Question about Cobra 29 LTD CB Radio
Please make sure you are tuning the antenna correctly.
It is important to tune your CB radio antenna to the proper length. The length must exactly match the wavelength of the frequency you transmit on. Or be really really close.
All Cb antenna's have a way to adjust the length of the antenna. If it's a mag mount, the metal whip can be slid in and out of a metal collar. Usually a set screw. If its a fiberglass whip, the way to tune it is on top. It either has a small metal rod with a set screw to adjust the length, or it threads in and out to adjust length.
Either type you have it will need to be adjusted for proper length. Here is how to tune the antenna to lowest SWR.
Some radios have a built in SWR meter. Some do not. If yours doesn't have an SWR meter, then you have to use an external SWR meter. Radio shack carries one, and you can find them cheap on ebay.
If you have a built in SWR meter or external, the procedure is the same:
1. Turn the radio to CH 20.(This is the center of the band.)
2. Switch the meter switch to CAL. (CAL stand for calibrate.
3. Key the radio. (Important. Do not talk while keying the radio.)
4. turn the CAL knob up until the meter hits the CAL mark.
5. Now while still keying the radio flip the meter switch to SWR.
Where the meter falls after that point is you SWR reading. If its above 3, that is real bad. 2 is not gonna kill your radio but its not the greatest. 1.5 and under is a good place to be, but the lower the better.
If your SWR is high, Here is how to find out if your antenna is too long or too short.
1. Turn to CH 1
2. Repeat SWR procedure. You must calibrate every time you do it.
3. Remember the SWR reading.
4. Turn to CH 40.
5. Again repeat SWR procedure.
6. Compare the reading between CH 1 and CH 40.
Now if the SWR is higher on channel 1 then channel 40 your antenna is too short. You must make it longer.
If the SWR is longer on channel 40 then on channel 1, then the antenna is too long, you need to make it shorter.
Make height adjustments about an eight inch at a time, and take reading each time.
once the SWR is significantly lower and the reading on channel 1 and 40 are about the same, you a about matched. At this point turn the radio to the center of the band, channel 20 and take an SWR reading.
You should now have a low SWR reading, 1.5 or under. If you are 1.5 or under, you are good to go.
If you cannot get the SWR to an appropriate level, there may be an antenna problem.
Now: If you have adjusted the antenna properly and you still get the antenna warning light, then the antenna warning circuit may be out of whack. Use an external SWR meter.
You may also be getting common mode current. Its when RF comes down the outside of the coax. If the swr goes up with modulation then it's common mode.
Here is how to stop common mode.
As close to the antenna as you can while still inside the vehicle, take about 6 to 8 inches of the coax, and coil it up into tight even coils and secure into shape with electrical tape.
This is an RF choke, and will stop the common mode current.
If all other radio's are fine and no ant light come one, and are all showing low swr, then turn VR6 back to disable the ant warning light. "only do this if the ant light comes on when SWR is lower then 1.5 and does not swing up with modulation showing that the ant warning light is miss aligned.
Posted on Nov 09, 2010
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Posted on Jan 02, 2017
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