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Old 07-15-2012, 08:35 PM   #1
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CB Antenna Problem

I have a 2010 Allegro Bus with a Cobra 75 radio installed at the factory. It currently has a very limited transmission range so I have been trying to troubleshoot the problem. The SWR reading runs from 8 to 10 on all channels which is as bad as it gets. I have an identical Cobra CB in another vehicle so I switched out the handsets and got the same SWR. I then removed by factory installed antenna, a Procomm Impulse and installed a Firestik on the factory spring mount on the roof. No improvement on the SWR, no change really at all. I removed the factory installed spring mount and repeated the same test with the Firestik antenna directly on the stud coming out of the roof. No improvement on SWR, no change. I am thinking I have a bad cable or open junction somewhere. Does anyone have any suggestions on where to start? Also, the rig has two roof top antennas. The second one has a Triple Vision label on it and I was gurssing that was the FM radio antenna. Is that correct? Or are the both CB antennas? If they both are CB, then where is the cable splitter located?

Thanks in advance for the help.

Jeb
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Old 07-15-2012, 08:49 PM   #2
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The factory would usually install one antenna I would say the other would be radio. the cable can be a issue check the connectors make sure they are cleanly installed the crimp not loose also if the cable needs to be replaced keep it as short as possiable and in length by 3's 9ft 12ft 15ft 18ft some thing about the wave if I remember correct. That fire stick should have a adjustment for SWR at the tip if you pull the tip off you will see it. Hope this will help
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Old 07-15-2012, 09:25 PM   #3
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Check continuity of the center conductor? I had a high SWR on my 4x4 and wound up shortening the coax but 8-10 is crazy high.
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Old 07-16-2012, 08:17 AM   #4
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Thats getting into the burn up radio range
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Old 07-16-2012, 08:25 AM   #5
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First thing I would do (because it's quick and easy) is replace the PL259 connector on the radio end of the coax. If that doesn't fix it then check the connection at the antenna. I'm betting though that you have an issue with the PL259.
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Old 07-16-2012, 09:04 AM   #6
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If the cable checks ok, and even if it doesn't make for sure the antenna mount is grounded.
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Old 07-16-2012, 09:10 AM   #7
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At the factory they just drill a hole and run the wire in then put the PL259 on afterward most of the time they just crimp with pliers not good
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Old 07-16-2012, 09:55 AM   #8
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On modern radios anything over 3/1 can blow your finals, Of course most modern radios know that and have protection systems installed (Reduce power)

If you are sure you changed the correct antenna.. Then you have bad coax, those are the options. Get new coax.. Get good coax though at 11 meters it's not as important as at, cay uhf or satellite frequencies, In your case RG 58U is good enough.
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Old 07-16-2012, 08:31 PM   #9
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Thanks for all the advice and feed back. Here is where I am at as of Monday evening. First, today I called Tiffin Service in Red Bay and they were their usual helpful selves. The technician who took my call, Danny, said the Allegro Bus comes with only one CB antenna mounted in the roof over the drivers seat. The antenna over the passenger seat is the AM/FM Radio antenna. He indicated the cable connections on the antenna end were accessible from the roof. He stated the cable was one continuous length with no splices, joints etc. I had to remove all the factory installed sealer around the mount on the roof but here is what I found. The antenna is mounted on a 2" by 6" aluminum plate which was screwed down with four self tapping screws through the fiberglass roof to the structural steel frame. A separate ground wire was provided grounding the plate under one of the screws. The antenna cable was connected through the plate using an insulated bulk head fitting. I disconnected the cable and checked for a short to the shield and found none. I checked the center cable for continuity and found the cable had continuity and observed no issues at both ends. The cable ends look electronics grade installed with soldered center posts. They look good, not home made by Tiffin. The through bulk head fitting was installed properly and the center antenna connection was not grounded to the aluminum plate. The outside shield connection was grounding properly to the aluminum plate. I removed the bulk head fitting from the plate and installed it directly on the cable and installed the factory antenna. The cable end was not grounded in this temporary method and the antenna was propped up unobstructed on the roof top. I am getting a 10 SWR on channel 1 and a 8 SWR on channel 40. I installed the spare Firestik I have from another vehicle. Same setup with the antennal just propped up free standing on the roof. I got the same SWR readings. This Firestik antenna is getting a 1.5 SWR on the other vehicle. I ran out of day light and will pick up with it on Wednesday. I plan on trying to vary the factory antenna lengths and see what happens. I may also try using the antenna cable from my other vehicle and see what happens with it. As before, I am wide open to suggestions. Thanks.
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Old 07-16-2012, 08:56 PM   #10
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Replacing the coax is your best option. Also use the same setup that you have in your truck,radio and everything, It could be the radio itself. Goodluck
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Old 07-16-2012, 09:56 PM   #11
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At this stage only two possibilities. 1. You are parked under or next to a structure such as a roof or wall of a building or under a tree. 2. Since the coax is not shorted or open it could be pinched badly and causing the vswr. As mentioned pulling a new one is in order. You might be lucky enough to tape the end of a new one onto the old one and pull it through. Tape them very tightly and make sure you taper the tape in the direction you will pull. Use some liquid dish soap to lube the cable for extra help.
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Old 07-17-2012, 12:27 AM   #12
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Put the coil and Firestick back on. Take the tip off the Firestick and you can tune it. It screws in and out to lengthen or shorten, whichever direction you need to go. You must put tip back on after every adjustment for testing. There's going to be a lot of up and down the ladder unless you have a second person in the seat.
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Old 07-17-2012, 10:24 AM   #13
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Did you check to make sure the cable SHIELD was connected end to end? in addition to the center conductor?

There is one other (Rare) issue called a ground loop, but let's not go there yet.

But both the center conductor and the shield (not the nut but the metal under the nut) need to be good end to end and when the antenna and radio are disconnected no conductivity between them. Staples through the wire do happen.

Easiest way to test this is to first check for continunity center to shield, other end disconnected if you get a low ohms reading (or a beep depending on your meter) BAD CABLE.. if not (if it indicates open circuit)

Put a jumper on the other end, (short it out) and if you still get open circuit BAD CABLE if you get low ohms (Like 1) and/or a beep, good.

Make sure the little nubs on the ring under the nut go into the matching "V"s in the fitting (SO-239) on both radio and antenna mount.

Make sure it is tight.
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Old 07-17-2012, 08:20 PM   #14
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Thanks for the input. I will resume checking tomorrow. I had another thought. Excess CB cables are not supposed to be coiled and stowed. They are supposed to be left loose and meandering about. I was wondering if the factory tech. coiled up the excess cable nice and neat and tied it up to the frame in the ceiling or in the front cabinet somewhere. Would coiling the cable give me this kind of SWR numbers? Also, CB cables come in very precise specific increments, I think in 3 foot increments to keep the cable length correct to match the antenna. I was wondering if the cable was cut and a new end put on getting this length out of whack? Do you think that would cause the SWR problem? I will check the cable ground as well as the center cable as noted above. I am familiar with the tuning of the Firestik antenna and I will see if it can bring it in to spec but I have never had one so far out on the SWR reading before and I suspect the antenna does not have enough adjustment built into it. I am anxious to try the alternate cable with both antennaes. That will tell me something. I will keep this post up to date and again thanks for all the suggestions. I am not quitting until I get it right, even if I have to go to all external mounts with a new cable and antenna as I have seen some one else did on this site previously.
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