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Old 07-20-2016, 05:15 AM
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Cobra 75, SWR and antenna issue

Need some assistance with CB tuning please. I installed a Cobra 75, Teraflex spare tire mount, 4' firestick antenna. I'm very proficient with electronics and this has me stumped. I installed everything and the best SWR reading I could get was 2.6 I took the bracket off and ground the powder coating from the underside where the coax connector touches and around the mounting bolts. I metered the grounds to bolts on the frame and have a good reading. I do not have a short between the antenna and ground. When I hooked the SWR meter back up and keyed the mic the tx led on the handset pegged like it should but stays there when I release the mic. The SWR meter shows a constant signal when the mic is keyed or not. If I shut the Jeep off and turn it back on it looks normal till you key the mic then it pegs. If I remove the SWR meter and short cable from the circuit the CB seems to work as it should. I'm thinking I screwed something up by messing with the grounds. I guess the meter could have gone bad, kind of unlikely though. Anyone have any ideas to try?

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Old 07-20-2016, 09:46 AM   #2
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power and ground for the CB itself is where?

What did you do with the coax between the antenna mount and the CB (don't loop it!)

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Old 07-20-2016, 10:21 AM
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power and ground for the CB itself is where?

What did you do with the coax between the antenna mount and the CB (don't loop it!)
Power and ground are tapped into the switched power outlet on the bottom right of the dash. '16 JK They aren't soldered, I skinned a small section (1/4") of the factory wiring and wound the CB wires (1/2" of wire) around that.

Coax was ran down the passenger side, up the kick panel and into the CB base. I'll have to check to see if the extra is looped in the kick panel, I don't believe it is. The base is temporarily wire tied to a gray bracket behind the glove compartment.
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Old 07-20-2016, 12:02 PM   #4
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Your power connections won't affect your SWR. Odds are good your high SWR is being caused by how the antenna mount was assembled, how it was mounted to the mounting bracket, or the mounting bracket isn't adequately grounded to the Jeep. Powder coating on a mounting bracket can prevent a ground connection too.

Can you post good sharp closeup photos of your antenna mount taken from both the top and the bottom? It's possible the problem is visible.

The last high SWR issue I worked on was from the antenna being shorted to ground through the mounting bracket. The hole in the bracket was drilled too small which caused the antenna to short to ground through the bracket. Other common antenna mount problems that cause a high SWR is if the nylon insulated shoulder washer is not installed correctly or even missing. Either of those will short the antenna to ground and cause a high SWR.

Don't worry about things like how the coax is routed or if any of it is coiled up, neither of those will cause that extra-high SWR you're experiencing.

The below is the most commonly used type of antenna mount and how it must be assembled... especially note the location of the insulated shoulder washer whose job is to insulate the threaded part the 3/8x24 threaded adapter is screwed onto from the mounting bracket. If those threads touch the mounting bracket, a high SWR and a dead antenna will be the result.

Note the mounting bracket must have any powder coating removed where the arrows indicate (but not from inside the hole) and where it attaches to the Jeep where it needs to have a bare metal connection.
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Old 07-20-2016, 08:08 PM
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Here are the photos. I also have a video of the behavior.
Photos:









The videos are here:

With the SWR meter attached http://www.youtube.com/watch?v=d5t01Fl8acM

With is disconnected:
http://www.youtube.com/watch?v=bCRhbtMhcEw


Thank you for the assistance.
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Old 07-20-2016, 10:01 PM   #6
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Your photos aren't showing up for me.
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Old 07-21-2016, 01:39 AM   #7
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Looks like the radio is getting locked up in Xmit most likely because the very high SWR.
Are you zeroing the meter correctly? Set the switch to FWD. Then adjust the meter until its on the "CAL" mark on the right of the meter. Then flip the switch to REV.

Take a reading on Ch40 and write it down.
Then go to Ch1, re-cal, take a reading then write it down.

If the reading is better on Ch1 then 40 then its to long. Turn the antenna screw in some then retest.
If its better on Ch40 then ch1 then its to SHORT. Adjust it out.
Once you get it ball parked then go to the channel you plan on using the most or just Ch20 and dial it in. Remember to re-cal every time you change channels.

If you can't get it at all I would unbolt that mount and use visegrips or a C-clamp to temporally clamp it to something big and metal with nothing actually next to the antenna itself. Like the edge of a metal table or a fence post. If you can get the SWR tuned with it mounted some place else then its an issue with the placement on the jeep.

That combo may not work that close to the jeep body.
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Old 07-21-2016, 05:29 AM
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Quote:
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Your photos aren't showing up for me.
Anything now? I had to re-verify my email address at imgur.
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Old 07-21-2016, 10:50 AM   #9
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Anything now? I had to re-verify my email address at imgur.
They're still not showing up for me on my desk computer or my cellphone browser. Try hosting them at www.photobucket.com which is a popular photo hosting site that I have been using without problem for years.
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Old 07-21-2016, 03:52 PM
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Thank You tarditi, junkman, and Jerry. There are times that I would rather be lucky than good. I went out one last time and randomly set the adjuster on the antenna, locked it down and checked it. It is now at an acceptable 1.1 SWR. I must have moved the adjuster 100 times and never hit this spot. All is well in the land of CBs now!
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Old 07-21-2016, 06:54 PM   #11
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Nice! glad you got it tuned. I am guessing the getting locked up in transmit thing went away once the SWR was fixed?
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Old 07-21-2016, 07:15 PM
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Yes, the lockup did quit once I got it close. It didn't have to be far off of tune for it to lockup.

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