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MOBILE ANTENNA GROUNDPLANE IDEAS NEEDED

unit_399

unit_399

EL CAPO
Jun 17, 2008
2,120
3,019
273
ALEJANDRIA, COLOMBIA SA
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
 

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
 
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.
 
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
 
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
 
"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.
 
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.
 
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...
 
Sorry... I didn't mean to say, "connect the antenna base to the battery ground." I was trying to ask if connecting the ground side of the antenna mount to the steel car body was a good thing or not, but I didn't do a very good job of saying it. Yesterday, I made an "X"-shaped groundplane. Items that are commonplace in the US (like copper flashing and aluminum tape) aren't available locally here, so I ran 4 lengths of braid inside the top from the antenna mount to each corner and down to the main body where I grounded it. My SWR is 1.4:1 on 26.955 which is plenty acceptable. I drove to the village this morning and talked with my wife at the ranch (about 22 km away) without any problem. Thanks to everyone for all your help.

- 399
 
"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,...."

say what you mean and mean what you say. you never know who you're dealing with and you don't know what they already know and what they don't.

accurate speaking (or writing) is a sign of accurate thinking.

"Connect the antenna to the 'groundplane'........"

No, and an antenna is NOT connected through the vehicle ground, the antenna is "mirrored" via capacitance to the vehicle ground, not to be confused with a "hard" connection. only in a shunt feed arrangement is there any "hard" connection between the antenna and ground.
 

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