antenna/swr frustration

Outlaw does, in my opinion, really know his stuff, based upon his analysis and suggestions for fixes. In the long run (excuse the pun) bonding grounds will help in a lot of areas. One alternative which may be easier, if there's enough spare space on your mount, is from a thread I submitted about two years ago. It'll only be cheaper, however, if you've got a spare CB whip which is shorter than the distance from your existing mount to whatever part of your truck is underneath it.
This will make your antenna into a "vertical dipole", even though the physical lengths may be unequal, as long as the one you'll add in this trick is actually a CB whip, and not some random length of stainless or aluminum:

http://www.thetruckersreport.com/tr...diy-antenna-solution-for-fiberglass-cabs.html

I've used it at the house for CB, a ham band at 14 MHz, and one at 24 MHz, since my loop won't tune easily to those bands.
Outlaw's ideas are terriric, and should solve most peoples' problems, as well as reducing noise in your received signal. FWIW, you can perform his ground bonding AND the vertical dipole conversion, and the combination might work better than either one alone. But every situation is different, and Outlaw's suggestions are very thorough advice for just about every large vehicle. On my minivan, I've bonded all the doors, hood, and hatch to the main body and chassis as he suggested, as the lower you go in frequency, the more important is your coupling the vehicle to the *actual* ground plane, which is the surface upon which you're driving.

Let us know how it works out, please.

Happy drilling,
-- Handlebar --
diddly dahdidah

 

What would the radiation pattern look like on that home made dipole setup? More or less forward/rearward gain?

I have a horrible ground plane.

 

Ideally it would be omnidirectional, with the caveat that any sizable panels of metal (or an engine block, f'r instance) will somewhat shield the signal in that direction. If you used this arrangement on the tanker in your sig file, you could expect to have a bit of a shadow immediately behind the antenna because of the shroud on the stack and, unless the top part of the antenna is significantly taller than your trailer, a significant shadow to the right-rear of your cab.
In a vertical dipole the side connected to the shield of the coax actually radiates, although a loaded/shortened "ground" side of it will radiate somewhat less, because loading coils tend to absorb energy, rather than radiating it (not completely, but largely so). If you want to try an experiment, visegrip or otherwise clamp the metal mounting stud from the "new" antenna onto the bracket with the new whip pointing out horizontally. It won't be perfect, and it'll sure be a hazard to walking around it or opening the door, but it'll simulate (badly) a ground plane antenna with just one radial. For your test, say, point it out at a 2 o'clock to 4 o'clock position, and see what it does to your match. In that case, your biggest lobe should be in the direction in which the "radial" points, but without an antenna test range, or someone in another mobile who can drive around you on a 3- or 4-mile course, and give you readings, it can be hard to quantify.
If you're lucky enough to have a clamp with wide enough jaws to hang the "radial" antenna straight down, thereby simulating the vertical dipole without drilling any holes, take it for a drive and see how it does.
On a calibrated antenna range, I've been able to see a signal strength increase of just over 4 dB compared to the single whip, on a quiet part of the 24 MHz band. And the feedpoint impedance improved from about 20 ohms to just under 58 ohms, meaning that a bigger portion of the signal sent to the antenna actually "exited" the antenna, rather than being dissipated in the coax by bouncing back & forth because of standing waves. Eventually, all the transmitter's power gets out through the antenna, except for the part that is used up heating the coax.

Sorry, I'm doing it again....you asked what time it was, and I told you how to build a clock......
-- Handlebar --
diddly dahdidah

 

No no. I really like antenna and coax theory. I'm enjoying this. So, my antenna is on my driver mirror. If I mounted another antenna off the bracket and laid it horizontal forward, with the normal antenna still vertical, I'd see larger forward gain while losing gain elsewhere?

 

What would happen if I put something like a 2 foot radial forward and and rearward? Would I see gain in both directions while losing side to side?

 

Your goal with the shorted to the bracket whip pointing down is merely improving the ground by creating a counterpoise. No different except lower efficiency than the radials you see on base antennas. The first time I tried this was in the 70's and through experience I found that the lower the magnetic field around the counterpoise the better it acted as a ground plane. In effect capacitance creates a better ground as the Army discovered by using screens of large area on the ground instead of stakes driven into it. For CB whips the design with the least magnetic field around it is the Francis 1974 patent, probably why the GPK1 kit for the Antron 99 uses a copy of this design in their radials. Of course the least inductance is a straight tubing length such as you see on say the Alpha V-5/8 or similar antenna. Length compression must be used in mobile setups unless you drive monster trucks high off the ground. Meaning a wire wound electrically shortened radiator such as a typical CB whip. The idea does not work so well when the radial is too strongly coupled to conductors. Especially surfaces with much loss like having it too close to the aluminum door plates on say a Mac. Believe it or not the downward radial works pretty good in the back on the grab bar but you may need to use something like the 3 foot CB 22 Francis for the radial. Not as good as a 4 or 4.5 foot but better than nothing.

 

Hmmm I have enough room to run a 4 foot antenna straight down behind my air cleaner. I'm interested now.

 

I'm gonna experiment this weekend. I'm gonna spend some time in my dads machine shop and play around with fabricating a true vertical dipole with a mount that mounts directly to the frame at the rear of the cab. Gonna use some pipe he has laying around, weld a cap on the end, install a connector and mount the antenna on top of that pipe. Then mount the pipe to a bracket on the frame at the rear. Then run the coax up through the pipe. Obviously the pipe would be the counter poise, and being bolted straight to the frame, should be a fantastic ground.

Is there a length the pipe should be at a minimum?

 

If you are trying to build an open counterpoise it needs to be a quarter wave using a velocity factor of around 95 percent depending upon diameter. In short you need around a 102" whip. And it must not be grounded anywhere but at the feedpoint. Sounds like you need to do some reading of theory, get an ARRL book or google it or something before you start trying to create your own components.

 

I have done sone reading, but I wanted to double check the length.

Two ideas, then.

Don't let the pipe ground to the frame, so insulate it at the mount point somehow.

What if I used a nonconductive pipe and wound the 1/4 wavelength of thick copper wire around it to shorten the length of the pipe? Or would this not be any more effective than using another mobile antenna for the counter poise?

I'm just playing around in order to do some hands on learning. I learn better that way.