Radio reception question

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FISHBREATH

FISHBREATH

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A question was raised in the Body & Interior section regarding reduced radio reception when using a fiberglass hood.

I remember something I heard in the distant past about AM radio needing the steel hood for improving "ground wave" reception. With that in mind, I am considering moving my radio antenna to the RR quarter because I will be using a fiberglass hood.

I'm sure some of you guys have knowledge of this.
 
That will help. Look at the Corvettes, which used a steel shield around the distributor and plug wires to help prevent radio noise. To be honest, I'd bet not many Vette owners listened to the radio much, and "back in my day" when AM was still "a thing," I doubt those cars enjoyed the long distance reception that us with steel cars had. Even with that, turning the engine off resulted in better reception!!!

If you want to get serious about this, you could bond a couple of metal attachment points to the rear your hood, sand the back bare, and paint the bottom of the hood with a conductive paint, like aluminum. Then run a couple of flex grounds at the back of the hood to the cowl.

Vette shielding:
 

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Thanks Dart, you are the man I thought would know this.

That is a great idea with the aluminum paint and ground wire. I guess most of the aluminum-coated underside could then be painted over with body color.

Is the crux of the problem shielding the ignition components or something to do with radio wave propagation or both?

I will share this info with the guys in the Body section, if you haven't done so already.
 
I knew about the Vette sheilding and all. Oh, and I really do hate all of that tinwork.

What did the T/A and AAR do?
 
Corvette distributor sheilding was installed because the Corvette body's firewall being fiberglass didn't isolate the radio from ignition noise, it had nothing to do with reception whatsoever.
The fiberglass body on the other hand DID reduce radio reception, so the GM engineers installed what is commonly called a "ground plate" UNDER the radio antenna mount on all mid-year & shark Vettes. It was mounted below the fiberglass quarter panel, and has a ground strap that connects the ground plate to the car's chassis.

George
 
There is truth to the ground plane theory. My personal opinion is that there is enough ground plane in the fender, inner fender and frame to get the job done and done well. That said, the ground plane theory (fact) is mostly for transmitters such as CB, Ham type of radios and has less, if no effect on receivers. There's a lot of other variables to narrow this down to the hood. I listen to AM in my truck while my wifes car doesn't pick up AM hardly at all. There's really no explanation for it. But, do you remember the Chevy antennas in the windshield? One half was the received and one half was the ground plane. Of course they stopped using them but I never had a problem with the one that I had. Maybe someone here will take off their hood and go for a spin so they can let you know how it works. It doesn't mean it will work for you though but I'd try it before you relocated your antenna.
 
Thanks for the info, George and Lead. I would be interested in the metal plate beneath the fender and the aluminum paint because I plan to also use fiberglass front fenders for better weight distribution.
 
You're welcome!
BTW, the ignition shielding pictured above is for 72 and newer Corvette with small block.
In case you were wondering.

:D
 
George R said:
Corvette distributor sheilding was installed because the Corvette body's firewall being fiberglass didn't isolate the radio from ignition noise, it had nothing to do with reception whatsoever.George
Click to expand...

That's a rather skewed way of looking at it!!! If radio noise and other static does not "affect reception," I don't know what does????

Just raising the metal hood on most any car/ pickup "back in" the AM days would result in a terrible noise from the speaker. The GM "good idea" of a windshield antenna later didn't help, either.


So far as a ground plane, that is completely dependent on antenna design. Not all antennas need a ground plane, as some Marine VHF/ UHF antennas for boats are what is called a '1/2 wave' design, and can be mounted anywhere on a glass/ wood boat.

In amateur radio, the Cushcraft R3/ R5 multiband vertical antennas are such a design, and can be operated without a ground plane
 
Here is the ground plate:

http://www.zip-corvette.com/ProductDetails/ProductDetails.aspx?pid=%7B910A7CCC-1BA0-4C21-8641-5D7CB8BD02B8%7D&gid=%7B3E1F4D33-E723-4014-B4DA-0090251AC152%7D&pname=69-73+Antenna+Groundplate&Referer=&Alias=&ptct=SGR-SR&CTitle=&

Bad reception means (to most people anyway) that radio stations won't "come in".
Radio interference (to most people anyway) means that there is noise in the radio like buzzing from the alternator or distributor. That is what the Corvette ignition shielding was made to controll; interference.
In fact, besides the pictures posted above of the shielding kit, there were 2 "V" shaped shields mounted to the engine block to cover the 2 forwardmost sparkplug wires where they ran to the plugs. Thats how important all that shielding was to stop the ignition noise in the radio. It was not for reception.

George
 
LOL. You can think of it that way if you want, but the REASON it was there is to help reduce radio noise and THEREFORE help reception. The two go together hand in hand.

