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Author Topic: Improvising ground plane for mobile mount?  (Read 4353 times)
W2BCD
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Posts: 37




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« on: May 06, 2014, 03:17:25 PM »

Currently I have my antenna mounted on a lip mount thats attached to the rear edge of my hood on the drivers side, so it kinda looks like a stock am/fm car antenna. It's the only decent place to put it, as i have a CB on the tire carrier on the rear already, and I carry a kayak and other goodies on the roof very often, plus its lifted so I already have clearance issues, let alone a 3 foot antenna on top lol.   PROBLEM is, that my hood and body panels are all fiberglass with small aluminum supports. Plus, since its on the lip edge of the hood, whatever groundplane that is provided, is really only on to the front left diagonal, with the windshield being the other direction and air to the other side.

if I made a quarter of a circular piece of sheet metal and painted it black and slide it under the antenna mount (which is only squeeze on the lip from underneath with 4 mini screws that tension the lip), would it help any? what diameter should I start with before cutting a 1/4 out? will it even be worth it? will it make it directional to that quarter?

thanks all!

http://i103.photobucket.com/albums/m159/mercinary1/zz04_zps0ab419f1.jpg
« Last Edit: May 06, 2014, 03:25:22 PM by W2BCD » Logged
M6GOM
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Posts: 1050




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« Reply #1 on: May 06, 2014, 06:51:42 PM »

If that's a dualbander it'll make little difference. Taping wire radials to the underside of the hood running from the mount would probably work better. Its going to be directional to some extent anyway no matter what you do.

I don't worry about height clearance issues, I just run antennas with flexible whips. My Little Tarheel 2 is mounted at the highest point of my car and has a 72" whip on it. I leave it on going through drive-thru height barriers etc. The whip does what its supposed to, bends then springs back once the overhead object is cleared.
« Last Edit: May 06, 2014, 06:56:07 PM by M6GOM » Logged
KA4NMA
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Posts: 591




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« Reply #2 on: May 06, 2014, 06:55:33 PM »

Check out k0bg.com. It would be better to bond the different car parts (hood, etc) to the chassis.  Alan's website has more info.

Randy ka4ma
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W2BCD
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Posts: 37




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« Reply #3 on: May 06, 2014, 06:59:06 PM »

Ha I'd be worried about grounding a service line in my neighborhood driving around like that lol.

How would I connect the radial wires, right to the antenna mount? How many would be enough for decent gain? Like, if I do a few from 1 oclock position then how many left swinging back around to the 9 oclock? Any any particular diameter wire to use? I have plenty of wire of all types sittin around. Lastly lol, I see via quick google search for radial calculator that they should be ~19 inchs for vhf, ~6 for uhf. Would having 19 hurt while using UHF? Or simply not gain anything by it.

Thanks!

ps. going to k0bg now
« Last Edit: May 06, 2014, 07:03:07 PM by W2BCD » Logged
M6GOM
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« Reply #4 on: May 07, 2014, 04:46:04 AM »

Ah I forgot that you still practice that installation method for power lines in the USA.  Grin

Yes, connect the radial wires directly to the mount. Four should be enough. There will be less gain in the directions there's no radials but there's nothing you can do about that and as I mentioned earlier, it would be directional located there even if you were mounting it on a metal bodied vehicle. The good thing about VHF/UHF is that we tend to use antennas that are 1/2 wave or longer so are voltage fed rather than current fed and therefore require far less grounding and its possible to get away with none at all. You could be in the situation where you install them and probably notice no difference. Because of the local terrain I use 1/4 wave whips so I do need a "ground plane" and get common mode issues if I don't use one as it uses the coax to make up for the missing half of the antenna.

19" will work for UHF - effectively for "elevated radials" they're a quarter wavelength but odd multiples work and 19" is roughly a 3/2 wavelength for UHF.
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KB4QAA
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« Reply #5 on: May 07, 2014, 10:59:05 AM »

It is silly to suggest adding 'radials' or additional metal to an 'edge clip' mount.

The best you can do is ensure the set screws on the mount are clean and shiny and digging into bare metal of the hood. Scrape off paint if needed.  I would add some oxygen excluding grease like GB Oxgard or Noalox.

Next, check Alans website  www.k0bg.com and Bond the hood with two or three straps, as well as the trunk/tailgate.  Those two horizontal planes will have the most effect.  To be completely thorough, bond the doors and the tail pipe, which will help more with noise reduction than actual antenna pattern/performance.

done.

[edit] I commend you an being 'brave' enough to put he mount on the hood.  It is perfectly reasonable and the next best choice to the roof top!  Certainly better than a magmount which has no direct ground connection.  The only better choice would be to drill a hole in the hood center.....  Smiley
« Last Edit: May 07, 2014, 11:14:41 AM by KB4QAA » Logged

KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
M6GOM
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Posts: 1050




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« Reply #6 on: May 07, 2014, 11:16:02 AM »

It is silly to suggest adding 'radials' or additional metal to an 'edge clip' mount.

Its on a fibreglass hood that has a few small aluminium plates bonded into it to provide support for the hinges and the hood latch.....

Quote from: W2BCD
PROBLEM is, that my hood and body panels are all fiberglass with small aluminum supports.

He could drill a hole in the centre of the hood but as its fibreglass it'll be useless both for a ground and at supporting the antenna. And last time I looked, magmounts don't work on fibreglass. Wink
« Last Edit: May 07, 2014, 11:18:25 AM by M6GOM » Logged
W2BCD
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Posts: 37




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« Reply #7 on: May 07, 2014, 02:51:11 PM »

I think I spoke in error... I checked in on a Land Rover site and I think the entire hood is aluminum. I'll open it up tomorrow and take a look at the underside to make sure, I just never bothered lol. its in the shop, left rear caliper siezed, and the thing was smoking hot.

 Actually right now, those tiny screws on the lip mount arent touching bare metal at all, I didnt think they where part of any ground. I'm still new at this crap, electrical grounding to me in a vehicle is a metallic path that finds its way back to the post on the battery... no currents going down my tires into the pavement right?! haha. i figured the coax is the electrical return and the ground return is the radios powerwires back to the battery terminals. rf grounding is still a new concept to me... brings up the scenerio in my head... how did apollo craft use radios with no rf ground = because the antennas where not monopoles i assume? (ie. i take it dipole antennas dont need rf grounding because they are there own grounds?)

anywho, thanks for all the input. that sights a big help. i have no problem putting two straps on the hood to the firewall or sidepanels and scraping back some bare spots for the lip screws. (mind you they are flat point and act as clamps, not actually screwing into the hood itself, just squeezing it solid clamp style. id love to put it on the roof, but it just aint happening, so im looking for the best ways to improve whats there.
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