DIY antenna solution for fiberglass cabs

Hiya, folks.
I don't know why this didn't occur to me sooner. Since so many trucks have big mirrors with tubing mounts, but are hanging off of cabs that have fiberglass roofs, it's hard to come up with a decent ground plane. Here's something I do at my ham station, and if I had a taller vehicle, I'd use it on that, too.

A regular mirror mount has the grooved clamps and a 3/8 x 24 stud mount that faces upward. The top nut of the stud, where the antenna screws in, is insulated from the bracket by a white nylon washer. Well, usually white. But it's gotta be there, and none on the bottom, so that the outer connector of the coax goes to the bracket, and the center pin goes to the top nut and antenna whip. So far, so good; it's stock.

A "ground plane" antenna, the kind where you've got that same stud mounted through a metal roof uses the roof as the "phantom half" of the antenna. What it's doing is simulating what would be the other half of a dipole antenna, which has two equal halves, fed in the middle by the coax, and stretched tight at the ends so it's all in line, with the coax coming off at a right angle at the midpoint.

Here's the cool part. We can make a tuned dipole out of our mobile antenna mount! Just drill a 7/16-inch hole in the flat part of the mount, out of the way of the existing stud or the bolts and clamp. Get a spare top nut (about an inch tall, 3/8 x 24 threads all the way through; truck stops have them with the other antenna parts). Also get a 3/8 x 24 bolt about 3/4-in long, and a lockwasher, ideally stainless.
Put the lockwasher on the bolt, stick it down through the hole you drilled in the bracket (the head is on the top of the bracket) and thread the spare antenna mount top nut tightly onto the bolt. Do not put an insulating washer on this. We want this to look like where the antenna gets mounted, except it must be "shorted" to the bracket.

Now, find a place on the mirror mount (or grab rail, someplace that has free space directly beneath) and clamp the mount in place. Take a second CB antenna whip of whatever length will best fit under the bracket, and screw it in to the nut that's facing down, using another stainless lockwasher to keep it tight.

Now, you should have a mount that looks stock, except that there's a sort of mirror image of the top half, underneath and pointing towards the ground. If you've got room for a 3-ft fiber glass antenna, great. 4 feet? Wonderful. Only have 2 feet before the bottom whip hits an air filter or horn or fender or something? Use a 2-footer. The important thing is that it be a CB antenna, and not some random chunk of wire or broken-off whip.

Now what you've got (with the regular antenna on top, of course) is a true dipole antenna. The bottom antenna will do essentially what a metal roof would, by restoring the "phantom half" -- except that in this case, what you have is the "real" other half!

Don't bother with trying to tune the bottom whip. It's going to be closer to the door or cab body than the upper half, so it will still be a little bit unbalanced, and will be impractical to try to tune the SWR by adjusting the bottom at all. However, your antenna system will likely show a different SWR than it did before, so go ahead and check the tuning and adjust the "regular" antenna (the top one) as usual, as if it were new.

If it will be awhile before you can get to a shop to have this done, don't worry; suddenly having your antenna more efficient won't hurt anything so it can wait. And, FWIW, the built-in SWR meters on radios are notoriously inaccurate, so wait until you can get to someone with an outboard meter to check and adjust the antenna if necessary. It might not need any changes at all.

Here's a pic of a modified bracket, below. The new stud is at the far left end. The bolt top is visible on top, and the "top nut" is partially visible underneath.

Hope this helps. If this seems at all unclear (wouldn't be the first time I didn't explain something correctly), feel free to ask.

-- Handlebar --

 

Yup, oughta be way easier to match, and more broadbanded. I've got two of these lashups here at the shack, one for 20M, one for 10M, as alternates to the 360-ft magnetic loop up in the treetops. Hamsticks work better in pairs.
-- Handlebar --

 

A little; like in any antenna, the more of the physical length that's shortened into a coil, the less the radiating cross section that's available to transmit or receive. Think of any antenna as a sail on a sailboat. If you only unwrap one quarter of it off of the boom and hoist it aloft, you're gonna "catch less air", even though the rest of the sail is physically there; it's just stowed where it can't do any good, but the boat still has to move the same weight of the sail
For a similar reason, a 5-ft fiberglass antenna will outperform a 2-ft fiberglass antenna. A true dipole has equal halves, but you *can* shorten one half with a loading coil, with some loss of performance.
But it's still gonna be *way* better than just an antenna element on one half and essentially nothing on the other.
If you can fit a 4-ft or 5-ft whip on the top, and a 3-ft on the bottom, you should be golden.

I'm sure someone with a better sense of commerce will market this as a trucker kit. I'm happy just to be able to help folks have better signals while they're out there doing their jobs. Especially when anyone with a drill and a wrench can do it themselves and save a fortune

Thanks for your kind words.
-- Handlebar --