Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection point in
the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

Thanks

--
Adam

Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection point
in
the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

That be 75!

Sparks...

On Sun, 22 Feb 2004 17:20:41 +0000, Sparks wrote:

Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection
point
in
the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

That be 75!

Sparks...

Car aerial cable is not normal coax, its very low capacitance.

Dave
--
And you were born knowing all about ms windows....??

On Sun, 22 Feb 2004 16:12:16 GMT, "ARWadsworth"
wrote:


What type of cable do I need? Is it the 50 or 75 ohm coax?

The correct physical size for the fittings. The impedance is
nominally 75 ohms but isn't at all critical. Whether you use 50 or
75 is of little practical significance.

--
Peter Parry.
http://www.wpp.ltd.uk/

In article ,
ARWadsworth wrote:
Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection
point in the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms. It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception. Is a complete aerial for your car that expensive?

--
*Growing old is inevitable, growing up is optional *

Dave Plowman London SW 12
RIP Acorn

On Mon, 23 Feb 2004 00:19:20 +0000 (GMT), Dave Plowman
wrote:


Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms.

The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.

cable impedance= 138 Log (D/d) (d=inner diameter, D=outer diameter)

If the space between the conductors is (as it usually is) filled with
an insulator with a dielectric constant greater than unity then the
impedance calculated must be divided by the square root of the
dielectric constant. The usual insulator is polythene with a
dielectric constant of 2.25 so most solid cable has an impedance
about 2/3 of that calculated using the formula above.

Although 300 ohm co-ax cable can be made it isn't used in cars (or
any other commercial application).

Automobile radios and antennae are designed (using the word loosely)
to match 50, 75 or 130/150 ohm antenna systems. Of these the latter
two are the most common. Most car radio suppliers do not supply the
antenna impedance figure they expect, neither do most antenna
suppliers quote nominal impedance figures. A few, Hirschman for
example, quote 130 or 150 ohms but it is neither standard nor
particularly important.

The feedpoint impedance of the antenna varies widely with frequency
and in car applications a large degree of mismatch is accepted as
accurately matching an end fed rod antenna to the feeder over a range
of frequencies in bands from 190kHz to 110 MHz would simply be too
complicated.

In most cases the link margin is adequate to allow for inefficient
antennas and the cable lengths involved are short enough to make
cable losses insignificant.

In order for a cable's characteristic impedance to make any
difference in the way the signal passes through it the cable must be
at least a large fraction of a wavelength long for the particular
frequency it is carrying. In car applications this only occurs at
VHF frequencies. As matching the antenna to the cable is impractical
most manufacturers try to match the radio to the cable. However, not
many radio manufacturers design to a specific input impedance more
accurately than "low".

As the impedance of the cable only becomes important when the cable
is long and the source and load have the same impedance it follows
that with a wide mismatch from the source and short runs the cable
impedance is pretty much inconsequential when used for broadcast
reception in cars.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.


--
Peter Parry.
http://www.wpp.ltd.uk/

"Peter Parry" wrote in message
news
On Mon, 23 Feb 2004 00:19:20 +0000 (GMT), Dave Plowman
wrote:


Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms.

The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.

cable impedance= 138 Log (D/d) (d=inner diameter, D=outer diameter)

If the space between the conductors is (as it usually is) filled with
an insulator with a dielectric constant greater than unity then the
impedance calculated must be divided by the square root of the
dielectric constant. The usual insulator is polythene with a
dielectric constant of 2.25 so most solid cable has an impedance
about 2/3 of that calculated using the formula above.

Although 300 ohm co-ax cable can be made it isn't used in cars (or
any other commercial application).

Automobile radios and antennae are designed (using the word loosely)
to match 50, 75 or 130/150 ohm antenna systems. Of these the latter
two are the most common. Most car radio suppliers do not supply the
antenna impedance figure they expect, neither do most antenna
suppliers quote nominal impedance figures. A few, Hirschman for
example, quote 130 or 150 ohms but it is neither standard nor
particularly important.

The feedpoint impedance of the antenna varies widely with frequency
and in car applications a large degree of mismatch is accepted as
accurately matching an end fed rod antenna to the feeder over a range
of frequencies in bands from 190kHz to 110 MHz would simply be too
complicated.

