I install many radios in off road vehicles such as trucks, farm machinery, dirt moving equipment etc. Many of the newer vehicles are coming out with fiberglass roofs. The standard antenna no longer can be used because they require a metal roof as a ground plane. Antennas such as"No Ground" plane and 1/2 wave antennas are available but they are taller and at more risk of damage with tree limbs etc.
I have come up with a solution that has proved to work very well. I use a concave disc blade as a ground plane. For you city folks, a disc blade is usually 12-18" in dia. where 20-40 or so are mounted on a farm implement that is pulled behind a tractor, that turns over the ground. All the iron junk yards have them.
A ground plane should be at least a 1/4 wave length. Longer is good. For 450-470 MHZ a quarter wave is around 6" So a disc blade of at least 12" dia. is needed. For 150-170 MHZ the blade would need to be 36" dia. I have not made any for this freq. band nor do I know of any blades this large but other objects would work. But for UHF I have taken 14" blades and brazed a good quality washer, like brass, or plated over the square hole. The washer hole dia must be 3/4" to accept a "M" mount. Thick washers won't work. I then use a standard 5 db gain antenna. I like the MaxRad MUF4505. The concave provides room for the mount and cable underneath, From the edge of the disc I cut a 2" long slit 3/8" wide towards the center. This allows clearence for the cable.
Find a good flat roof surface and use "Floor and Deck" adhesive to fasten the disc down. On the outer rim I lay 4 beads of 4" length @ 90 deg apart. Then press the disc down into the gupe. Then spread some more over the edge.
VERY IMPORTANT. The antenna must be cut 3/4" longer than the cutting chart shows. Then trim down from there for best VSWR. The concave shape of the disc needs the extra length. VSWR readings have been excellent when the correct length has been reached.
Fiberglass roofs and ground planes
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windmillsforsale
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Or you could take aluminum foil tape used to seal insulation board or heating ducts and make an x on the inside of the fiberglass roof and use a regular antenna mount with the tape as a ground plane. If you want a better conducting tape go to the marine store and buy the sticky backed copper foil used for ground planes in fiberglass boats.
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spareparts
- Batboard $upporter
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Fiberglass Roof Groundplane
If you need a ground plane cut for a fiberglass roof, I can recommend a really good fabricator in Metuchen NJ. He has a complete CNC driven laser cutting system that can do just about anything. The metal can be glued or mechanically fastened to the fiberglass, either inside or outside the vehicle. PM if you need contact details
Martin
Martin
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raymond345
- Posts: 268
- Joined: Sat Sep 22, 2001 4:00 pm
Why a Ground PLANE?
Why look at a ground plane antenna?
we use a zero ground plane antenna's .
Just install the antenna and go.
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