Dave's notes on the Bobtail are an excellent supplement to the subject.
The Bobtail was "invented" first, with the half-square later, although the
half square is--from the perspective of basic theory of SCV antennas
(SCV=self-contained vertically polarized 1 wl wire antennas)--the more
fundamental. Hence, I tend to alk in terms of the HS, and think of the
bobtail as two HSs with a common vertical element (just as the open double
rectangle with a common wire is a double reactangle--called an open double
DMS in the Antenna Compendium).
The bobtail is the best of the entire double SCV group in terms of gain
and side null. Max gain in my models over average ground was about 5.5
dBi, compared to about 3.8 dBi for the half square.
Models suggest that for lower HF use, they work best with the horizontal
wire up, placing the high current regions above most RF-eating ground
clutter. This is not necessary if you adapt the design of the Bobtail and
HS for VHF work, where orientation up or down no longer makes a
difference.
National Contest Journal will run a 5-part series on the entire SCV
family. Since NCJ does not pay (like QST/CQ/etc) rights will be
non-exclusive, letting me place the articles after they appear one by one
at the web site for those who do not subscribe to that journal.
Unfortunately, the Bobtail does not appear until episode 5, except for a
brief appearance in the first article as part of the family protrait.
Also, expect a couple of articles in Communications Quarterly on VHF
applications. If you need vertical polarization but the very tiny waisted
figure 8 pattern (for direction finding, etc.), the HS works very
well--and the follow-up will be some 2-meter parasitic beams, vertically
polarized but with narrow beam widths, using the HS and the Bobtail as
basic elements. A 3-element bobtail looks like a porcupine with a punk
hairdo. Since at VHF, pointing the verticals up is very practical,
current node feeding is also very practical.
AT HF, with the horizontal line high, current feeding requires care in
keeping currents off the outside of the coax line, while voltage feeding
has its own challenges. Carrol Allen, AA2NN, noted an interesting feed
system he has used allowing both vertical and horizontal polarization of
an SCV loop that might be interesting to those who want both DX and near
QSOs.--applicable to closed loop SCVs.
Hope these notes supplement the discussion.
-73-
LB, W4RNL
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