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CB aerial mounting


martyn668
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Hello there - I bought a Super Stud Mount from Thunderpole last week. Just need to know how to mount it.

Obviously they're closed today, so I can't ask them, then I posted the query on Landrovernet, which has a special CB section (my first post there), and about 5 minutes later they closed teh forum for updates and maintenance! So it's back to LR4x4. I know there are lots of experts out there.

I've got a bracket welded to a roll cage on my Defender, with a 3/8" hole in it to take the Super Stud Mount. This will therefore provide a good earth (groundplane?) if required. I also bought a Thunderpole Orbitor Aerial and 3m of cable. The question is regarding the mount.

The metal stud with copper/brass insert which goes through the 3/8" hole appears to have a rubber inner which sits slightly proud of the nut. (I guess this insulates one part of the coax from the other). This would prevent the outer touching the mounting bracket of a perfectly dimensioned hole, UNLESS I use the included washer underneath the bracket. However, I suspect the included washer goes above the bracket, because the large rubber washer is moulded to fit the metal washer. However, if the metal washer goes above the bracket, the whole assembly is insulated from thh roll cage bracket by virtue of the fact that the rubber washer is moulded to insulate it.

So do I need to make a good earth. Between the roll cage and what? Reading some of the other posts, they talk about the rubber being a conductor. Is this the case? If so, I guess it's special rubber, charged with metal particles. In which case a good earth between the metal washer and the roll cage would seem to do it.

Thanks in advance for your help. Why don't they put instructions with these things for beginners like me?

Thanks

Martyn

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However you mount it, you want good metal to metal contact between the outer part of the mount, and the vehicle earth. this means checking the connection to the roll cage, and also the cage to chassis mounts, for electrical conductivity.

Being pedantic, and probably beyond your requirements;

'Earth' and 'Ground Plane' aren't the same thing. whilst creating a connection to earth is relatively easy, a ground plane takes more thought. Ideally, an aerial would be mounted on to a flat metal surface, of large area and uniform shape. A roof isn't bad if you mount the aerial in the middle. If you were to mount the aerial at either end of the roof, the shape of the roof/ground plane would shape the performance of the aerial in different directions. If you're using the roll cage, the above doesn't really apply apart from the fact the earth plane is far from perfect.

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Thanks Landymanluke.

Thanks also for the explanation of groundplane and earth. Now here's a follow up question to that. Supposing the rollcage has a good (electrical) connection to the chassis, and the chassis to the body and roof, does the roof then still act as a ground plane? Or is the electrical (radio signal) route to the roof too convoluted?

Thanks

Martyn

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A ground plane just needs to be a lump of metal, it does not matter if it is connected to the rest of the chassis.

Its got nothing to do with currents flowing from CB ground to the plane.

Think of it like a reflector...

The size an shape are important, if the antenna is in one corner, it will not work well in all directions.

If you cant get a good plane, then there are 'ground plane less' antenna bases available (used on boats etc)

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or you can mount it wherever you like but buy an electronic groundplane which does the job of a nice large sheet of metal.

Here's one; http://www.4x4cb.com/public/menu.cfm?Category=516

Do those artificial ground planes work for all types of personal radios? CB's, UHF, VHF etc

I have a 144meg Icom set that would definatly benefit from one of these.

Thx,

S.

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Do those artificial ground planes work for all types of personal radios? CB's, UHF, VHF etc

I have a 144meg Icom set that would definatly benefit from one of these.

Thx,

S.

I think they work on all, but it would be worth checking with the vendor if it's a specific application you're after.

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I've heard comments (from an RF engineer) that they are not the fantastic cure-all they may seem. I think the old adage "If it seems to good to be true, it probably is" may apply. They probably help, but are not a substitute for a decent ground plane.

That said, I don't think it's super-critical unless you need to transmit for miles.

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Yeah no for sure the 'electronic'* ones are a last resort. The SWR is never great & transmission range suffers.

But usually for our activities, we are only looking for a few hundred metres to a few km.

If you have metalwork - use it.

