I have an external vertical antenna on my 2001 Ford F-250 pickup.
It's OEM, for the AM-FM broadcast radio in the cab of the truck.
This is a metal antenna, tapered from base to tip.

This antenna is about 32 inches long, and sticks straight up in the
air. It appears to have something spirally wrapped on the exterior of
it, helically wound, with about 1 turn per inch of antenna length.

Does this antenna serve both AM and FM? I can't tell for sure, but
it looks as though there is coax running from the radio to the base of
the antenna. No markings on the coax that I can discern, but it hard
to see up under the dash, with a flashlight!

What would be a probable ohmic value for this coax, if indeed that
black cable IS coax!? Would it most likely be approx. 50 ohm........or
75 ohm........or what?

What is the likely (or "common") configuration of the above
antenna? Would I call it a ¼ wave vertical for the FM freqs? As you
can tell.......I don't know much about how an AM-FM auto radio and
antenna are designed.

Any chance that the AM part of the radio uses a ferrite rod antenna,
and that the external, visible antenna is FM only?

This radio is a VERY good receiver, compared to 3 other AM-FM receivers
that I have.....Sony, GE, and a Grundig. It "pulls in" weak
stations in an impressive manner.

Please, someone fill me in a bit here, or direct me to a good website.
I _HAVE_ GOOGLED around, but found mostly nothing pertinent to my
questions.

Thank you so much........ Lee Carkenord

wrote:
I have an external vertical antenna on my 2001 Ford F-250 pickup.
It's OEM, for the AM-FM broadcast radio in the cab of the truck.
This is a metal antenna, tapered from base to tip.

This antenna is about 32 inches long, and sticks straight up in the
air. It appears to have something spirally wrapped on the exterior of
it, helically wound, with about 1 turn per inch of antenna length.

Does this antenna serve both AM and FM? I can't tell for sure, but
it looks as though there is coax running from the radio to the base of
the antenna. No markings on the coax that I can discern, but it hard
to see up under the dash, with a flashlight!

What would be a probable ohmic value for this coax, if indeed that
black cable IS coax!? Would it most likely be approx. 50 ohm........or
75 ohm........or what?

What is the likely (or "common") configuration of the above
antenna? Would I call it a ¼ wave vertical for the FM freqs? As you
can tell.......I don't know much about how an AM-FM auto radio and
antenna are designed.

Any chance that the AM part of the radio uses a ferrite rod antenna,
and that the external, visible antenna is FM only?

This radio is a VERY good receiver, compared to 3 other AM-FM receivers
that I have.....Sony, GE, and a Grundig. It "pulls in" weak
stations in an impressive manner.

Please, someone fill me in a bit here, or direct me to a good website.
I _HAVE_ GOOGLED around, but found mostly nothing pertinent to my
questions.

Thank you so much........ Lee Carkenord


While there antennas that are wrapped with wire, such as some shortened
CB antennas, most auto radio antennas that have this spiral appearance
are made that way to cut down on wind noise - it isn't wire.

- 73 de Mike KB3EIA -

I think the AM receiver input on car stereos is VERY VERY high
impedance; the antenna on AM is basically a voltage probe.

I don't know if the FM RX input is the same way, but the antenna is
just a stick of stainless steel.

The car stereo uses the single antenna for both bands as far as I
remember; a ferrite loop doesn't work in a car - - - you'd get fades
going around corners, etc. because of the directionality.

The spiral wrap is plastic. It does something to break up the vortices
that would usually be shed periodically off the antenna in the wind.


Dan

The cable, btw, is designed to be very low capacitance. It's not
normal coax.

That used to be true back in the old vacuum tube radio days. I don't
believe low-C cable is being used these days ... just plain old 50/75 ohm
coax.

Jim



wrote in message
oups.com...
The cable, btw, is designed to be very low capacitance. It's not
normal coax.

wrote in message
oups.com...
The cable, btw, is designed to be very low capacitance. It's not
normal coax.

I measured a piece of auto radio coax, it was 8pf per foot.

"amdx" wrote in message
...

wrote in message
oups.com...
The cable, btw, is designed to be very low capacitance. It's not
normal coax.

I measured a piece of auto radio coax, it was 8pf per foot.


FWIW, my Dodge van's installed radio antenna went bad (open shield, way out
of reach), so I put a Moto-to-BNC adapter on the back of the radio and
dropped a pigtail of RG-58 down by the heater outlet. I connected my MFJ
2m/440 magmount to the radio via the pigtail. Works quite well. If I want
to use it for ham radio, a couple of quick connector twists is all it takes.

Added bonus: When driving out of town, away from strong signals and low
overheads, I unscrew the stock MFJ whip, exchange it for a straight whip
several feet long and get outstanding broadcast reception, both AM & FM.