In most cases the link margin is adequate to allow for inefficient
antennas and the cable lengths involved are short enough to make
cable losses insignificant.

In order for a cable's characteristic impedance to make any
difference in the way the signal passes through it the cable must be
at least a large fraction of a wavelength long for the particular
frequency it is carrying. In car applications this only occurs at
VHF frequencies. As matching the antenna to the cable is impractical
most manufacturers try to match the radio to the cable. However, not
many radio manufacturers design to a specific input impedance more
accurately than "low".

As the impedance of the cable only becomes important when the cable
is long and the source and load have the same impedance it follows
that with a wide mismatch from the source and short runs the cable
impedance is pretty much inconsequential when used for broadcast
reception in cars.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.


--
Peter Parry.
http://www.wpp.ltd.uk/

How do I repair a dent in the wall behind me?
That lot went over my head at some speed I can tell you!
Frankly I'd buy another arial.

Drew

In article ,
Peter Parry wrote:
Think you'll have problems finding the correct stuff. IIRC, it's
actually 300 ohms.

The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.

snip

Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't. You could well be right about it being 150
ohm rather than 300 - I wasn't sure hence the IIRC. But I *am* sure it
ain't 75.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.

But I think you'll find in practice that neither is an exact match -
receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.

--
*It was all so different before everything changed.

Dave Plowman London SW 12
RIP Acorn

"Dave Plowman" wrote in message
...
In article ,
ARWadsworth wrote:
Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection
point in the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms. It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.


Is a complete aerial for your car that expensive?

Not an option. The aerial itself is fine. The motorised aerial at the back
of the car connects to the cable (I know that the cable connects to the
aerial and is not a complete unit as I swapped my aerial to the motor
version from a friends car after he had tested his airbags). The biggest
problem is that the coax is part of the wiring loom (or certainly enclosed
with the loom inside some sticky foam) and the inner core of the coax is now
missing.

--
Adam

"Dave Plowman" wrote in message
...
In article ,
Peter Parry wrote:
Think you'll have problems finding the correct stuff. IIRC, it's
actually 300 ohms.

The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.

snip

Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't. You could well be right about it being 150
ohm rather than 300 - I wasn't sure hence the IIRC. But I *am* sure it
ain't 75.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly,
especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.

But I think you'll find in practice that neither is an exact match -
receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.

So if I was to buy a bog standard car aerial with cable and then cannibalise
this cable for my needs, do you think it will work? Now you mention it I do
remember the car radios where you had to tune into a (was it a weak) MW
station and tweak a pot to match the aerial to the tuner.


--
Adam

On Mon, 23 Feb 2004 18:22:44 GMT, "ARWadsworth"
wrote:


So if I was to buy a bog standard car aerial with cable and then cannibalise
this cable for my needs, do you think it will work?

Go and get a length of TV Coax from a a local shop and use that. If
you buy an aerial with a cable most will be 75ohm cable anyway
(especially motorised aerials).

--
Peter Parry.
http://www.wpp.ltd.uk/

On Mon, 23 Feb 2004 17:22:03 +0000 (GMT), Dave Plowman
wrote:


Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't.

It won't make a blind bit of difference. I'll measure the difference
later this week and post the results.

You could well be right about it being 150 ohm rather than 300

I am - there is no commercially available 300 ohm co-ax.

I wasn't sure hence the IIRC. But I *am* sure it ain't 75.

Well measuring a bit of supposedly "150 ohm" cable I happen to have
lying around I've come up with a figure of 110 ohms. This cable has
an insulator which looks awfully like solid polyethylene so assuming
the same permissivity would give an impedance of about 75 ohms.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.

But I think you'll find in practice that neither is an exact match -

Neither is any form of match at all.

receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.

No - it really doesn't matter - it isn't the same as sticking a
capacitor of that value across the output - it's a ladder network and
the fundamental capacitance is less important than the mismatch
(which isn't that important anyway).

--
Peter Parry.
http://www.wpp.ltd.uk/

"Peter Parry" wrote in message
...
On Mon, 23 Feb 2004 17:22:03 +0000 (GMT), Dave Plowman
wrote:


Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't.

It won't make a blind bit of difference. I'll measure the difference
later this week and post the results.

You could well be right about it being 150 ohm rather than 300

I am - there is no commercially available 300 ohm co-ax.

