstances. Remember, in transmitting, you determine
the polarization, so it's not a factor in efficiency. In receiving, you have
to take what you get. On the other side, my vague recollection is that
attenuation from mismatched polarization at AM broadcast frequencies is
quite low. But you asked about the theory...

Beyond that, in practice, communications receivers and consumer-type
broadcast receivers, like car receivers, are quite different from each
other. The front end of a broadcast receiver usually is low-Q, and gain in
the receiver overall usually is more than you need. You aren't nearly as
worried about actual coupling efficiency and the result of a low-Q front end
is that there is much less current fall-off in the first stage as you tune
the dial, because you aren't tuning the first stage.

That's good, because broadcast receiver front ends (with rf amplifiers,
which car radios used to have -- I assume they still do) don't tune. They're
broadband amps, if they're amps at all. And their impedance swings widely
from one end of the dial to the other. Which is OK, because of the low-Q
front end and high gain of most of those receivers.

In other words, you're wasting your time to fine-tune an antenna for one of
those receivers. A perfectly-tuned, impedance-matched transmitting antenna
*with gain* will beat a random wire. At the same time, you can get more
current flowing through the output end of a long antenna, if the impedance
its seeing isn't such that it's reflecting a lot of power back up the
antenna, than you can from a short-element transmitting antenna, such as a
loaded whip. With low-Q or loosely coupled front ends on the receivers,
that's generally the case.

Now, car antennas aren't "long" antennas in the sense antenna engineers use
the term, which is, IIRC, 3.5 X the wavelength or more (at 3.5 X wavelength,
a long wire has the same gain as a dipole). But, in practice, wire-type
antennas for AM broadcast tend to work better when they're longer,
particularly when they're feeding crappy receivers.

Since it's been 32 years, I think, since I took my test for First Class
Radiotelephone Operator (with radar endorsement), it would be wise to
double-check my statements before putting your money on the table. My last
edition of the ARRL Handbook is 1990. I don't trust my memory that well,
frankly. But that's my recollection. I used to have some good antenna
engineering books but they appear to be long gone.

--
Ed Huntress

In article , Ed Huntress says...

It doesn't *have* to be a resonant system for the antenna
to work.

True enough, but the effect of a small ground plane may or may not be
helpful.

Experimental comments given in another post in this thread
indicate it's actually needed.

Jim


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Ed

You are missing the point. There is no difference in the *antenna* when
used as either a transmitting antenna or as a receiving antenna. You
persist in getting involved with how the antenna is used. Antenna Theory is
the same for any given antenna whether it is used as a transmitting antenna
as or as a receiving antenna.
If you want to relieve the specs for a receiving antenna because some
aspects of antenna design arent important, thats OK. But, when you post
information that indicates that THEORY changes dependent on if the antenna
is transmitting or receiving, you are wrong. I have tried to get you to
think it over with my previous posts. But you seem to want to bring
radiation of harmonics into this 'weak AM signal' discussion. Whats that
all about.

And, since there is no need for transmitting radio waves from Andrew's
site, the transmitting antenna theory probably doesnt find much application
to Andrew's situation.

Jerry

Jerry







That refers to the fact that you don't want to transmit harmonics if you
can
avoid it. You want to transmit a single frequency. In receiving, it
doesn't
usually matter. In fact, antennas made strictly for receiving aren't
usually
tuned at all.

That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600 kHz)
without tuning. If you tried that with a transmitting antenna, your
impedance at the feedline end would be all over the map, and the reflected
power would, at some point, probably blow up your output stage.



Ed, antennas are reciprocal devices.

A transmitting antenna will be capable of transmitting and receiving. A
receiving antenna usually is not capable of it, unless it's also designed
for transmitting. Car antennas are not.

One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

Again, receiving antennas for the AM band usually work better the longer
they are. Most receiving antennas are low-Q designs so impedance matching
is
less of an issue. Still, they can have a gain advantage due to length
(longwire effect), and the gain may outweigh the loss due to mismatch.

In any case, the point is that, in practice, you can get good performance
in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used
for
transmitting.

--
Ed Huntress

In article , Ed Huntress says...

No, no. the interelectrode distance is much too great for there to be much
capacitance.

