Fiberglass Jeep top and 2 meter antenna.

[KF7NTH]

I have a magnet mount MR77 antenna which I plan to use on several vehicals one being a Jeep with a fiberglass top.The only flat metal surface is the hood and I don,t want to put the antenna there.I'am considering making a mounting bracket out of 1" Sq. tubing connected to the rear bumper and tailgate bumpout half up the tubing, with around plate welded to the top of tube to stick the antenna to just above the top of the Jeep.I do not want to drill any holes in top which may cause the top to crack. I read in another post to get as much metal as possable under your antenna.With the fiberglass and small mounting plate will that create any problems. Is there any other way to do this or will my bracket idea work. Thanks for any imput.

Jeff KF7NTH

[K0BG]

You'd be much better off with a 1/4 wave on a hood seam mount. They're made specifically for Jeeps, and Ram trucks.

[W9GB]

I read in another post to get as much metal as possable under your antenna.
With the fiberglass and small mounting plate will that create any problems.
Is there any other way to do this or will my bracket idea work.

Jeff -

This is a problem for radio installers with newer municipal and school buses (fiberglass and composite body parts).

PCTel (MAXRAD, ANTENNA SPECIALISTS, BLUEWAVE, WISYS AND SPARCO WIRELESS brand names)
sells a metal disk for installation under the antenna, to serve as a ground plane for these antennas.
http://www.antenna.com/apg_product_lines.cgi?id_num=12

Part number: K332
6-inch ground plane disk with .38" OD hole provides ground plane for fiberglass-bodied vehicles.
http://www.antenna.com/apg_products.cgi?id_num=10999

PCTEL, Inc.
471 Brighton Drive
Bloomingdale, IL 60108-3102
Sales Toll Free: 1-800-323-9122
Phone: 1-630-372-6800
==
w9gb

[KF6NFW]

I would suggest using a construction adheisve and adhere a metal plat of desired shape and size to the fiberglass top. No holes and is magnetic!

However, the best is a hole, second down from that is a lip/gutter mount with setscrews, after that comes good old magnets and window glass mounts, just pathetic.

If you dont do the best, then at least do the next best, height wont be the issue as much as reliability. With a good lip mount it will work just fine also.

RAM makes some awesome mounts, we use them in some of our storm chase vehicles.

[K0BG]

Actually Gerry, that's not quite correct.

At VHF and up, the body of a vehicle is more or less an adequate ground plane, depending on how, and where you mount the antenna. At VHF and above, placement is much more important that the type of the antenna (1/4, 1/2, 5/8 wave).

At HF, the body of the vehicle really isn't any sort of counterpoise. In fact, the term counterpoise is typically misused, and misconstrued, especially in this context.

The body of the vehicle, is capacitively coupled to the surface under it. Between the body and the surface, are standing waves which are the actual cause of the ground loss prevalent in every HF mobile installation. Bonding doesn't provide a more congruent surface necessarily, but rather tends to lessen the standing waves—a worthwhile endeavor.

When we mount antennas at trailer hitch height as an example, we force a larger percent of the RF return current to flow through the lossy surface, rather than through the less lossy superstructure of the vehicle. Thus higher mounting reduces ground losses, but there will always be some ground loss at any HF frequency, including 10, and (near VHF) 6 meters. At roughly 100 MHz, the ground losses become so slight, they typically can be ignored, but some ground loss is still there.

The common thought is, that as the frequency is reduced, the ground losses increase. That isn't necessarily so. In fact, 40 meter ground losses can be somewhat more than on 80 meters all else being equal. Again, it is the standing waves which determine the ground loss, and it is a whole lot more complicated than a linear function.

Someone in the thread mentioned using a 1/2 wave antenna on plastic vehicles. They do work in a fashion, but unfortunately, the coax cable does a fair amount of radiating. You can choke it off, but there are still ground losses which might not be RF radiated, and thus are radiated as heat.

[KX7MFH]

For what it's worth. Why not leave the mag mount for the other vehicles with the steel roof/trunks and do a permanent mount for the Jeep? If you had both of the same mounts for permanent/mag mount, then you could just cap one off while you used the antenna on the other. Or have two antenna.

Here are a couple pics of what I did on my Jeep that has the same type of fiberglass top, sometimes a soft top or nothing.

They have worked great, with great reports, even on distant repeaters. 2m and 440.





And for the 11 meter...



~Mike

[KD8DEY]

For what it's worth. Why not leave the mag mount for the other vehicles with the steel roof/trunks and do a permanent mount for the Jeep? If you had both of the same mounts for permanent/mag mount, then you could just cap one off while you used the antenna on the other. Or have two antenna.

Here are a couple pics of what I did on my Jeep that has the same type of fiberglass top, sometimes a soft top or nothing.

They have worked great, with great reports, even on distant repeaters. 2m and 440.





And for the 11 meter...



~Mike

Nice Install

[K9STH]

You might want to consider a coaxial type of antenna such as the Antenna Specialists ASP-358.

http://www.datasheetarchive.com/ASP-...tml#datasheets

Click on the PDF icon for the actual data sheet.

This antenna can be bumper mounted and definitely does not require any metal under it since the counterpoise is included.

Glen, K9STH

[KE5PPH]

tiny Jeep.jpg this is still the best place, NMO on the back seam of the hood. NO visual obstruction.

[VE3HOH]

Glue some aluminum foil to the underside of the fibreglass roof. It's cheap and readily available. You can paint over it . Then drill a hole and put in a real antenna mount that will accept an NMO mount antenna. You will do fine with a 1/4 wave vertical for 2M. The excellent ground plane will be fine . You would do well to scrap the lossy RG58 coacx and use RGU400 . Silvered wire with teflon insulation. It works wonders for UHF installations. .
The many pictures of hardware intensive antennas in this forum are beautiful and wonderful , but the RF is just looking for an easy way in and out of the radio. The RF could care less about the fancy and shiny hardware in some of the pics.
Larsen used to copper coat their commercial VHF whips for better conductivity ! That copper may be a key to a low loss antenna. Look at the HF hamsticks. They are more than half copper wire and work very well. That may just be due to the use of a good conductor called copper wire !
I use a 3/8 x 24 stud mount in a 1 rack unit panel that is bent up so I can use 4 hose clamps to secure it to my roof rack on my Jeep Grand Cherokee. I then run a 3 inch wire to my NMOP mount and secure it with teh set screw on the nmo antenna mount,.
I have run 75m thru 6m hamsticks on there for thousands of miles and they work well. The SIdekick screwdriver antenna works well up there too!
Experiment to see what works for you.
Its not rocket science!

73 Pete ve3hoh/w3