I wasn't sure hence the IIRC. But I *am* sure it ain't 75.

Well measuring a bit of supposedly "150 ohm" cable I happen to have
lying around I've come up with a figure of 110 ohms. This cable has
an insulator which looks awfully like solid polyethylene so assuming
the same permissivity would give an impedance of about 75 ohms.

It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly,
especially
for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.

But I think you'll find in practice that neither is an exact match -

Neither is any form of match at all.

receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.

No - it really doesn't matter - it isn't the same as sticking a
capacitor of that value across the output - it's a ladder network and
the fundamental capacitance is less important than the mismatch
(which isn't that important anyway).

--
Peter Parry.
http://www.wpp.ltd.uk/

Let's cut all this ****. Car radio aerial cable is specially made and has a
characteristic impedence of 240R. It also has a dense braid screen. By all
means buy TV co-ax but (a) you will get too much loss especially on LW/MW
and (b) unacceptably high interference. What is more whilst you may be able
to get it to fit the radio aerial plug it won't fit the plug now used for
the aerial end in most vehicles.

The original requester should buy a radio-end plug from their local car
radio or electronics shop and fit it to the damaged cable, then use a
ready-made short extension to make it reach the radio.


--

Woody

On Tue, 24 Feb 2004 07:02:20 -0000, "Woody"
wrote:


Let's cut all this ****. Car radio aerial cable is specially made and has a
characteristic impedence of 240R.

Where on earth do you get that from?

It also has a dense braid screen. By all
means buy TV co-ax but (a) you will get too much loss especially on LW/MW

Cable rated at 300MHz will give high loss at 1MHz? How?

and (b) unacceptably high interference.

Where from?


--
Peter Parry.
http://www.wpp.ltd.uk/

On Sun, 22 Feb 2004 16:12:16 +0000, ARWadsworth wrote:

Having had unwelcome visitors last night I now need to replace the cable
from the car stereo to the aerial (or more probably to a connection point
in the cable as I am sure that it will not be one complete run).

What type of cable do I need? Is it the 50 or 75 ohm coax?

Thanks

can I suggest posting this to uk.tech.digital-tv ?

Martin Warby

On Tue, 24 Feb 2004 21:36:29 +0000, Martin Warby
wrote:


can I suggest posting this to uk.tech.digital-tv ?

uk.rec.sheds would be a closer match.


--
Peter Parry.
http://www.wpp.ltd.uk/

In article ,
Martin Warby wrote:
I now need to replace the cable from the car stereo to the aerial

can I suggest posting this to uk.tech.digital-tv ?

Don't they object to off topic postings?

An ICE group might be a better choice.

--
*Caution: I drive like you do.

Dave Plowman London SW 12
RIP Acorn

ARWadsworth wrote:

"Dave Plowman" wrote in message
...

In article ,
Peter Parry wrote:

Think you'll have problems finding the correct stuff. IIRC, it's
actually 300 ohms.

The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.

snip

Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't. You could well be right about it being 150
ohm rather than 300 - I wasn't sure hence the IIRC. But I *am* sure it
ain't 75.


It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly,

especially

for MF reception.

Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.

But I think you'll find in practice that neither is an exact match -
receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.


So if I was to buy a bog standard car aerial with cable and then cannibalise
this cable for my needs, do you think it will work? Now you mention it I do
remember the car radios where you had to tune into a (was it a weak) MW
station and tweak a pot to match the aerial to the tuner.



Not a pot, a capacitor. ON MW as Peter Parry said, there is considerable
mismatch, and the aerial 'looks' capacitative. The variable cap tunes in
the RF stage to more or less use teh cable resonance to tne the frnt end.

On VHF the antenna is nearly matched as a quarter wave whip, and the
cable will be more or less non-reactive.



--
Adam

On Wed, 25 Feb 2004 00:29:13 +0000, Dave Plowman wrote:

In article ,
Martin Warby wrote:
I now need to replace the cable from the car stereo to the aerial

can I suggest posting this to uk.tech.digital-tv ?

Don't they object to off topic postings?

An ICE group might be a better choice.

there seems to be a fair few OT topics (theres one running about motorway
speed signs at the moment.Was thinking it might not be so OT as there are
quite a few people (like say Bill)in there for whom aerial installation is
thier business and know there stuff about coax cable.

Martin Warby