New CB user Cobra 29LX high swr and antenna grounding issue

[Quickbird144]

Hello everybody. Fairly new to the CB scene. Picked up a used Cobra 29LX from a pawn shop. Found a 4' antenna at my shop and fabricated an antenna mount for my jeep XJ. CB power and ground are ran straight to the battery, using a 18' coax cable with a firestik stud.

The best tune I could get was 2:2 with the built in swr meter. I'm not able to transmit but can hear others talking although not that great.
I found a diagram about checking the connections for continuity and found with my coax cable connected to the radio I'm shorting between the outer connector of the coax and the antenna. Unplugging the coax from the radio, I get an open.

My ground plane is well grounded. Am I missing something or is the CB radio grounding the coax hot side internally? Any help would greatly be appreciated.

 

[WA0CBW]

All continuity measurements should be made with the antenna disconnected from the radio.
BB

 

[Quickbird144]

All continuity measurements should be made with the antenna disconnected from the radio.
BB

Thanks for the quick reply. So it is normal to see continuity between these connectors with the antenna connected?

 

[jonwienke]

Having the antenna a few inches from the body of the vehicle like that will always result in high SWR and crappy performance. If SWR is only 2.2, the wiring is probably OK, but the antenna location is the problem. If you want decent performance, you need to relocate it away from the metal body panel. The best place is on the roof, in the center.

 

[prcguy]

Some radios show a short at the antenna connector, you are getting the correct readings when the coax is unplugged from the radio. Built in SWR meters in radios can be inaccurate, check it with a real meter. Its best to check ch 1 and 40 to see if the antenna is too long or too short. A better match on ch 1 means the antenna is on the long side. If its better on 40 its too short, etc.

Another potential problem is lack of ground plane and this has nothing to do with DC ground as measured with an ohmeter. The antenna wants a lot of sheet metal right below it like an entire car roof. The location you chose is lacking that large surface area being at the rear side of the vehicle. If the antenna is of adequate length to give the best match in the center of the CB band and its still high, then lack of ground plane is probably at fault. In this case you can fool the antenna by placing a small capacitor at the antenna mount, something between about 25pf and 50pf usually does the trick.
prcguy

 

[JamesO]

So are you saying nobody can hear you if you transmit? Or are you not transmitting due the SWR value?

Disconnect the coax from the radio and measure the resistance between the antenna stud and ground, it should show as "Open".

You could also check the continuity between the center pin of the connect and the antenna stud and the outer connector and the ground as well.

 

[Ubbe]

I agree with prcguy, you totally lack any groundplane and the groundplane from the car goes besides the antenna element in parallell ruining the antenna performance.

Trucker sometimes put antennas on the side mirror that also lack groundplane but works somewhat better due to the distance between antenna and the metal of the truck door.

/Ubbe

 

[mmckenna]

As others have said, it's likely your ground plane.

My ground plane is well grounded.

There's a difference between a DC ground from the antenna mount to the vehicle body and a proper ground plane. DC grounds and RF ground planes are different things.

 

[Quickbird144]

I appreciate all the replies!

Not sure why I thought ground plane was DC, that's the newbie mistake. I've got a roof rack and will probably eventually move the antenna setup there with a shorter antenna.

With the coax disconnected from the radio and measuring bank at the antenna, I got open from the coax outside connector to the antenna stud. Perfect. Plug the coax back in, I get a short.

I bought a cheaper Cobra over the weekend to test my setup and since I wasn't sure if the 29 LX was fully functional. Plugged the coax into the cheap Cobra, go back to the antenna mount and Bam. I get open at the coax outside connector to the antenna stud. Turn to channel 19 and got a clear radio check from another user.

So in the end turns out the 29 LX was grounding the hot side of the coax internally. I've no clue of the insides of these so couldn't say exactly is what but those were my findings. All is well now. I greatly appreciate all the help!

 

[jonwienke]

Some radios have coils across the antenna connector, and read shorted with a meter. Doesn't mean anything is wrong with your radio.

 

[cmdrwill]

Some radios have coils across the antenna connector, and read shorted with a meter. Doesn't mean anything is wrong with your radio.

Very true the coil that "grounds out" the antenna is like a open circuit to the radio signals in the CB band. The coil also drains static off the antenna.

Do not expect much out of an antenna on the roof rack, not enough ground plane/ counterpoise at CB frequencies. Jeeps are not CB or Ham radio antenna friendly, the cab is all fiber glass.

A CB 27 Larson NMO mount antenna on a hood edge mount is way better.

 

[Quickbird144]

You guys are correct as also mentioned earlier about the ground.

I received an exact model, brand new, and the antenna is grounded from the CB. However, I got completely different performance from the new one. I was out of tune on ch40 and got an antenna warning whereas the old radio I just got 2.2 across all channels.

I tuned the antenna and now all channels are reading .5? If that makes sense. This was using the onboad swr meter but I did purchase an actual meter so will be recalibrating.

 

[Quickbird144]

Do not expect much out of an antenna on the roof rack, not enough ground plane/ counterpoise at CB frequencies. Jeeps are not CB or Ham radio antenna friendly, the cab is all fiber glass.

I believe you may be referring to the wranglers? I have a 1999 Cherokee. Steel all around.