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MOBILE ANTENNA GROUNDPLANE IDEAS NEEDED

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  #1  
Old 07-10-2008, 08:53 PM
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Default MOBILE ANTENNA GROUNDPLANE IDEAS NEEDED


I'm putting a mobile unit in the '79 Daihatsu F50 4x4 that we use on our ranch, and everything is ready to go except the antenna. The only mobile antenna I have available is a 5' Firestick, and I want to mount it in the center of the roof. Trouble is... the Daihatsu has a removeable FIBERGLASS top. Do I need to construct some sort of groundplane, or will grounding the base of the antenna with braid to the steel body of the car suffice? Can I install 4 copper wire "radials" like a base antenna? If so, how long should each radial be to work properly with the Firestick. I'm using a Galaxy 2100 radio, a 300 watt linear, RG213 coax, and the Firestick. Any suggestions?

- 399


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Old 07-11-2008, 05:40 AM
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The 'problem' is that fibreglass instead of a metal top. So, think about adding metal to that fibreglass until you have something approaching a 'real live' metal top, sort of. Any of your suggestions ought to work. Won't be the most absoluteliest 'best' thing in the world, but most other 'small' cars aren't either, big deal. Making a sort of 'X' from corner to corner, then connecting the radials to the rest of the body at those corners, will probably be as practical as anything else. Making those 'radials' out of copper flashing instead of wire (increasing surface area) would also probably be a good idea. Doesn't really have to be copper, any conductive metal ought'a work just dandy (gold plate it! yeah, right). As long as you have a good connection at the 'cross over' point, and at the corners, the rest doesn't matter a lot (as long as it doesn't come loose and wrap around your neck). Mount the antenna at that cross over point, naturally.
Good luck.
- 'Doc

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Old 07-11-2008, 02:37 PM
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I would recommend mounting the antenna on some metal part of the vehicle chassis.
If you try to ground the antenna mounted on fiberglass, you're going to have alot of work for something that will break and look ugly.

I think the windshield supported by metal which is part of the chassis.
The rear of the vehicle may be similar.
The counterpoise is important since you want to run 300 watts.

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Old 07-11-2008, 10:04 PM
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I've made a mount that is large enough to distribute the load and not tear up the fiberglass. And... the top is very thick and pretty tough. I think I will try Doc's suggestion and make an X with copper flashing. If it doesn't work out, I will try something else. We really need 2-way communication on the ranch and, because we're in the mountains, I need the power. I have added a second alternator and a Caterpillar tractor battery to the Daihatsu to power the cb, the winch, and the off-road lites.

If I use the "X" approach, should I connect the antenna base to the Cat battery ground or just leave it grounded to the copper flashing?

I really don't care about "ugly" because a Daihatsu F50 ain't gonna win no beauty contest anyway.

- 399

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Old 07-11-2008, 10:28 PM
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Connect the antenna to the 'groundplane', the 'groundplane to the vehicle's metal body/frame. An antenna doesn't care beans about a battery (it's connected through the vehicle ground anyway).
- 'Doc

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Old 07-12-2008, 10:35 AM
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"Connect the antenna to the 'groundplane', the 'groundplane to the vehicle's metal body/frame. An antenna doesn't care beans about a battery (it's connected through the vehicle ground anyway)."

yeah, i can just picture that.

the antenna is connected to the center terminal of the antenna mount which is isolated from the ground plane. the ground plane (copper flashing or adhesive backed aluminum 2" wide ground plane tape) is connected to the external shell of the antenna mount which is isolated from the antenna connection contained in the center of the mount. the antenna is only ever connected to the ground plane in a shunt feed network, nothing you have to concern yourself with here.

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Old 07-12-2008, 05:59 PM
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built a 'mast' from an external/tailgate mounted spare tire carrier that is mechanically connected to the metal body, or build a mast to a chassis mounted trailer hitch. use something with some 'meat' to it, like 3"-6" channel steel with a flat plate on top, maybe 6"x6", maybe bigger. make sure the 'mast' is sufficiently 'connected' to the metal body & frame. it WILL work JUST FINE with 300 watts. even 16 transistors ;-) this was done on numerous chevy blazers, including 1975 & earlier where there is NO metal roof at all, 'glas is windshield back. swr can be adjusted to 1.1:1. analyzer readings will change with size of mast, plate & where real metal roof is in relation to antenna base. but, my later installs when using an analyzer always got acceptable #'s....as well as skip contacts & long distance local.

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Old 07-12-2008, 07:24 PM
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Groundplane = 'other half' of the antenna. Therefore, since that 'other half' would be grounded to the vehicle's body/frame/whatever, and since the battery's negative terminal is also grounded to that vehicle's body/frame/whatever, at least 'half' of that antenna is connected to the battery. Since 'antenna' usually means all parts of the antenna, the battery is therefore connected to the antenna.
I think it's a fairly safe assumption that the vertical radiating element wasn't meant to be connected to a battery, wouldn't you say, freecell? I sort of doubt if he meant it that way, don't you think? At least, I took it to mean the 'groundplane' thingy, not the vertical whip thingy, and that was my intent. Of course, that was an assumption on my part, so I could certainly have been wrong.
I guess I should make certain that my answer isn't misunderstood. So, change the word "antenna" to the particular part of the antenna usually called the groundplane. I am terribly sorry if it was misunderstood.
- 'Doc


Excuse me while I mop up the sarcasm that dripped off of this post. Have to clean that stuff up pretty quick, it makes a terrible mess if you don't...

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