Watch out ed, I have a capacitence meter and I know
how to use it!

You have your choice of vehicles to measure, a '93 toyota
2-wheel drive short bed pickup, or an 84 toyota camry.
Take your pick.

My guess is that the whole system is functioning as an unbalanced dipole.
I'll see if I can find anything on it later tonight. I don't recall any
discussions about it in the various ARRL handbooks.

The term 'unbalanced dipole' fits this pretty well I think.

Jim


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Ed Huntress wrote:



That refers to the fact that you don't want to transmit harmonics if you can
avoid it. You want to transmit a single frequency.

I fairly regularly send out coherent signals with a 4 MHz bandwidth.
You're saying you like to use a narrowband antenna to limit your out of
band radiation. Coherence doesn't have a lot to do with it.

In receiving, it doesn't
usually matter. In fact, antennas made strictly for receiving aren't usually
tuned at all.

You might notice that the TV aerial still on your neighbor's house seems
to be made to pick up a certain band of frequencies.


That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600 kHz)
without tuning. If you tried that with a transmitting antenna, your
impedance at the feedline end would be all over the map, and the reflected
power would, at some point, probably blow up your output stage.

Here we get to the crux of the matter. Low frequencies are a special
case for receive antennas, in that the receive signal is noise limited,
i.e., if you gather more signal you will also gather more noise. Galatic
noise is very high a low frequencies, so you use a lousy receive antenna
because a good one won't buy you anything. As you go up in frequency (FM
broadcast for instance) Galactic noise falls off and you see better
antennas.


A transmitting antenna will be capable of transmitting and receiving. A
receiving antenna usually is not capable of it, unless it's also designed
for transmitting. Car antennas are not.

An antenna is what it is. I could pump a respectable amount of power out
of the AM/FM antenna at FM VHF frequencies.

Again, receiving antennas for the AM band usually work better the longer
they are. Most receiving antennas are low-Q designs so impedance matching is
less of an issue. Still, they can have a gain advantage due to length
(longwire effect), and the gain may outweigh the loss due to mismatch.

See above

In any case, the point is that, in practice, you can get good performance in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used for
transmitting.

If this were true you could rip out your satellite dish and hook up your
longwire in its' place. And I would be out of a job.

Kevin Gallimore

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In article , axolotl says...

Ed, antennas are reciprocal devices. One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

I think maybe what ed was getting at was, if somebody goofs up
the impedance matching on a large transmitter output stage, bad
things happen. For a receiver, the signal is simply poor in that
case.

Jim


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In article , Robert Swinney says...

... The car, although not an efficient ground plane at AM
freqs., does in fact, act as part of the antenna. The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its radiation
below the whip (probe). That type of antenna is "tuned" by varying it's
length until it presents 50 ohms to the transmission line. A whip shorter
than 1/4 wave length, AM broadcast on a car for example, can't be tuned to
exhibit 50 ohms - i.e., it can't be a 1/4 wave dipole because there is
simply not enough room. Such an antenna presents a much higher capacitive
impedance. The cable feeding such an antenna is not a transmission line in
the truest sense; it is more like a driven conductor inside a shield.

Not to nit-pick here, but real quarter wave dipoles don't
present 50 ohms to the feedpoint at the center. It's a somewhat
higher impedance, more like 300 ohms.

One can tweak this a bit by drooping the *ends* of the
antenna down towards the ground, or by making a sloping end
fed (zepp) antenna.

This having been said, I always used to feed dipoles without
using a balun, with RG-8. Never had any trouble getting
the antenna to tune up or trouble making contacts.

Jim


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Ed

I read your post and now want to apologize for anything I might have
written about your grasp of the theory of antennas and the use of it to
design antennas.
I didnt realize that you are an old guy and we all know how that can
effect our grasp.

Jerry


"Ed Huntress" wrote in message
...
"Jerry Martes" wrote in message
news:gQ8Ud.31503$uc.15851@trnddc01...
Ed

I must have misread your previous comment that ANTENNA THEORY is
different
for transmit antennas than for receive antennas. I'm pretty sure the
Antenna Theory is the same for both Transmit and Receive.

