Antenna mount on fiberglass

I would glue a bunch of copper mesh or foil to the underside of your tonneau cover and then I would use short ground straps to ground the corners of the mesh or foil under the tonneau cover to the bed of the truck and then I would use short ground straps on each corner of the bed to ground the bed to the frame. Personally I would go out and purchase a Gem Top tonneau cover which is made out of steel but that's my personal preference.

 

The problem with running a long ground strap is that you might start to transmit off the ground strap it's best to keep the ground straps as short as possible and if you did not line the interior of the tonneau cover with metal it would force you to run an extremely long ground strap to reach the antenna mount

 

I've done a similar fix on installs in ambulance bodies using kits that used to be available from Antenna Specialists Company, back when they existed as Antenna Specialists Company. Essentially the kit was a roll of about 40 feet of 2-inch wide adhesive-backed copper foil. It would be applied on the inside of the roof so they would intersect at the antenna's mounting hole where the foil would connect to the ground side of the mount.. Depending upon how much space one had to work with, and using the loose formula of radial length = quarterwavelength + 5%. Somewhere from four to six would be applied for UHF and "high band VHF", or the ~148-174 MHz band. There was just enough foil to do one low band (25-50 MHz), as its wavelength is so much longer. The kit also came with some installable shorting clips to make sure all the copper was electrically a dead short from any point to any other point. The antenna mount (I've always preferred NMO) typically needed a "thick roof mount", with a longer connector body because the Fibreglas® roofs were just enough thicker than a metal roof to not quite reach the threads.

As long as you're able to do Rabbi's suggestion of using enough mesh to cover the entire bottom surface of the lid, than I frankly would skip the idea of trying to bond the truck bed through some piece of wire up to the now-meshed lid bottom. Even though you don't have what should be around 9 feet in all directions from your intended mount location, it'll still be a huge advantage over anyone with the same antenna just slapped anywhere on a plastic roof.

I also agree with one of my learned brethren that an intuitively installed from the mount base to the truck bed in that it's going to foul up the loading by presenting a chunk of conductor that's a significant fraction of a wavelength. This would probably become an "unintentional radiator" of RF, and account for a drastically unmatched OCFD ("off-center fed dipole).

I would support installing a couple of *short* pieces of braid at the front end of the topper, screwing the other end into a hidden piece of the front bed rail after prepping the screws' locations by sanding or wirebrushing to bare metal. That will allow the lined bottom of the bed to incorporate the adjacent truck bed rail, effectively increasing the area of metal making up the counterpoise.

BTW, any screen or mesh will do for the inside lining, but think of long term crudproofing. Copper or brass seem like easy choices until you whip out your wallet. Consider using aluminum screen as long as you can come up with a way to keep the seams tight and somewhat protected against having all those little points of contact turn into diodes, transmitting lots of broadband noise every time. Compounds like NoAlOx (or similar sp?) are supposed to cut down on electrolysis at places where aluminum comes into contact with copper in some electrical installations.

Or, have a decently equipped two-way shop put an NMO mount in the middle of your roof and you can change your antenna on a whim or as needed for garaging, required radio range, and the like. Come sell or trade-in time, you can leave the mount in place, over the mount with a purpose-made rain cap after removing your antenna. Let the presence of a professionally installed antenna mount that will not leak water and can take any antenna from HF to beyond 1 GHz be a selling point.

The mount you'd be leaving behind costs less than $20, and rain caps are $2-3. If you're really intent on keeping that mount and coax, there are well made flanged hole plugs that show just a low profile flat black rubber surface where the mount used to be.

Not precisely what the OP desires/requires. I hope there's enough useful stuff in this missive (my fingertips are beginning to smoke & bleed from typing this long), even the idea of going for the NMO hole & mount.

73

 

You will find lot's i mean LOT'S of info's here, this is the pot of gold in regard to installation buying tips etc etc etc much easier than screening the net to find what you are looking for.
Also lot's of knowledgeable folks who are more than happy to share & educate.

Good luck!

 

Lots of opinions also

 

Here is a suggestion that works.

Put the mount where you want, leaving the idea of grounding the lid alone.

I would use the NMO mount, it is simply the best one out there.

Then take a couple ferrite beads ( http://palomar-engineers.com/ferrite-products/ferrite-beads ) and put them on the coax near the mount.

Run a two inch copper strap from the frame to the bed, make sure your grounding is good for the vehicle itself, but if you want add another one to that strap mount point to the mount itself.

I use copper bolts for my last antenna experiment which worked well. Got them from a plumbing place for a couple bucks each.

You really won't need the second one because you already decoupled the mount RF speaking.

Then tune the antenna.

Why?

Well first off any mesh or any other thing seems to defeat the purpose of the vehicle being a ground plane itself, there is much more surface area with the bed itself than with mesh applied to the lid.

Second with the ferrite beads, you decouple the coax ground from the RF so to speak (meaning less RF feedback through the coax), leaving the RF to radiate into the antenna, this eliminates the problems that people have with the ground straps being part of the radiator of RF and not being part of the ground.

The best system I have yet come up with is a mount at the back of the cab, elevating the antenna just above the cab. People think that you need it all to be even and such but in truth RF is a funny creature and it sometimes has it own way of moving about.

 

Don't ruin your bed cover mount a top-loading antenna center of the bed rail at the sliding window with a mirror mount bracket 18 foot coax between a cab in the bad running underneath the cab come back up in the cab with the coax hook it to your radio