Ham help please, Dipole/Ground plane?

Hiya, Dude.
It's nice to get remembered for sumpin', even if it's sumpin' that didn't necessarily work out for everyone ;-(

Anyhoo --
You've got the theory pretty much correct. And when I went to look up my old post to which you referred:

http://www.thetruckersreport.com/tr...tion-for-fiberglass-cabs.135122/#post-1787334

I realized I'd been sorta tunnel-visioned about the mounting considerations. When I suggested simply grounding the down-facing element, I realized I'd assumed *zero* connected metal, like a metal mirror frame, or a previously bonded bracket-to-door-to-frame, etc. Having a bunch of indiscriminate metal attached to the ground side of the dipole will detune that "half" of the dipole.

With the clarity of hindsight (which is, as everyone knows, an exact science....) I submit herewith my amendment to the original post, with apologies to anyone who ran afoul of the laws of physics whilst attempting to use my directions.

If there were *no* other connected or immediately adjacent metal to the mount, those directions would likely have been fine. And that version is slightly easier to build than the following, which will make for a true and isolated dipole.

In this scenario, the downward facing element will have its attachment to the bracket exactly as the upward facing one: nylon washer to isolate it from the bracket, a proper 3/8" mount nut, and a ring terminal for the coax shield, which should be insulated (heat shrink, tape, etc.) out to the ring, which is then fastened under the insulated bolt that holds the downward element to the bracket. IOW, the two elements *should* look about identical, except that one is fed by the shield and the other by the center lead of the coax.

As to radiation pattern, if there were no objects near the vertical dipole at all -- to include such annoyances as the truck's cab, chassis, engine, driver, etc. -- the signal would radiate in much the same pattern as if a grossly overinflated truck inner tube were dropped over the whole shebang, with the mounting bracket sitting at the midpoint the tube, sorta like a bagel that's been evenly sliced. In that case, the maximum signal strength will be perpendicular to the axis of the antenna --- parallel to the road. If you could see the radiation field, it would diminish as it approached the elements of the antenna, with (ideally) zero radiation straight up and down. In actual practice in the real world, the pattern can be visualized by a pretty neat computer program called EzNEC, and it shows a huge lobe equally up the road and out to the sides and rear, a couple of smaller lobes ("intensity") as you look increasingly upwards to the front and rear (and sides), and some deeply lower strengths in-between.

You asked about directing the signal more along the axis of the road, if I understand correctly. The droop in elements helps match the feed point impedance between the ~35 ohms or so for a ground plane and the 72 ohms or so for an ideal dipole. Speaking of which, that's why looking for a 1:1 SWR is sorta misleading, as a perfect ground plane, at 35 ohms, with a radio expecting 50 ohms, is the ratio (the "R" in SWR) of 50:35, or about 1.43:1 at resonance. A typical SWR bridge doesn't show when an antenna is resonant, only the logarithmic (like a slide rule) of forward power and so-called reverse power.

Turns out that in theory a 45 degree droop angle for the ground plane radials tunes that impedance to the place between 35 and 72 ohms. And an antenna of that style assumes a minimum of three such radials.

While you could try to skew the pattern more nearly forward and aft with just two radials, I don't know what it would do to the pattern and impedance. I believe it's also unlikely that, in a real-world situation, you'd see enough of a benefit by trying to narrow the pattern if it screwed up (technical term there) the impedance or resonant frequency of the antenna at the feed point (connection place at the bracket).

I'm away from the shop right now, and trying to hammer this out on a too-small screen on my Android tablet. I don't have EzNEC on this tab, or I'd try to model your question in the software.

What would help in your experimentation, since hardly any actual antennas at these low frequencies begin to approach the theoretical ones, is find a shop or person with an antenna analyzer to check your progress in a cut-and-try session with different radial configurations. With a wavelength of around 33 feet, CB signals and their antennas are going to be affected a lot more by things like engines, trailers, and bodies than would signals at, say, UHF with wavelengths of around two feet. Nearly *everything* about a typical truck will be in what's called the near field of a CB antenna, wildly affecting how the radio signals interact with their environment. With a height limit of 13'6" for the roadway, it's hard to come up with an HF antenna that can behave anything like the theory says it might if it were up at, say, 33 feet above a perfectly conductive ground (the actual Mother Earth, or a body of water directly under the antenna).