There hasnt been a question that any given antenna might be difficult
to
use as either a receive antenna or a transmit antenna. I'm convinced
that
it would very unusual to find an antenna who's radiation pattern or its
terminal impedance is different for transmit than for receive. Where
have
I
gone wrong?

Oh, boy, now I have to shake 30 years of cobwebs out. g

The *basic* theory is the same. In practice, there are both theoretical
and
practical differences.

In receiving, harmonics don't matter as much, if at all. So you aren't
worried about the impedance of the receiving antenna at harmonics to the
frequency you're interested in.

Another theoretical matter is that you may experience different
polarization
in receiving, and a cockeyed wire may actually do better than a highly
polarized and tuned antenna. For example, in the daytime, AM broadcast
waves, transmitted by ground wave, are vertical-polarized. At night, when
the waves are skipped off the ionosphere, the polarization varies in an
(apparently) random way. A low-Q, non-directional antenna may be better
for
receiving in such circumstances. Remember, in transmitting, you determine
the polarization, so it's not a factor in efficiency. In receiving, you
have
to take what you get. On the other side, my vague recollection is that
attenuation from mismatched polarization at AM broadcast frequencies is
quite low. But you asked about the theory...

Beyond that, in practice, communications receivers and consumer-type
broadcast receivers, like car receivers, are quite different from each
other. The front end of a broadcast receiver usually is low-Q, and gain in
the receiver overall usually is more than you need. You aren't nearly as
worried about actual coupling efficiency and the result of a low-Q front
end
is that there is much less current fall-off in the first stage as you tune
the dial, because you aren't tuning the first stage.

That's good, because broadcast receiver front ends (with rf amplifiers,
which car radios used to have -- I assume they still do) don't tune.
They're
broadband amps, if they're amps at all. And their impedance swings widely
from one end of the dial to the other. Which is OK, because of the low-Q
front end and high gain of most of those receivers.

In other words, you're wasting your time to fine-tune an antenna for one
of
those receivers. A perfectly-tuned, impedance-matched transmitting antenna
*with gain* will beat a random wire. At the same time, you can get more
current flowing through the output end of a long antenna, if the impedance
its seeing isn't such that it's reflecting a lot of power back up the
antenna, than you can from a short-element transmitting antenna, such as a
loaded whip. With low-Q or loosely coupled front ends on the receivers,
that's generally the case.

Now, car antennas aren't "long" antennas in the sense antenna engineers
use
the term, which is, IIRC, 3.5 X the wavelength or more (at 3.5 X
wavelength,
a long wire has the same gain as a dipole). But, in practice, wire-type
antennas for AM broadcast tend to work better when they're longer,
particularly when they're feeding crappy receivers.

Since it's been 32 years, I think, since I took my test for First Class
Radiotelephone Operator (with radar endorsement), it would be wise to
double-check my statements before putting your money on the table. My last
edition of the ARRL Handbook is 1990. I don't trust my memory that well,
frankly. But that's my recollection. I used to have some good antenna
engineering books but they appear to be long gone.

--
Ed Huntress

"Jerry Martes" wrote in message
news:idaUd.14101$QQ3.10031@trnddc02...
If you want to relieve the specs for a receiving antenna because some
aspects of antenna design arent important, thats OK. But, when you post
information that indicates that THEORY changes dependent on if the antenna
is transmitting or receiving, you are wrong. I have tried to get you to
think it over with my previous posts. But you seem to want to bring
radiation of harmonics into this 'weak AM signal' discussion. Whats that
all about.

'Sorry if I took it off-track, Jerry, but I responded to the issue of
groundplanes, without having seen the earlier posts. I was just jumping in
on that point.

As for the theory, I may have left the wrong impression, but see my comments
on polarization and front-end Q as well as harmonics. I didn't know what you
know or not, and you asked about theory, so...

I didn't mean to confuse the OP but it looked like this discussion had
branched out. As Robert says, there's no way that a car is acting like a
ground plane (in antenna design terms) at AM broadcast frequencies. And the
discussion seemed to long-gone at that point, anyway.

--
Ed Huntress

In article , Ed Huntress says...

In any case, the point is that, in practice, you can get good performance in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used for
transmitting.