A low noise floor at the antenna terminals of ANY receiver equates to better reception, better signal to noise ratio. It doesn't matter if you turn the engine off or shield the thing, "less noise" at the radio means you are going to be able to RECEIVE weaker stations than you did when the higher noise was present.

This is exactly why receivers are measured in terms of signal to noise -- because the noise makes a difference.
 
Good info, gents, on a subject rarely discussed anymore.

I remember reading in old service manuals that the best AM and FM reception qualities were obtained by changing the mast height on the old stainless steel antennas. Apparently, a matter of six-or-so inches made a big difference in reception.

I suppose that the marine antennas are much higher priced than standard old-style antennas.

By the way, the new-car antennas seem to be of a rather short mast height with spiral-wound wire as the antenna. Any idea about how they perform compared to the old-style antennas of the vintage of our older cars?
 
AM (Amplitude Modulation) antenna had adjustable length to trim to wave amplitude . Low (560) end of band has a different height wave compared to high end (1650). FM, (Frequency Modulation) or length of wave does not require adjustable antenna length for opptamal reception.

Most older car Am radios have a small trim adjustment screw located somewhere on case of radio that sometimes needs to be turned to tune or adjust radio’s electronics to antenna’s impedance when a change in antenna or radio is made. I don’t recall just which frequency one tunes to in order to perform this trim procedure, but I’m sure it’s on line somewhere. Once antenna is properly trimmed, reception will generally improve.

More on trimming here: http://www.electronicspoint.com/am-fm-car-radio-antenna-trimmer-t75872.html

One other reason AM reception is inconsistent throughout the broadcast day is federal regulation of station signal out-put has to be lowered after sun down excepting a few grandfathered 50,000 watt clear channel stations such as WBZ, WOR, WABC, etc. which are generally the pioneer station first to broadcast in North America.

Here in Maine one can pick up most of these clear channel stations once the ionosphere settles down after sunset within a 2000 mile radius, best reception being just before sun rise.

More on this here: http://en.wikipedia.org/wiki/Clear-channel_station
 
Good point. Antenna length of AM is very poor in cars, which had traditionally GREAT receivers with POOR antennas. Bear in mind that the low end of the band, approx 500 kcs, er excuse me HERTZ is a 3:1 frequency / wavelength change compared to the high end

Coupled with the low frequencies, where a good performing antenna would be quite long, and this 3:1 change, it is nearly a miracle "back in the days" that we got the reception we did. I can remember late night weekends when (in the Spokane area) it was a "big thing" to be able to receive KJR from Seattle. There also was a popular station in Canada, but I now longer remember the callsign.

And sometimes you could receive San Francisco and other "far off" stuff. This could be GREATLY improved by careful selection of the parking area (LOL) and of course turning the engine off.

Yup. Antenna trimmers. I've forgotten, found one post that says to adjust them down around 6-700 KHz, but I don't remember. Our "favorite station" back then was 790/ KNEW, later KJRB (sister to KJR) so I always set mine there on 790

Car radios would have been MUCH better performers if the trimmer had been a dashboard control. They make a tremendous difference in performance.

A friend of mine scored a big 'ol CB whip (102") which he hooked to his AM radio, and that thing REALLY worked!!!!

(You "new guys" have to understand, that where I grew up as a kid, there WERE no FM stations we could receive, 4 track tapes had barely been introduced, and the "fad" was those awful "reverb" units)
 
wjajr said:
AM (Amplitude Modulation) antenna had adjustable length to trim to wave amplitude . Low (560) end of band has a different height wave compared to high end (1650). FM, (Frequency Modulation) or length of wave does not require adjustable antenna length for opptamal reception.

Most older car Am radios have a small trim adjustment screw located somewhere on case of radio that sometimes needs to be turned to tune or adjust radio’s electronics to antenna’s impedance when a change in antenna or radio is made. I don’t recall just which frequency one tunes to in order to perform this trim procedure, but I’m sure it’s on line somewhere. Once antenna is properly trimmed, reception will generally improve.

More on trimming here: http://www.electronicspoint.com/am-fm-car-radio-antenna-trimmer-t75872.html

One other reason AM reception is inconsistent throughout the broadcast day is federal regulation of station signal out-put has to be lowered after sun down excepting a few grandfathered 50,000 watt clear channel stations such as WBZ, WOR, WABC, etc. which are generally the pioneer station first to broadcast in North America.

Here in Maine one can pick up most of these clear channel stations once the ionosphere settles down after sunset within a 2000 mile radius, best reception being just before sun rise.

More on this here: http://en.wikipedia.org/wiki/Clear-channel_station
Click to expand...


Now, that brings back memories. I was in college in Daytona Beach, FL in the early 1970s. I used to listen to WABC radio on a cheap bedside clock-radio at night.
 
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