* In fact theres nothing electronic in them, just a tuned coil. So I'm pretty sure they are for the specified frequency range only.

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Hi Martyn,

I have exactly the same issue as you. To be honest I figured that for off roading ataching to the roll cage aint a big deal as I am not interested in miles of distance.

The connection is a problem as I want a waterproof connection but only have a small cable hole in the bulkhead to get out of the bodywork.

I found an excellent cable from Thunderpole (I think) but it seems to be only sold in the US. The description of your mount sounds like it is open to the elements?

If you find a solution please post as it will help me too :D

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why not just use the standard cable, and terminate it yourself, once you're outside the body work, or even buy a waterproof lead, for the outside, and cut the other end off, and re-make the internal connection.

You can buy RG58 by the metre, and make up ends using whatever style/model conenctor you like.

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You can buy RG58 by the metre, and make up ends using whatever style/model conenctor you like.

If its RG58 you want then drop me a line, I'm sure I've got the remains of a drum knocking round somewhere. I'm sure (if I can find it) I can stick some in the post for a small fee.

HTH

Glue

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Had a long chat with the guy from Thunderpole today. Very helpful. He had to explain it about three times. How difficult is it to describe over the phone connecting a 3-dimensional object to another 3-dimensional object only getting electrical conductivity in the required places? Basically he confirmed what Landymanluke said about the connections. He was convinced the roll cage would give me a decent enough groundplane.

I'll probably have a go tomorrow (Wednesday) evening, and post up my results. It'll get a proper test this weekend all being well, as I'm off for a weekend in Wales (with Trailmasters).

The mount is more or less open to the elements. I guess if you drilled a hole in a panel to fit it, it would be element proof on the underneath where it counts. Having said that, I think it should be OK. Might rig up a plumbers tape type doodah around it when it's on to waterproof it?

I just took the cable out of the door - Land rover panel gaps and all that, then ran it outside along the roll cage, as I don't have time to drill the body work and to rejig the end of the cable.

Martyn

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I've wired up 27Mhz antennas in the past and left the co-ax conections exposed or just covered with a rubber boot but self amalgamating tape, silicone sealant and a few other things will seal it or help to keep corrosion at bay adequately if you are that worried.

Get the mount on, get the SWR as low as poss and Bob is your long lost uncle who is getting slightly concerned that you may be worrying unduly about waterproofing. Keep a regular check on the SWR and the cleanliness of the connections ;)

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Well, got it all mounted tonight. Good continuity between the outer of the coax and the roll cage/chassis, and no continuity between the innner of the coax and the roll cage. Now to test the SWR. Wired in the SWR meter. Couldn't get the needle to move at all on send or receive. I suspect the SWR meter is kaputt. Switched on and listened on all channels. I overheard a couple of faint crackly conversations, but it wasn't worth butting into ask if they could hear me, as I wouldn't have understood their answers. No idea how far away they were - could have been 10 miles or 10 meters.

I may be able to drive past a CB shop on my way to Wales, to see if they can do a quick SWR. Otherwise, I'll suck it and see while in Wales.

The CB (Harvard 400M) has an analogue SR-F power meter. No idea what one of those is, but it seems to be something to do with transmission power. It goes nicely all the way up to the top of the green when I transmit, just short of the red. No idea what it's doing, but top of the green area must be good!

Martyn

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Well the bloke who tested it on Friday said it was good. He plugged in a SWR meter, and tested all sorts of things, pronouncing them "a little long", but generally good. Off I trotted up the M6 to Wales. I was able to listen in to lots of truckers talking normal trucker stuff - never did see the woman in the silver Merceedes they were talking about. However, when I tried to talk to them, no one seemed to hear. Then on the Trailmasters weekend in Wales, same thing - I coudl hear everyone, even Captain Incredible going to Infinity and Beyond or whatever he does and all the mobile phone ring tones. But no one could hear me. I suspect the mike, so that's going to be taken apart to look for loose wires before I consult an expert again.

Watch this space.

Martyn

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