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Earthing an aerial


 
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Jimbob
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Joined: 28 Jun 2008
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Location: High Wycombe

PostPosted: Tue Apr 05, 2011 10:00    Post subject: Earthing an aerial Reply with quote

Hi,  I'm fitting a CB to my trooper and have a springer on a mirror mount attached to my roof rack.   I have been reading that for the CB to work properly the aerial needs a good earth.  

The mount is an aluminium right angle bracket and the rack is powder coated.

What's the best way to get a good earth in this situation?

Would put up some pics of this setup but I'm unable to post them.

Any advice would be great.
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TrooperDan
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PostPosted: Tue Apr 05, 2011 13:04    Post subject: Reply with quote

I have my arial on the plastic door mirror and can get 2 miles in town. For Green laning it is more than adequate.  Smile
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hillbilly col
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PostPosted: Thu Apr 07, 2011 17:24    Post subject: Reply with quote

what you have should be ample enough...i have mine mounted to rear door and has rubber pads inbetween bracket and door to preserve paint/bodywork and have no probs  Wink
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MikT725
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PostPosted: Thu Apr 07, 2011 19:57    Post subject: Reply with quote

If you really wanted to you can get an artificial groundplane for insulated or non metallic mounting points
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Matt Quinn
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PostPosted: Mon Apr 11, 2011 19:22    Post subject: Reply with quote

The "good earth" idea for CB aerials is one that's often misunderstood...  What's needed is in fact a good 'ground plane' for the antenna; the Wikipedia entry offers as good a basic explanation of what that is as any. Can't post the link sadly but it's easy enough to find.

...the ground plane need not be electrically earthed. - Although in practical terms it almost-certainly will be as the braid of your co-ax lead is generally connected to the chassis of your rig which in turn is connected to the chassis of the vehicle. Magnetically mounted aerials for instance will have a pretty poor connection to 'earth' (if any!) at their base, but still the 'ground plane effect' of the car body is more than adequate.    

That said, most aerial mounts ARE designed to be properly earthed... A lower SWR can usually be achieved with a 'good earth', there are other more complex reasons why it's a good idea.  And my own instinct would be to try and connect the rack electrically to the aerial bracket.  This could of course be achieved by simply breaking the powder coating under the bracket. However a better solution might be to run a short piece of single-core cable from the bracket's mounting screws to one of the screws or bolts that hold the rack on...  Use a copper 'hoop' connector and just trap it under the bolt...

Or even a small self-tapper screwed into the rack at some point would do...

Radio Hams would no doubt disagree... All in all though it's not something to get too concerned about... So long as you can achive an 'acceptable' SWR reading and the rig isn't going to get damaged the couple of miles needed for vehicle-to-vehicle communication is relatively easy to achieve.
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MikT725
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PostPosted: Mon Apr 11, 2011 21:37    Post subject: Reply with quote

I believe the ground plane is basically to give a zero potential for the radio signal to be compared to i.e a zero point to measure against  Question
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Matt Quinn
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PostPosted: Tue Apr 12, 2011 0:30    Post subject: Reply with quote

Not quite...  The term 'Ground Plane' is used in other contexts; a PCB for instance might be designed with a ground plane - basically an area tied to a zero reference point as you describe - in order to reduce noise and ensure that (say) a number of I.C.s on a circuit board are working to a common reference point.

It is true that you would measure the signal against the 'ground plane'... It's the chassis side of the rig, and connected to the vehicle chassis.  Just as you'd perhaps measure whether a particular wire was 'live' or not by referencing your meter to 'ground'... i.e. attaching the negative lead to the bodywork. But that's not the purpose of the ground plane; it's merely one of its characteristics.

The simplest form of practical antenna is what's called a Dipole...  Those 'T' shaped ribbon aerials you sometimes get with stereo systems are an example. It's called a dipole because it has two 'legs'... One leg is 'driven'...    A Dipole is approximately one-half wavelength long overall.

A monopole antenna (such as a CB aerial)  is a single element antenna fed at one end, that behaves as if it were a dipole antenna. It's formed by a conductor (electrically) 1/4 wavelength long which is near a conductive surface (the ground plane) that works as a reflector.  The current in the reflected image has the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together effectively form a half-wave dipole that radiates only in the upper half of space.

So, the ground plane (crudely, and grossly over simplifying) could be thought of as the 'mirror' that completes the 'illusion' of a 1/4 wave monopole actually being a 1/2 wave dipole...  How 'convincing' that illusion is depends on the quality of your ground plane.

...And just to make it all really confusing you'll find that being pedantic about it, A vehicle is not actually a ground plane in itself!  It acts like a capacitor between the antenna, and the surface under the vehicle which is the ground plane!  Life really is too short for considering all that jazz though... IMHO that is!
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markymoan
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PostPosted: Tue Apr 12, 2011 3:03    Post subject: Reply with quote

I just have a short springer on a mag mount and it works fine  Laughing
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Matt Quinn
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PostPosted: Tue Apr 12, 2011 9:56    Post subject: Reply with quote

It will do Marky...  I've used mag mounts myself for a variety of comms antenna and they do work well.

My own rig is attached to a little compact CB aerial bought in France. It's maybe 14" long, and attached to the wing via one of those plastic 'body mounts'...   Most folk wouldn't even think it WAS a CB aerial. The main reason for choosing this position was that I have a very slight dent in the wing and this covers it up!

Theoretically it's NOT the best position, or the best aerial.  But with a little care adjusting the SWR  (can't even remember what I got it down to) is within safe limits and the rig works well.  Average range on the road is about five or six miles, and anything up to fifteen from high ground. During 'long skip' conditions I've heard folk as far away as Stuttgart on the EU band while travelling along the M8!

If you're going to get into the whole Ham/CB thing as a hobby in itself you'd maybe have a different set of concerns and priorities... But for most 4x4 enthusiasts the 'working solutions' are pretty straightforward IMHO and can be quite inexpensive.  - My guess is that the O/P's 'question' can be answered with two little copper 'ring' connectors and about 6" of single core wire.
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Jimbob
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PostPosted: Sun Apr 17, 2011 21:21    Post subject: Reply with quote

Thanks for the advise guys,

I will ground out the aerial to achieve the best results, will let you know how it goes.
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