Strange SWR Question

[dapayne12]

Hi,

This is my second tuning an antenna with an SWR meter. The first time I had no problems, but I will describe bellow.

I have two radios in two vehicles. The first is a Uniden Pro 510xl in a car. With a separate SWR meter I was able to adjust the SWR to almost 1 on channel 20 and around 1.4/1.3 on channels 1 and 40. I recently installed a 29 NW LTD Classic with NightWatch Display in a truck. The Cobra 29 has a built in SWR meter. I am having a much more difficult time tuning the SWR on the Cobra radio. The strangest thing that I notice, the SWR depends on where I hold the microphone. I can tune it that the SWR is near 1 when I hold the microphone near the floor of the vehicle but as soon as I bring the microphone up to where one would talk with it the SWR shoots up to just bellow 3. If I continue to use it like this the SWR will slowly move up past 3 activating the antenna warning light without doing anything to the antenna itself. I find this strange, with the Uniden radio it doesn't mater where I hold the microphone, the SWR reading is always the same and it doesn't ever move up like it does on the Cobra radio. Does anyone know what could be causing this? I read a few different web pages on how to tune with an SWR meter and never did it say anything about where the microphone is positioned can effect the SWR reading, or about the SWR reading moving after time without doing anything to the antenna.

Thanks,

David

[Red Warrior]

Sounds like the microphone cable is acting like a counterpoise for your antenna. This would point to a grounding issue. Make sure the antenna is adequately grounded and run a seperate ground wire from the case of the radio to ground.

[Grump]

The meters in the radios aren't very accurate you may want to get your meter and a jumper and use the external meter and go off of that on that is what I would do

[Foxhunter]

You may already know this but still let me suggest an obvious consideration that is possible. Make sure you have the "Mic Gain" knob turned completely down. If not you may be picking up a bit of feedback from the radio back into the mic's recieve. Remember when you are checking SWR the mic gain should be completely turned down/off and the operator remain silent, any noise will affect and raise the SWR reading. You always check SWR with an "unmodulated carrier" (ie--no noise/don't speak). The use of a good external meter, if you have one, is always suggested over the one on the radio.

['Doc]

The quick-n-dirty answer is that since the radio and mic seem to be having that much affect on things from an SWR stand point, then there's something 'not right' about the antenna, where/how it's mounted. Won't try to guess what, cuz I have no idea what the antenna/truck/mounting is like. But that's where I'd start looking.
- 'Doc

[dapayne12]

Hi,

Thanks for all the tips. After reading I think it's the ground, but I will try to give some more details. I had the mic gain all the way down because that's what was suggested in the manual for the CB when measuring the SWR. I could use the SWR meter I have borrowed from a friend, but it's very old, I'm not sure if it's very good or not. The antenna is a Cobra CB 28-in. Antenna, magnetic mount placed in the centre of the roof on the truck. I currently have the coaxial cable going out through the passenger door. A friend of mine assisted me in connecting the power cables for the radio. We currently have the ground wire secured under a bolt. I'm not 100% what the bolt is attached to so I am going to take another look at it tomorrow to see if I can find a better grounding spot on the truck.

Thanks for the help,

David

[dapayne12]

Hi,

Just an update. After taking the information gained here and talking to someone who once used CB radios at work he agreed that it was most likely a grounding issue. After looking at my setup he suggested running a grounding wire from the mounting bracket to the frame of the vehicle. After doing this I now have very low SWR readings.

Thanks,

David

[treetall]

Glad to hear it. hope to hear you in dx land soon. im back quiet