New to the forum and help required

General Mobile CB antenna advice and troubleshooting

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diggygun
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Joined: Sat Feb 18, 2017 7:04 am

Sat Feb 18, 2017 7:32 am

Hi all

I'm new to the forum and looking for some help for installing my system into a Land Rover Defender Hard Top with a roof rack.

Over the years I have self installed these with varying success, usually not on the good side, always had issues with grounding, its got by but then the CB Radio burns out or the antenna breaks off.

Does anyone know of a company in the Milton Keynes area or surrounds that offer an installation service so that I can get it installed properly and not have the problems that I have had before

The CB radio is a Midland 210 DS and I have previously used Gutter mounts, Roof rack mounts and Side mounts, but as said, never really had any success.

If any one knows of an installer or if anyone could help me with the practicals that would be very much appreciated

Many thanks in advance

lars
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Joined: Wed Feb 08, 2017 12:40 pm

Mon Feb 20, 2017 12:13 pm

Hi

I've struggled with a similar situation to yours for the last two months, and got no real help from anybody. I've finally -- after hours and hours of fiddling -- got the CB working pretty well, with low and stable SWR across the band. The Thunderpole guys told me that it wouldn't hurt to have the SWR in the red zone, at least for short transmissions, but I wasn't very happy about that, and I got crappy range.

I can only tell you what finally worked for me; I've discovered, unfortunately, that everybody's installation, and installation-related problems, seem to be different. Perhaps that's why nobody wants to take on installation work commercially?

Anyway, I have a Jeep (about the same size and shape as your Defender) with a metal roof, and roof bars that stretch between roof rails. The roof bars are steel, but they are not electrically connected to the car body in any way -- the connectors are rubber. Mounted on the rear roof bar I have a 1.5m coil-loaded antenna. There is a sound metal-to-metal contact between the antenna mount and the roof bar -- I cut away all the plastic waterpoofing stuff and filed it to a mirror finish to make sure of that.

Now, received wisdom is that a full-size roof rack might make an adequate ground-plane for a coil-loaded antenna, but a single roof bar won't. So in such situations (apparently) you need to make an electrical bond from the antenna mount to a nearby piece of metal bodywork. This was deadly advice to me -- when I did this it because absolutely impossible to get low SWR anywhere. On the other hand, if I did not connect the antenna mount to the bodywork, I got an SWR that varied from minute to minute, across the entire range of the meter. It was extremely frustrating, because I couldn't adjust anything.

What worked in the end was to leave the antenna mount not connected to the car body, so that the roof rail was the only ground-plane (which should not work, in theory). I used a coax cable that was about four feet too long, and made a tight coil of coax of about 4 inches in diameter, strapped directly under the antenna mount. I think I have made a sort of common-mode current choke by doing this. Usually chokes like this are supposed to reduce currents induced by the antenna into the coax cable. It took a lot of trial and error to find the best size of the coil and where to mount it. When I had it inside the car, it was very sensitive to objects (including people) moving around near it. Mounting on the roof near the antenna worked best. My coil has about ten turns of coax in it. It's strapped to the roof bar using cable-ties, so it doesn't wiggle about. My home-made choke is probably radiating RF energy furiously, so it makes sense to have it on the outside of the vehicle, so (with luck) that energy goes somewhere useful.

Along the way I tried all sorts of other things -- bonding bits of the car bodywork together using braid, moving the mount around on the bar, moving the bar around on the roof, running extra ground wires, and so on; I replaced every part of the system apart from the radio itself, thinking something might be faulty.

All I can suggest, I'm afraid, is that you do what I did -- get an SWR meter, and then try every possible combination of grounding, antenna location, and cable routing that you can think of. Although low SWR does not, by itself, mean that that antenna will perform well, high (> 2.5-ish) SWR more-or-less guarantees it will perform badly. At least with a low SWR you can be reasonably be sure that the radio won't actually catch fire in use.

It's odd, because I remember using CB back in the 70s, and we just threw a mag-mount antenna on the roof and got on with it. It all just seemed to work. Maybe we were just lucky? Certainly my most recent installation has been a bloody horrible job.

Best wishes
Lars

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