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Thread: Earthing Defender bonnet for HF ground plane

  1. 17th November 2014, 12:09 PM #1
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    Earthing Defender bonnet for HF ground plane

    I'm trying to nut out antenna options as my antique Moonraker antenna gave very poor performance on our remote trip this year. An online article I read by Codan engineers indicated that by far the best radio wave propagation with mobile antennas is achieved with a front-mounted antenna at top-of- bullbar height, so I'm fabricating a bracket to weld to the bullbar to achieve this (the article was comparing bullbar top mount, bumper bar mount, and rear mount- the results were quite an eye-opener, the rear wheel-carrier mount reduced the signal strength similarly to reducing power from 100 to 5 watts). It occurred to me that the Defender bonnet is effectively insulated from the body at the hinges by the plastic bushes, so would it be necessary to add an earth strap to the bonnet to get an effective ground plane ('terminology?) ?
    I'm assuming the answer will be 'yes', mainly wondering how others have done this.
    Given the cost of a more modern auto-tuner, I'll probably be installing a multi-tap whip antenna, leaving the spring base installed on the bar and only fitting the whip when needed.
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  2. 17th November 2014, 12:20 PM #2
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    Be a little careful, bullbar mounted HF antenna are illegal in South Australia.

    BTW: you should ground every opening panel in the vehicle with a dedicated cable, including the ATU and bullbar to the chassis, the body to the chassis and all the doors to the body.

    You should also shield all the sources of electrical noise with braided or foil wrap and ferrites.

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  3. 17th November 2014, 03:56 PM #3
    oldyella 76
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    You are right re the placing of the moonraker. I have a codan 9350 antenna and I had a bracket down at the side of the bull bar on the drivers, side my friend had the same radio and aerial on the top of his bull bar. When out in the scrub near the W.A. border, he could get someone in central Vic and I couldn't. So I shifted mine to the top of the bar and the performance was far better. So now I have 2 brackets, one at the bottom at the side that I use now ( this is just legal in Vic) and then and when I get out of the way I place it on top of the bar so I can shift if needed. With regards to the legality VicRoads have a document on it's web site and it places certain restrictions including the size of aerials within the drivers view. Have a look at Victorian Standards Information 29. I have been in Queensland and seen them on top of the bull bar there and in another state as well I think it may have been in the N.T. So the regs are all over the place. I will take my chances in the bush.
    Lindsay.
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  4. 17th November 2014, 06:30 PM #4
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    Quote Originally Posted by Lotz-A-Landies View Post
    Be a little careful, bullbar mounted HF antenna are illegal in South Australia.

    BTW: you should ground every opening panel in the vehicle with a dedicated cable, including the ATU and bullbar to the chassis, the body to the chassis and all the doors to the body.

    You should also shield all the sources of electrical noise with braided or foil wrap and ferrites.
    Sounds like I've got a bit of work ahead of me with earth straps. I can't see myself shielding all the electrickery though, as I don't plan to use the HF set whilst driving. It is mounted in the rear box of the vehicle anyway.

    Another query, how important is the length of the coax to the antenna? Is there some ideal length, or should I just route it and rim it neatly to length like I would when wiring accessories?
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  5. 17th November 2014, 08:23 PM #5
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    The only things that you really need to shield are impulse electrical sources.

    The alternator wiring, ignition/injector wiring in petrol engines and the common rail wiring in diesels. Electric fuel pumps can sometimes cause interference but first up is the previously mentioned items..

    For the grounding, all you need to do is a thin braided earth cable type wire between one of the hinge bolts and the door frame on each door or tailgate (upper and lower).

    You won't find me on: faceplant; Scipe; Infragam; LumpedIn; ShapCnat or Twitting. I'm just not that interesting.
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