Car radio antenna problem

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Car radio antenna problem Empty Car radio antenna problem

Post by Wildcat445 on Sun May 19, 2013 12:17 pm

I have (had) a car antenna mounted on the roof of my garage to use in testing and repairing car radios. Over the winter, it got damaged by being broken off, presumably by ice rolling down the roof. I am not able to climb the 23 foot height that the antenna is mounted, so I hired a local handyman to replace it. Since the replacement, I have horrible noise in car radios. Previously, there was little, if any, noise in the radio output. I appears to be vibrator hash. I got my binoculars out, and it appears the handyman mounted the antenna to the wood facia, not on the metal roof itself. Could there be a ground problem with the antenna causing this? I can't see that the antenna touches the metal at all. Could something in my power supply have taken a dump, like electrolytic caps? I use an old Electro power supply that weighs a ton. I have used it for 30 years, and my stepdad used it for 30 years before that. It is all original and has not been touched, certainly since I have owned it. The caps in the power supply look like they are as large as a beer bottle,probably unobtanium. It has some nice big selenium rectifiers, too. It develops 0-32 volts at 15 amps continuous. I need to get this figured out. I like to shoot skip at night with an old Buick radio.

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WC

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Post by 75X11 on Sun May 19, 2013 12:54 pm

Might you have a short, or water damage in your antenna line? I would try an alternate antenna and see if the noise reduces any.
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Post by Wildcat445 on Sun May 19, 2013 5:15 pm

It is a new antenna, and the problem appeared immediately after it was installed. I don't have another antenna to try.

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WC

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Post by 75X11 on Sun May 19, 2013 5:44 pm

Are yousing the original antenna connector with a coax line to the outside antenna?
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Post by tuberadiogeek on Sun May 19, 2013 6:43 pm

Use a piece of wire or a metal coat hanger for an antenna and see if you get the same results. Either one of those will suffice as a temporary antenna. Although seeing as that PSU is at least 60 years old, it probably needs recapped. Them electrolytics are likely dried out.
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Post by Motorola man on Sun May 19, 2013 10:24 pm

Car antennas typically require a ground plane. I would start there.
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Post by Wildcat445 on Sun May 19, 2013 10:45 pm

What kind of noise could I expect due to faulty capacitors in the PSU? The origianl antenna was thru the metal roof of the garage. We left the stub there and tarred around it so the roof would not leak. The handyman could not get it off. He attached the new antenna to a bracket that is screwed to the facia. So the theory is that I need the antenna to go back thru the metal roof like the old one. Or could I run a ground wire of some kind to the antenna from the roof? Thanks for the help, everyone.

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WC

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Post by 75X11 on Mon May 20, 2013 1:01 am

Usually bad caps will give a 60 cycle hum. Vibrator noise is more of a buzz. For your antenna, the usual installation is with a shielded line with the shield connected to the bracket or ground plane and the shielded line to the whip. It might be the easiest to start using maybe about a yard of solid wire connected to the center connector of your radio's antenna connector to see if you get quieting with local stations.
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Post by tuberadiogeek on Mon May 20, 2013 1:23 am

You can mount it on a piece of steel, that would act as a ground plane aka the "body" of the car. Without a ground plane it will still work, but not as well. I totally forgot car radio antennas needed a ground..
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Post by Wildcat445 on Mon May 20, 2013 2:33 pm

This antenna is mounted on a homemade metal angle bracket, but is attached to the wood facia, not the metal roof. The fact that there is no ground on the antenna is where my noise is coming from. I could take a screw out of the metal roofing, and attach the antenna bracket to that. Would that provide an adequate ground plane? Thanks.

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WC

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Post by terrydec on Mon May 20, 2013 3:54 pm

Gremlins
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Post by tuberadiogeek on Mon May 20, 2013 4:00 pm

I would think that metal bracket would do the trick, but it may not.. I think some others here may know more about then i do.. However a few years ago i was using a mobile mirror mount CB antenna at my house and i attached a piece of steel vent cover to the wood then screwed the antenna to the piece of steel and it worked good enough to keep my SWR low enough to safely transmit. I would think your car antenna would behave similarly..
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Post by Wildcat445 on Mon May 20, 2013 4:10 pm

Terry, IIRC, the last Gremlin I worked on was a '71 with a leaky transmission and rust over the rear wheel wells.

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WC

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Post by 75X11 on Mon May 20, 2013 4:56 pm

A ground plane is not quite as important on a receive only antenna as on a transmitter's antenna. What kind of cable are you using to connect to the antenna? I do know about receive antennas, from setting up antennas for fire department scanning receivers. I've also set up auto antennae for car radio testing.
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Post by terrydec on Mon May 20, 2013 5:55 pm

Wildcat445 wrote:Terry, IIRC, the last Gremlin I worked on was a '71 with a leaky transmission and rust over the rear wheel wells.

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WC

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Post by Wildcat445 on Mon May 20, 2013 8:01 pm

The antenna I am using is just a common replacement antenna that I got at NAPA. It uses a shielded lead-in cable and I bought an extension cable for it. I use the balance of the original lead-in that worked fine.

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WC

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Post by 75X11 on Mon May 20, 2013 8:43 pm

The reason I asked is that the shield is to provide interference shielding. If there is any damage to the line at the antenna end and the shield has a short or near short, it will channel any noise back to your radio. If the line at the base of your antenna had been kinked that may be the problem. That is the reason I suggested trying the radio with a couple or 3 feet of wire to the center antenna connector alone, to see if the noise reduces any. this would help eliminate the antenna as the source of the noise.
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