I can recall that most of my receivers worked a *lot* better
when the antenna was tuned up to get good matching for transmitting.
A *lot* better. Like, they didn't really work at all unless
the transmatch was set right.

I think that probably most folks who say they are getting good
performance from a random length longwire are either lucky
or they don't really know how good it *could* be.

Jim


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In article 5v7Ud.45587$uc.4010@trnddc08, Jerry Martes says...

I'd submit that your experience with transferring the car's "antenna" to
the bench (without the car) gives credence to my theory that the car is the
antenna, and what we call a car antenna is a probe that senses the currents
induced in the car by the radio wave as it passes (at AM frequencies).

Interesting idea. I'm not quite sure how one would test this.
Wouldn't that say that the whip antenna would work best if it
were bend over to lie near the body of the car?

Jim


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Ed

Now I have really gone wrong. I think I'd better sign off on antennas.
I just posted to your previous thread with the impression that you were
rambling and probably didnt have a grasp. Now that you write this very
coherent post to this thread I realize that I have gone too far with my
criticism of you and your comment about the difference between antennas.

I actually beleive that you have alot of info on antennas and matching
devices. I could have stopped posting yesterday and be better off than I
now am.

Jerry


"Ed Huntress" wrote in message
...
"Jerry Martes" wrote in message
news:idaUd.14101$QQ3.10031@trnddc02...
If you want to relieve the specs for a receiving antenna because some
aspects of antenna design arent important, thats OK. But, when you post
information that indicates that THEORY changes dependent on if the
antenna
is transmitting or receiving, you are wrong. I have tried to get you to
think it over with my previous posts. But you seem to want to bring
radiation of harmonics into this 'weak AM signal' discussion. Whats that
all about.

'Sorry if I took it off-track, Jerry, but I responded to the issue of
groundplanes, without having seen the earlier posts. I was just jumping in
on that point.

As for the theory, I may have left the wrong impression, but see my
comments
on polarization and front-end Q as well as harmonics. I didn't know what
you
know or not, and you asked about theory, so...

I didn't mean to confuse the OP but it looked like this discussion had
branched out. As Robert says, there's no way that a car is acting like a
ground plane (in antenna design terms) at AM broadcast frequencies. And
the
discussion seemed to long-gone at that point, anyway.

--
Ed Huntress

In article , jim rozen says...

In article , Ed Huntress says...

No, no. the interelectrode distance is much too great for there to be much
capacitance.

Preliminary calculations indicate that the number will
be somewhere around 1000 pF. Pretty low.

Jim


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jim rozen wrote:



Not to nit-pick here, but real quarter wave dipoles don't
present 50 ohms to the feedpoint at the center. It's a somewhat
higher impedance, more like 300 ohms.

A thin dipole is about 73 Ohms. If you fold it you will get about 300.

Kevin Gallimore

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On Sat, 26 Feb 2005 19:14:02 -0600, "Robert Swinney"
wrote:

Jim sez: It won't be a resonant antenna. But the car's body
will improve the signal seen at the input to the radio.

Tha's what Jerry has been trying to tell you. If by resonant you mean an
antenna that presents a transmission line with its characteristic
impedance -- then a whip, even though it is too short to be a 1/4 wave
dipole, will still be a better antenna if it has an approximation to a
ground plane under it. Jerry's car is part of the antenna!

The ground plane /EFFECT/ of the car body on the signal strength
from a 1/4 wave radiator proves to me that it has to be part of a
receiving antenna system - otherwise it wouldn't have worked back in
the bad old days, when I used to put a 102" whip on the very back
center of the car and drive around in tight circles fine-tuning the
Squelch and RF Gain settings. Do it in a few widely spaced locations,
and you can easily triangulate the source.

Hell, it even worked when I hung that same 102" antenna off the back
fender of a Schwinn Corvette 3-speed beach-cruiser bicycle, and
adjusted a 23-channel handheld mounted on the handlebars. Just took
longer (and a nice aerobic workout) to get to the Hidden Tee...

(One install where you were guaranteed no ignition noise. And
totally stealth, as long as you weren't winded.) ;-)

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

Jim

I dont know how far you want to go with this antenna thread.

