On Sat, 24 Jul 2021 10:06:23 -0400, micky <
NONONO...@fmguy.com>
wrote:
>The urls people have posted here (before electronics.repair was added)
>make clear that the ground plane in the car makes a difference, and that
>cars without one need a special antenna cable, but a) they're mostly
>pushed for CB radios, b) it's not at all clear that the special antenna
>is as good c) when shopping for an antenna, any with ground plane
>provision probably make note of it, but those without do not, afaik,
>warn people what is missing.
I beg to differ. Nobody seems to have mentioned the bane of all MF
(medium frequency) reception, RF noise. A metal ground plane (car
body) does a tolerable job of isolating the AM antenna from the noise
generating ignition, black boxes, and gizmos. With a plastic body,
the antenna will pickup more noise from the engine.
The typical car antenna is sometimes located as far away from the
noisy engine as possible and connected to the AM receiver with RG-62/u
93 ohm low-capacitance coaxial cable. There is an adjustable
capacitor between the antenna and the receiver input capacitance to
resonate the antenna system. The coax cable capacitance and the
receiver input capacitance act as a voltage divider. The more coax
cable capacitance to ground, the less signal and noise arrive at the
receiver. Choose your coax cable type and length carefully.
You can have a really sensitive AM receiver, and still not be able to
hear much. The threshold of sensitivity is atmospheric and man-made
noise.
<
https://en.wikipedia.org/wiki/Atmospheric_noise>
Note the graph. At 1MHz, the RF noise (mostly from thunderstorms) is
huge.
<
https://map.blitzortung.org/#3.91/39.62/-91.39>
RF noise from neon signs, motors, sparking of any kind, etc just makes
it worse. If you simply build a bigger antenna, or add an RF
amplifier, you increase both the desired signal and the noise
proportionally. If a receiver and antenna produce some SNR (signal to
noise ratio), and I add more antenna gain, or more RF amplification,
the resultant SNR will be the same. In other words, a bigger antenna
or a "signal booster" don't buy you anything. The trick is to somehow
improve the SNR, which is not easy. See various articles on the
PA0RDT mini-whip antenna for clues:
<
https://www.google.com/search?q=pa0rdt+mini+whip>
Car AM radios tend to have the minimum sensitivity and RF front end
gain needed to function in a strong signal environment. They're not
made for digging signals out of the noise. That's NOT because AM car
radios are made to be inexpensive. It's because the receiver is
sitting next to a very noisy car engine. Were it designed to be as
sensitive as an LF or HF receiver, all you would hear is engine noise.
Try it. Build yourself a BCB (broadcast band) RF amplifier and attach
it to your car radio antenna input. In most cases, you'll hear your
engine, pump motors, and atmospherics quite well, but the distant AM
stations will still be buried under the noise.
I don't have any suggestions to improve your mobile AM reception.
Well, maybe the obvious suggestion to do what you can to eliminate,
move, shield, or isolate sources of RF noise. If weak AM signals
magically appear when you turn off the engine, the source of the noise
is obvious. The problem is that you might do a wonderful job of noise
reduction on your car, such as buy a diesel, but that does nothing if
you're stuck in traffic and surrounded by other noisy vehicles. Notice
that the ultimate noise generator, the all electric car, usually does
not come with an AM radio. For example, Tesla will sell you an
optional overpriced infotainment package that includes AM:
<
https://electrek.co/2020/10/28/tesla-brings-back-radio-infotainment-retrofit/>
Vendors used to sell rubber grounding straps, that discharge any
static buildup on the car body. That should get rid of some noise.
However, I believe carbon doped car tires have largely eliminated the
need for those straps.
So, to answer your question, yes a plastic body gives lousy AM
reception if your engine belches lots of RF noise and your receiver is
reasonably sensitive. If your receiver is stone deaf, it doesn't
matter.
Good luck.
--
Jeff Liebermann
je...@cruzio.com
PO Box 272
http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS
831-336-2558