One experiment will be my brothers 10 wheeler. It suffers from nearfield reflection because he goes under hoppers frequently and wanted to mount the antenna low enough so that it would never get hit on
the metal plate that's between the drivers door and the hood. The RF ground is adequate, but the best SWR possible is around 4:1. His seems like a textbook case where the only option is to go for the conjugate match.
One question I had is: which is the commonly used matcher for low power applications? I looked around, and the PDC-30 looks to be the best overall choice for the application.
These questions will be handled here.
I did the experiments on my brothers truck and would like to share the results with all of you here.
My brothers truck (as mentioned above) uses a 4 foot Wilson Silver Load on a ball mount that is mounted down low between the drivers door and the hood. The result is a very adequate (probably
better than most) ground plane. The problem is on the hot (visual) side. Almost all of the antenna itself is within 12 inches of a parallel metallic surface. That results in a horrendous nearfield reflection problem. In other words, it's more or less like the entire antenna
is capacitively coupled to ground.
Here's the history (there's an interesting twist).
When we first installed the antenna the best SWR
we could achieve was about 4:1--and that required the use of the grounding wire (impedance transformer)
on the Silver Load.
As it turned out (at least in this application) the grounding wire lowered the SWR (by increasing the resistive part of the complex impedance) but also lowered the field strength. This is not the first instance where I've detected RF on the grounding wire. If that wire is hot with
RF, grounding it will simply delete some amount of RF that could otherwise
be radiated. That's exactly how it was in this case. The grounding
wire lowered the SWR at the expense of lowered field strength. You now know of a real life situation where lowering the SWR does NOT result in increased field strength (more power on the air).
Now for the latest news.
Today we performed some new experiments. The goal was maximum field strength. I used my tried and true tuned-circuit Lafayette Transceiver Tester in the field strength mode. The field strength is measured in arbitrary (relative) units from zero to 100. The first thing we did was set up a baseline reading. That reading was 20 units (5 watt dead-key, no modulation, antenna ground wire grounded, 4:1 SWR).
The first thing we did was disconnect the grounding wire. That resulted in an SWR of 7:1, but the field strength INCREASED to 45 units. The SWR went up, but so did the field strength. That right there tells you that SWR is NOT the last word in antenna performance. The next thing we did was achieve a conjugate match by inserting a Gold Line tuner between the antenna and radio and tuning it to achieve as close to a non-reactive 50 ohm load as possible. That effort resulted in a 1.1:1 SWR (couldn't quite tune all the reactance out) using an MFJ259B. The field strength at that point was PEGGED (100+ units In a nutshell, achieving a conjugate match is DEFINITELY worth it when all else fails.
I didn't want to give away my Gold Line antenna tuner, so we went and bought a PDC-30. On the outside the PDC seems better built than the Gold Line, but it won't fully compensate a 7:1 SWR like the Gold Line would. The best I could do was a 2.5:1 SWR. That's definitely useable, but I can't recommend the PDC-30 as the best overall tuner/ matcher if it can't pull a 7:1 SWR down to 1:1 like the Gold Line unit did. Fortunately, if the PDC-30 is the best you can get, it's possible to open it up and easily experiment with the values of capacitance and inductance necessary to achieve the goal. The first thing to try is reversing the "load" and "in" connections. Secondly, you can parallel fixed caps across either mica cap to increase
the maximum possible capacitance (that's what would be required to achieve a 1:1 SWR in the above case). The inductors are easily adjustable as well if need be.
I hope this helps.
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-=[Bill Eitner]=-
Link to A.M. Tutorial, rec.radio.cb FAQ
and The Dark Side: http://kd6tas.conk.com