I have submitted that it is more the car thats radiating than the
"antenna". Further, I submit that what we normally refer to as the "car
antenna" is really something that senses, or probes the field induced in the
car body by the passing (AM) radio wave. I supose that if the "antenna
could be made about twice the height of the car body, the "antenna" would
become more than a probe. But, most car antennas are fairly short, so it is
the body that has the major effect on the reception. And the placement of
the probe becomes important.

Jerry




"jim rozen" wrote in message
...
In article 5v7Ud.45587$uc.4010@trnddc08, Jerry Martes says...

I'd submit that your experience with transferring the car's "antenna" to
the bench (without the car) gives credence to my theory that the car is
the
antenna, and what we call a car antenna is a probe that senses the
currents
induced in the car by the radio wave as it passes (at AM frequencies).

Interesting idea. I'm not quite sure how one would test this.
Wouldn't that say that the whip antenna would work best if it
were bend over to lie near the body of the car?

Jim


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On Sat, 26 Feb 2005 15:01:41 -0800, Eric R Snow
wrote:



If I understand it correctly, (which I doubt ), since the car doesn't
act as the ground plane, the ground must. If this so, how come a car
radio that worked very well at recieving faint stations, hardly picks
up anything when I tried to use on the bench with a regular car
antenna? The antenna was connected properly, with the cable plugged
into the radio and the antenna mounting screw connected to the ground
screw on the radio.
Thanks,
Eric

Try connecting the radio chassis to an "earth ground", like an outlet
box. This is an "earth ground" to 60 Hz power, but there is
probably an appreciable run between your bench and an actual earth
ground. That wire or conduit may serve as the "car body".

"axolotl" wrote in message
...
Ed Huntress wrote:



That refers to the fact that you don't want to transmit harmonics if you
can
avoid it. You want to transmit a single frequency.

I fairly regularly send out coherent signals with a 4 MHz bandwidth.
You're saying you like to use a narrowband antenna to limit your out of
band radiation. Coherence doesn't have a lot to do with it.

Aack. I'm going to be sorry I got into this. I knew I shouldn't have used
the word "coherent" as soon as I sent that message. g

Here's what I was talking about: A resonant transmitting antenna, which
often is physically tuned to length in the case of commercial broadcast
transmitting antennas, has induced and reflected waveforms that are in-phase
along its length. A receiving antenna almost never does. The incoming
signals are of different frequencies and, except in those specific cases
where the antenna is physically resonant (no add-on capacitive or inductive
loading), the reflected waves are out-of-phase. Thus, the receiving antenna
has to deal with non-coherent waves along its length, while a transmitting
antenna, if physically tuned to a single frequency, has only coherent waves
*along the antenna*, with no out-of-phase reflectance from the terminal
ends.

Thinking more in terms of physics at the time than of radio terms, I used
"coherence" to express the whole mess. And I was thinking about the waves
*within* the antenna rather than the radiation. That was a mistake, as I
quickly realized that I had bought trouble when you questioned it. g
Instead, I should have said that a transmitting antenna, ideally, is
designed so it severely attenuates harmonic waves (if it's cut to the
half-wavelength resonance or odd multiples of it). Ideally, its induced
waves and reflected waves are in-phase: coherent. That, indirectly, also
aids in the suppression of harmonics.

A receiving antenna, usually, has to deal with fundamentals, harmonics of so
me of those fundamentals, and everything in between. A car antenna has to do
precisely that, in fact, when it tunes between 700 and 1400 kHz, for
example.

I was trying to avoid all of that in explaining what I meant by "coherent,"
and so I chickened out in my first attempt to explain it. d8-)



In receiving, it doesn't
usually matter. In fact, antennas made strictly for receiving aren't
usually
tuned at all.

You might notice that the TV aerial still on your neighbor's house seems
to be made to pick up a certain band of frequencies.

Several bands, actually: two within the VHF band. And that's a function of a
highly compromised design. It's very broadly "tuned," in two separate
segments. All of that complication, all of those tubes, are necessary to
achieve some gain over a range of frequencies from 54 to 216 MHz. If it's a
VHF/UHF combination, it has to range up to 890 MHz.

