Morrison,
Here is a quick and incomplete overview of the various 2-element antennas
you noted in your message.
The 8JK is usually constructed as a fixed wire antenna, since it is
bi-directional. Dimensions are not critical, so general guidance in
manuals should suffice to construct one. Calculating a set of line
lengths for a convenient feed is a little time consuming, but parallel
feedline and a balanced ATU would take care of the situation.
Recently, the ZL Special has been constructed in the US mostly from
300-ohm parallel line, following the guidance in W7EL's article of
several years ago in QST called "The Field Day Special." This version
uses 300-ohm line for both the elements and the phasing line--along with
some ideas on using 1/2 wl lines from each junction so that the antenna
can be reversed when hung in a fixed position. So far as I know,
Lewallen's article was one of the first, if not the first, published
pieces on the antenna to point out that it is not the impedance phase
shift that is crucial, but the current phase shift to get a desired
current magnitude and phase on the rear element relative to the forward
element values. I did a little further work on this in a piece for
Communications Quarterly a few years back, but my work was on 10 meters.
The HB9CV is better known in Europe than in the US, and construction
details appear in Rothammel's Antennenbuch, recently released by DARC as a
2nd Edition. Essentially, the phasing sustem used, whether balanced or
unbalanced, uses gammas or Tees at the elements for a match, since for
single tube elements, the native impedance is quite low. The variables
for the phasing are selected to produce a compromise between max gain and
max F-B, to get about .3-.5 dB additional forward gain while keeping the
F-B in the 20 dB range. The Railbeam is a version of the HB9CV.
If F-B and low SWR coverage of the entire band is more important than raw
gain, you might consider a Moxon rectangle, which would be about 70% of a
full length 2-el Yagi, since the ends fold back toward each other.
Details start in Les' book and continue in various articles, along with a
couple of notes at my site.
For coverage of only part of the band, A director/DE version of the 2-el
Yagi will yield as much gain as the HB9CV with 20 dB F-B and potential for
beta matching at about 0.07 to 0.08 wl spacing for compactness.
I suspect that before you go further, you may wish to refine what you want
from the 20 meter antenna. That will help focus your further efforts
before cutting up any of the tubing.
Hope these notes help.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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