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Cobra CB, Antenna, and Autocom troubleshooting


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The Autocom Super AVI is almost there. I have a pair of windsocks coming to resolve the final turbulence noise for our helmets and the ground loop engine whine is all gone now thanks to a couple of filters.


I went to go for a ride on my own this weekend and so exchanged the BTB radio for the CB. I connected my 3' Wilson antenna to Toolman's antenna mount and then the 9' of coax to my Cobra 38WXST handheld CB using battery power. Then I plugged in the 2-pin Autocom cable and started the bike. I got immediate engine whine that got's louder when I rev'd the throttle. As soon as I unplugged the Autocom cable from the CB, the noise disappeared.


Here are some pictures of the setup.










I figured that since the CB was battery powered there wouldn't be any ground loop problems like I was experiencing with the Stereo and GPS.


Could the antenna/coax be producing the whine? Any ideas on what to try next?


Once I get this resolved, I will have some SWR questions next.



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Try it with the "rubber ducky" antenna and see if the engine noise goes away. If it does, you might want to try a higher quality (better shielded) antenna cable and re-route it away from any power lines.

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Try it with the "rubber ducky" antenna and see if the engine noise goes away. If it does, you might want to try a higher quality (better shielded) antenna cable and re-route it away from any power lines.


Yup... That solved it. You'd think I would have thought of that. dopeslap.gif Such a simple test.


I think I have a better coax cable that I will try when we return from vacation. If I'm running the coax over the top right side of the tank, I should be OK from a power interference front?


From a shielding perspective, I have a lot of extra flat, braided grounding cable (see pictures above). Does anyone think it would help/hurt if I ran the coax cable down the middle of this to add shielding?



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  • 2 weeks later...

The flat braided grounding cable worked perfectly as additional shielding. I ran the coax cable down the center of it and I don't any engine whine anymore. cool.gif


Now for my SWR questions... Forgive me if I'm missing the obvious... I plugged in my SWR meter between the CB and the coax connector per the instructions to test and adjust the antenna length. I have tried this with both a Wilson 3' and Firestik 3'.


If the concept of SWR adjustment is to get to as low a reading as possible and the way that I get there is by shortening the adjustment screw... Why would I ever NOT want to keep the screw as short as possible? confused.gif


In other words, I assume I would never want the screw to be out so that I get a reading of 2 or 3. And it would be absolutely ideal if I could ge to the smallest number possible. Does that make sense?


I tried to do some reading on the topic but I guess I'm still missing the point of SWR. I think I understand the concept but not the practical solution.


Just curious.

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SWR, or Standing Wave Ratio, is the result of the energy being relected back from the antenna and mixing with the forward energy. When the energy is 'in phase' it adds to give a peak, when antiphase it cancels to give a trough. It is the ratio of thes that you measure. The amount of reflected energy is a factor of antenna effiency, the more you radiate the less there is to reflect. The adjustment is to match the effective length of the antenna to 1/4 of the wavelength of the transmitted frequency. It seems that the manufactureres make the antenna a bit long, to allow simplified adjustment instructions. What you are doing is getting the length as close as possible to 'perfect'. In the thoretical 'perfect 1/4 wave pole' al the energy is transmitted, none returned giving a SWR of 1.

CB's operate over a narrow band of frequencies so setting a mid-frequency SWR to its lowest value is close enough over the band.

I have always been anal in this adjustment and have often trimmed the wire type antennas to get an almost perfect match - UK CB is FM and a bit higher frequency so needs a slightly shorter antenna.


Broad band HF radios have an Antenna Tuning Unit (ATU) to adjust the effective length of the antenna for each frequency tuned to. The better ones use SWR monitoring in a feedback loop to achieve this.


Cya, Andy thumbsup.gif

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