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CB install issue, short exists

7K views 27 replies 11 participants last post by  Jerry Bransford 
#1 ·
HI All,
I installed a Cobra 75 in my JK over the weekend. All went well, I thought.
Attempted to tune the antenna and could not get out of the red on SWR meter.

Here is my setup.
Cobra 75 installed behind the glove box. Power and negative wired directly to the battery. I originally had it wired to my sPOD, but eliminated that to simplify the troubleshooting.
I have a Firering Coax - MU-8R18(18 ft.), Teraflex Antenna mount on my Teraflex hinge. Firestick 4ft. Firefly antenna.



The Teraflex Mount is grounded well and good continuity exists to the frame. I ground the paint off the mount under the bolt that mounts to the hinge.

The issue looking at the "testing for continuity" on the Firestick web site, is that i have a short somewhere. Do I have a bad coax cable?

I have continuity between the antenna stud and the Tearflex mount.

If i unplug the antenna from the radio the continuity does not exist.
 

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#2 ·
Yikes. Did you disconnect from your CB when checking the wiring? Disconnect the coax at both ends, if you get continuity between shield and conductor then the cable is bad.

Further bad news, keying up the transmitter with a shorted cable can blow out the amplifier. Might be protected in some way on newer CB's not sure.
 
#3 ·
The issue looking at the "testing for continuity" on the Firestick web site, is that i have a short somewhere. Do I have a bad coax cable?

I have continuity between the antenna stud and the Tearflex mount.

If i unplug the antenna from the radio the continuity does not exist.
If taking the antenna off of the mount removes the short, that points to the mount not being installed correctly. Take another look at the instructions and be sure the insulating washers are in there proper locations and orentation.

Your coax is fine.
 
#4 ·
I have it connected as the instructions say, like this.


I do not (correctly) have continuity between the stud and the mount when the coax is not plugged in the radio unit. When I plug the antenna coax into the radio unit, then I get continuity between the stud and the mount.
 
#6 ·
Ok... I disconnected the control unit from the battery.
I unbolted the Coax from the antenna mount. I have continuity from both sides of the firering. (that's bad.) I disconnect the coax from the radio control unit, and there is no continuity at the firering. Do I have a bad control unit?
 
#7 ·
Continuity check with the CB connected to the COAX isn't likely a valid test at all. Inside the radio the conductor and ground are likely connected via the final amp circuitry, there should be some resistance but a continuity meter may not detect it. I don't have a radio handy to double check though.

At this point I'd suggest hooking things back up and testing the CB (got anyone you can talk to?). If there's chatter on any channels ask for a radio check and give general location.
 
#9 ·
I am currently working on the same set up right now. You will have continuity through the control box. Disconnect the cable at the box and check between the shell and the pin on the connector. You should show an open circuit and it sounds like that is what you are seeing. An additional check would be to check between the connector shell and the ground on the body (there are points right near the glove box on the passenger side). If you can, check to see if you have continuity between the pin on the connector and the antenna stud. I used a battery jumper cable to get close enough to the glove box to check that.

Once you are there, you are in the same spot I am in. I checked a buddy’s set up of the same equipment on his FJ this weekend and he is getting the same problems. Good ground, no shorts and a high SWR. I will get to the bottom of this, but I am waiting on some additional parts at this point. I have a new cable and stud coming in today. I will try that. That will eliminate the Fire Ring cable and stud. I will also try putting the antenna on a bracket mounted on the edge of my hood. If I am still not getting a good SWR I will put someone else’s radio in the rig to see if that solves the problem. This should not be this hard!
 
#11 ·
Update: Improvement, but not perfect.
I pulled everything apart. Using the troubleshooting guide here helped step through things -> SWR Troubleshooting Flowchart | Right Channel Radios

I bailed on my earlier continuity tester and borrowed my brother's multi-meter.
My Antenna is good. Turns out the ground from the Firering to the mount was restricted but still showing continuity with my tester. I ground down more of the paint on the mount and reassembled. Checked the radio end of the coax for ground from the Jeep body the outer ring of the coax with the multi-meter and I got a good ground with low resistance.

On to the antenna tuning.... SWR is a fickle beast. After playing around the lowest I can get channel 1 and 40 to match at is 1.8.

So I'm functional, but not optimal. I'll keep tinkering to see if I can get the SWR down.
 
#24 ·
Why do people say only check ch.20? If you don't check the lowest and highest channels you don't know whether to lengthen or shorten the antenna. If it's already too short and don't know it you're fighting a losing battle.
 
