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Old 03-05-2009, 02:12 PM   #1
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CB Gremlins

I just installed a Cobra 29 and the SWR meter is reading between high and off the meter. I've been swapping antennas, antenna bases and antenna cords to try to determine the problem.

Best I can figure is the small ground plane. The hood is fiberglass, there is a big fantastic fan hole in my roof, and from the fan back there is the PH.

To try to get better results I moved the magnetic antenna to as near the center of the roof as I could, which means putting it next to the rear corner of the fantastic fan. This is the opposite corner from the power knob to raise the fan cover, and it is on the top not on the fan.

The power sources are isolated- CB goes directly to the starter battery (+) and passenger A pillar (-) with the antenna cord loose. The fantastic fan is powered from the fused distribution block I just put in, and that comes from the house batteries via 10 gauge wire in the overhead.

I set to channel 1 and it was off the meter, so I decided to get the needle back on the meter and turned the SWR calibration all the way down. Then, this happened:


I've reproduced this several times, you have to listen closely the first two times before I zoom in, it's hard to see it move in the video until I zoom.
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File Type: wmv cbgremlins.wmv (1.46 MB, 137 views)
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Old 03-05-2009, 04:09 PM   #2
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Re: CB Gremlins

Is that knob a servo motor control or is it just a mechanical knob? If its a servo control induction can cause it to move. You got a 500 watt linear in there?
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Old 03-06-2009, 12:42 AM   #3
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Re: CB Gremlins

SMB West would trim 1 1/4" from the end of the K-40 antenna, and then adjust it in the base to obtain a correct SWR for the Cobra 18RV that was typically installed. Having the correct length antenna cable is important too. Here is an interesting article that might help;
http://www.stu-offroad.com/misc/myth-1.htm

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Old 03-06-2009, 10:26 AM   #4
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Re: CB Gremlins

I just got off the phone with my CB guy. He said it's the RF.... simple radiation. Get that antenna and coax away from the fan as far as possible. The more you get it away from it, the more it will decrease.

He said you can also light up a fluorescent light bulb too the same way... I want to try that right now.
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Old 03-07-2009, 07:54 PM   #5
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Re: CB Gremlins

With higher power amplifiers you can actually hear transmissions out of speakers that are not hooked up to anything. Simple induction from RF transmissions and harmonics. If you stick the antenna to your ear and key up the mic it can burn you. This depends on the output of the CB's amp. That's why I was wondering what the sound was in the video. Sounded like a motor. If it's close enough some wierd things can happen. With high power it can cause a host of problems hundreds of feet away. Big radio systems can even set off certain alarm systems. I'm sure some of you HAM guys have had an experiance with this.
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Old 03-07-2009, 09:35 PM   #6
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Re: CB Gremlins

Yeah that grind was the motor in the fantastic fan.
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