My suggestion is to try some of these ideas out and see what actually works best for you. But certainly, nearly any tuned "other half" of the antenna will work better than just *any* CB antenna, regardless of the hype about coils and such, without such counterpoise. Just remember that, whether it's upright or downward facing, the longer the actual tuned length of the elements are, and the less that's compromised by coiling it up, the more efficient it will be.

For anyone still here -- thanks for your patience.

And Dude -- you *do* know you'll be ostracized by nearly everyone here by asking for specific help from hams on the board, right? :-O

73

 

Dipole would be useless next to a massive steel body. It works great next to a plastic bodied truck like KW t2000 or Peterbilt 387....
Volvo has excellent ground potential if you think outside the box. check dc ohms from back of cab to frame....

 

The signal will always be absorbed or reflected by too many things to take into consideration to any accurate degree. The idea is to minimize as much as you can, at least in the directions you care about most.

If there were no height restricions for CMVs (or anything else on the road, now that I think of it), a guy/gal/sumpin'-else could put a couple of sloped dipoles on the roof of one's enclosed trailer, phased to give some directivity to the pattern, and up in the clear of significant obstructions on the vehicle. Or a 108" whip or two down the length of the trailer's top.... <sigh>

[Note: the careful reader will observe my use of 108 inches as a quarter-wave antenna for CB. 102 inches is about smack in the 10 Meter Amateur Band. Using one for CB usually requires either a 6-inch potbelly spring or an extension to get it resonant in the CB segment.]

If the cab and nearby stuff is all "plastic", it won't significantly impact your signal towards the front, although certainly the engine will be in the near field of the bottom half of the antenna's pattern. Even if trailers were constructed of Fibreglas® panels instead of aluminum or steel, nearly any kind of freight is going to be in the way of signals heading rearward. Well, unless you're spending all your driving time making turns, like at a NASCAR race.....

All the dipole design does for you is help eliminate the need for a metallic roof as a counterpoise for a typical mobile antenna. After that, though, the need to be high and not-all-coiled-up still stands.

This is one of those areas, though, where the nature of HF (CB is between the ranges of 3 and 30 MHz, which defines High Frequency) has an advantage over VHF (30 to 300 MHz, to include the Amateur 2-meter band) and UHF (the frequencies between 300 and 3,000 MHz). The higher one goes in frequency, the more the signals depend upon actual line-of-sight between antennas to work well. Lower frequencies, like CB and lower, have more of a ground wave, and count partially upon the signal to kind of crawl over the terrain in its path to the receiving station. And those lower frequencies also are more likely to be reflected off the ionosphere, giving rise to skip, than VHF and UHF, although the higher frequencies do have some odd reflecting mechanisms at times. A CB signal is somewhat more likely to get through to the other station with a trailer or other obstruction than a VHF one when a large object is in the path. Sure, the VHF or UHF signal will bounce around more predictably, but watt for watt, higher frequencies don't go as far than lower ones.


The vertical dipole approach is to simply get around the lack of a metal roof through (or upon) which to mount a typical mobile whip antenna. All the rest of the considerations about tuning and proximity to unwanted reflectors still stand.

Did I help, or just cornfooze the issue further?

73

 

The Volvo is one of the best trucks ever from which to broadcast. From the pinch weld above the door All the way down to the bottom edge of the door frame the whole body is made out of steel all the way around. from the pinch weld above the door all the way to the top is all fiberglass. when I was driving a Volvo 770 I had a flat stainless plate bent into a 90 degree angle on each fairing and each plate was grounded directly to the metal bracket which was bolted directly to the steel body on the rear of the cab on each side. if you measured the resistance from the plate to the frame of the truck you would have seen 0 ohms. I've never been able to duplicate that on any other truck although every time I switch to another Volvo it is always the same. if you try using the factory mirror brackets Which are usually made out of aluminum or have a black coating on them you are going to run into problems.

 

Dunno. Could be that your radiated field strength actually jumped whilst holding the short whip up there. Equally likely is that your tumescent digits added enough capacitance to the whip so that it appeared to the system as a perfect 50 ohm load *without* affecting the radiated field at all.
Either that, or something else. <----- Handlebar's Universal Disclaimer, °1981