Antenna plate

[Elwyn]

Hiya folks

So, I've got a Land Rover Discovery 2 (Mk1) and I'm UK based.

I don't want to drill many holes into the roof but would like to fit some antennas.

So, how about fitting a metal plate on top of the roofbars.

At the front there are two that run from above the mid passenger row of seats to the front.

What I had thought was to put a metal plate up there and secured to the edges of the roof bars.

I could then mount antenna sockets to it, and lowered so there is less height above the rack when there aren't the main antennas attached. Of course, I may then end up with a smaller more basic antenna and use the APRS feature more. But I'd use it for mounting the HF ATAS120 to it as well as the 2m/70cm antenna for ham/2 way use. I've also got a CB that needs to be added at some point and and I may even mount a LED beacon to it at some stage too. But it (the plate) would take the abuse, and not the roof.

What do you think? Would it be enough ground plane for the ATAS120 and also how far apart would i need to mount the antennas so they wouldn't interfere with each other?

Currently the Kenwood D710 I have is mounted in the boot, with the head on the dash and a magmount antenna. But the ATAS needs a decent ground and so can't be used as a magmount...

Suggestions please

 

[DaveInDenver]

The ATAS, like any HF antenna, doesn't necessarily see a car as a super effective counterpoise. It's barely a fraction of the size necessary, if you're lucky 1/10th of wavelength except on the high bands. So what you need is the mount to be electrically solid to the same place as the radio, which is the chassis of the car, which in turn needs to have a good coupling to the environment around you. This means having a low capacitive path to the Earth.

Our off highway cars are terrible in this respect, large rubber insulation in big tires and high clearance. None-the-less, the plate idea is fine for HF as long as you take the time to get a secure path to the vehicle's body and frame. You can use the rack as the path if you take the time to remove paint and ground the rack to the car (it's likely not from the factory as they use rubber or plastic in the mounts).

Alternatively you can use braided strap or the shielding braid from coax to connect the plate itself to the car's body. Point really is that you need to make sure the plate upon which the antenna is mounted is at the same RF potential as the ground the radio is using.

On VHF and UHF you can make the plate very effective just by making it large enough, 19"/0.5m around would make it an ideal 1/4 wavelength ground plane.

 

[Elwyn]

Excellent advice Dave Thank you

 

[AlbanyTom]

If I understand what Dave is saying, I agree. That is - a metal plate 24" or so on the short side, with the antenna in the middle, would be a ground plane for 2m/440, but just a conductor for CB/HF. Those you have to bond to the car chassis somehow...it may be a lousy counterpoise, but it's something.

Lots of people have sites dedicated to HF mobile, lots of good info out there...some good, some seem to be questionable in terms of real science/physics. No matter how you slice it, HF mobile is a compromise, but lots of people do it and have success and fun. Best of luck!

 

[DaveInDenver]

You didn't misunderstand Tom, exactly what I suggested.

 

[uli2000]

For HF, bonding, bonding, bonding will be the mantra you need to chant to make it all work. Even with that, I don't think the roof rack will be the best place for the ATAS. A guy I know locally runs one on a lip mount on the hood with little problems, well, other than the ATAS itself, but thats a whole different issue. There are better options for mobile HF for sure.

I'd recommend checking out Alan Applegate's website, www.k0bg.com, which is the unofficial bible of mobile instalations.

 

[Elwyn]

So when installing this I'd need to earth it to the body as well as the chassis if I can. I can do that

 

[CaliMobber]

yea that should work fine for 2m/440. ideally the ground place should be as wide as the antenna is high. So a little quater length antenna would be perfect. Like others has said just make sure its grounded good to the body. Its all about getting that ground back to the radio ground and get an swr meter or see a shop after install to test it.

If you have to mount the cb then just mount it to the bumper.

 

[Elwyn]

Alas no, rest is all stock d2...