#17 ·
Lot's of good info in here. I'm having a similar issue with High SWR, where I can't get anything to drop down below 3. I've checked everything with my multimeter, and ground and continuity all check out and appear to be optimal. Everything seems functional from the antenna, down the coax, to the radio.
Here's where I think I went wrong. When I first installed the antenna stud, I had the plastic isolation washer on the wrong side of the mount, which means I would have shorted the antenna stud to the mount. I'm guessing that this might have caused some damage, but not exactly sure what it would have damaged specifically. Is it possible that I fried my control box, and would that cause high SWR? I imagine if I damaged the box, the SWR meter would read nothing... Thanks Guys.
 
#18 ·
Here's where I think I went wrong. When I first installed the antenna stud, I had the plastic isolation washer on the wrong side of the mount, which means I would have shorted the antenna stud to the mount.
That's exactly what happened, installing that insulating shoulder washer on the wrong side of the mount dead shorted the antenna to ground through the mount which is what caused the high SWR. It's doubtful you harmed the transmitter though, most modern CB radios have protection against that. Reinstalling the insulating shoulder washer correctly and tuning the antenna properly would get things working again.

It is absolutely critical to assemble that type of mount exactly as shown below.
 

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#19 ·
Thanks Jerry. I should have been a bit more clear. When I posted, I had already realized my mistake. I had already moved the insulating washer to the proper position, isolating the stud and antenna from the mount. After I moved it, I'm still getting a reading of 3-4 SWR... That is why I wonder if I might have caused permanent damage to the radio (or perhaps the SWR meter), when I checked the SWR the first time around with the shorted mount. Would using the transmitter to check the SWR with the insulating washer cause damage, or would it just not function properly? Thanks!
 
#20 ·
Would using the transmitter to check the SWR with the insulating washer cause damage
Doubtful with a modern CB which, as mentioned above, will have an automatic protection circuit to prevent damage to the transmitter.

Is your mounting bracket well grounded, and is the bottom half of the antenna mount touching bare metal as it must under the mounting bracket? Most antenna mounting brackets are powder-coated which prevents the antenna mount from receiving its required ground.

The antenna mounting bracket MUST have a bare metal connection to the tub, and the bottom half (labeled SO-239 below) of the antenna mount must be touching bare metal on the bottom-side of the mounting bracket. If those conditions aren't being met, a high SWR like a 3+ will be the result.

Think of your antenna as being 50% antenna and 50% Jeep.... if the screw-on barrel of the coax cable connector isn't getting a good ground connection to the Jeep through the antenna mount's SO-239 connection to the tub via the mounting bracket, your SWR is going to be sky-high.
 

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#21 ·
Thank you for the continued feedback. I've definitely scraped off plenty of paint on the bracket to allow the bottom of the stud to ground itself to the mount, and to the gate. I even tried running a ground wire to the frame under the back bumper, but that didn't seem to change the readings at all. I haven't tried running a wire to the tub, and grounding it there specifically, but I figured tub vs. frame I'd essentially have the same results. My next step will be to clean the mount up even more with my Dremel, and I will run a wire through the gate to the ground mount on the tub above the subwoofer in the back. Beyond that I might open the control box up and check the lead wires going to the battery from end to end to make sure nothing is shorting, or possibly damaged there.
 
#22 ·
Maybe this is a crazy question, but I'm just realizing something.

I purchased a 4' Fibreglass Firestick Antenna KW4 5/8 Wave - Rated at 400w.

I then purchased an Astatic SWR Meter - Rated for 100w.

Seeing as how the transmitters are limited to 4w, I figured this wasn't an issue. I assumed that the antenna is capable of up to 400w, but assumed that might be for use on different frequencies, and that a CB tuner would keep it at 4w transmission max. Am I wrong here? Do you think I need a 300w capable SWR meter to measure this antenna properly?
 
#23 ·
Nope, pay no attention to those wattage ratings. All of those are fine and they all work well together. And no the tuner is not limiting the cb's 4 watt output. The antenna works fine with any power up to 400 watts but it's actually only getting 4 watts max from the CB transmitter.
 
#25 ·
I may have found my issue, but not sure. I've checked continuity everywhere on the jeep to make sure everything that's an open circuit is supposed to be, and vise versa. Here's a new issue that I've just come across. For reference, I'm using the Cobra 75 WX ST, which has a removable radio that connects to a control box under the dash. With the system completely wired (antenna, mount, coax, control box, radio, and leads to battery) I am getting the slightest amount of resistance between the stud (the hot) and the mount/tub/etc (the ground). Radio is turned off while I meter this. As soon as I disconnect the radio from the control box, I'm getting a completely open circuit. No continuity. Possible short inside the radio, or is this normal behavior?
 
#26 ·
You're likely just seeing the normal resistance of the transmitter's output transformer secondary side... One side connects to the antenna and the other side connects to ground. Though I haven't seen the schematic of your actual 75WX, that's how most are wired.
 
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