I think if you examined the design electronically it would look more like a
coupled set of band-pass filters than a tuned circuit.

AM broadcast radio receiving antennas, as I said, usually aren't even
broad-tuned. They're completely un-tuned.


That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600
kHz)
without tuning. If you tried that with a transmitting antenna, your
impedance at the feedline end would be all over the map, and the
reflected
power would, at some point, probably blow up your output stage.

Here we get to the crux of the matter. Low frequencies are a special
case for receive antennas, in that the receive signal is noise limited,
i.e., if you gather more signal you will also gather more noise. Galatic
noise is very high a low frequencies, so you use a lousy receive antenna
because a good one won't buy you anything. As you go up in frequency (FM
broadcast for instance) Galactic noise falls off and you see better
antennas.

That's part of it, and the much greater ease and lower cost of making better
antennas at FM broadcast frequencies is another part of it. And the
combination of widely varying input impedances of receivers as they tune a
3:1 frequency range, as in AM broadcast reception, is still another part of
it. Finally, the low Q of the front ends of AM receivers is another part of
it.



A transmitting antenna will be capable of transmitting and receiving. A
receiving antenna usually is not capable of it, unless it's also
designed
for transmitting. Car antennas are not.

An antenna is what it is. I could pump a respectable amount of power out
of the AM/FM antenna at FM VHF frequencies.

'Depends on what you mean by "respectable," and how much power you had
available to put in. Or if you loaded it with the proper inductance or
capacitance -- or if you got lucky and happened to hit a current antinode
where you fed the antenna. If you hit the wrong spot, you'd just have a very
warm antenna.


Again, receiving antennas for the AM band usually work better the longer
they are. Most receiving antennas are low-Q designs so impedance
matching is
less of an issue. Still, they can have a gain advantage due to length
(longwire effect), and the gain may outweigh the loss due to mismatch.

See above

See above. g Yes, I'm aware of the noise issue. But try it. I spent a lot
of time fooling with broadcast-band DX as a kid. One of my antennas was 250'
long, and it was the best at night. In the daytime, I used one about 30'
long.


In any case, the point is that, in practice, you can get good
performance in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used
for
transmitting.

If this were true you could rip out your satellite dish and hook up your
longwire in its' place. And I would be out of a job.

That has little to do with antennas at AM broadcast frequencies, as you well
know. But here's a related point that some people don't know: A longwire of
5 wavelengths, for example, gives a 5 dB gain over a dipole. As the interest
in radio has gone up the frequency scale, that fact has gotten lost in the
history.

--
Ed Huntress

On 26 Feb 2005 18:27:43 -0800, jim rozen
wrote:

In article , Ed Huntress says...

No, no. the interelectrode distance is much too great for there to be much
capacitance.

Watch out ed, I have a capacitence meter and I know
how to use it!

You have your choice of vehicles to measure, a '93 toyota
2-wheel drive short bed pickup, or an 84 toyota camry.
Take your pick.

I was thinking the same thing! But where do you get the ground to
connect the other side of your cap meter? Cold water pipe maybe?

To start the pool, my guess is 500 pF.

On 26 Feb 2005 19:02:31 -0800, jim rozen
wrote:

In article , jim rozen says...

In article , Ed Huntress says...


Preliminary calculations indicate that the number will
be somewhere around 1000 pF. Pretty low.

That's if the ground is a pretty conductive surface, e.g. wet ground.
I'm guessing it would look like an air capacitor in series with a
leaky capacitor to an effectively zero potential surface somewhere
below the actual surface.

"axolotl" wrote in message
...
Robert Swinney wrote:

The basic 50 ohm
communications antenna (whip) is a 1/4 wave dipole with 1/2 of its
radiation below the whip (probe).

Are you confusing a dipole with a monopole?

No, I don't think so. One of the RF sages, H.A. Wheeler, said "All small
antennas are dipoles, loops, or combinations of these two canonical types".
When a dipole (or monopole over a large ground plane) is short the current
is essentially linear from feed to end. When the dipole is along the z -
axis of a standard spherical coordinate system, E and H fields are produced.
The mathamatical description of these fields can be gotten from an
engineering reference data handbook.

Bob Swinney


Kevin Gallimore

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