DIY antenna solution for fiberglass cabs

Windmill Hill · Aug 25, 2019

craig_sez said: ↑

Only way to make an ant work on that topper is to glue a big piece of metal on the bottom side of the toppers roof..If you ask me,just use a good big (4way)mag mount if you wanna runna big ant..If you just want a basic mag mount then a wilson or sirio will do just fine on trucks cab roof..
No drilling no fuss..
Click to expand...

craig_sez · Aug 25, 2019

Guess i missed that somewhere....Didnt work?
Sorry playin catch up here

Windmill Hill · Aug 25, 2019

craig_sez said: ↑

Guess i missed that somewhere....Didnt work?
Sorry playin catch up here
Click to expand...

No, you're good...This was in a different thread I started a while back. My SWR is below 1.4 across the board, but it's inconsistent. Seems fine one day and not so much another. Even when I have real good SWR I don't hear a single word, even on side bands. And that's when I have the 102" whip on.

Windmill Hill · Aug 25, 2019

I've tried multiple antennas, coax, radios, mics and bonding.

craig_sez · Aug 25, 2019

Hhhhhmmmm your in a ca fuffle aint ya...
If radios/coax/ant are not the issue then that leaves it mount issue eh...
Have you tried gnd straps?
How bout testing a mag mount just to test on the topper then on the cab?

Windmill Hill · Aug 25, 2019

craig_sez said: ↑

Hhhhhmmmm your in a ca fuffle aint ya...
If radios/coax/ant are not the issue then that leaves it mount issue eh...
Have you tried gnd straps?
How bout testing a mag mount just to test on the topper then on the cab?
Click to expand...

Just tried two different mag mounts on the cab before reading this and they both came up with too high SWR >2.4
Didn't bother trying on the fiberglass.
Put the 102 back on and talked to a driver 4 miles out clear as a bell. Haven't made any contact before that all morning. Go figure. I'm off to bed...talk again tonight or tomorrow.

craig_sez · Aug 25, 2019

Taller whip on the mag mount..I know my sirio needs roughly 72" of whip to tune..

rabbiporkchop · Aug 25, 2019

Not much ground plane to work with.
It's more like a skeleton with an antenna mounted to the top. It needs more ground surface. If you line the whole shell with aluminum foil it would work better. Ground straps from bed to frame and bed to cab should help.

mike5511 · Aug 26, 2019

Drill a hole in the truck roof and do it right. It won’t hurt the value one bit.

russbrill · Aug 26, 2019

handlebar said: ↑

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 --
Click to expand...

Try one of these antennas, they're made for situations like yours...
No-Ground Plane CB Antennas | Right Channel Radios

73,
Russ

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