This note is prompted by Yuri's comments on the measurements Wayne, N6NB,
made on multi element quads and Yagis published in the now defunct (mourn
gently here) Ham Radio magazine.
Wayne's landmark article put to rest a number of myths about quads. While
much has changed in antennas since that time, I believe the results are
still valid.
Wayne could find no quad of 3 or more elements that outperformed a Yagi of
similar length. He tested quads of several different designs around
Southern California. Wayne did not make systematic measurements on a 2
element quad though. The two element quad he used was a 3 band quad with
fixed physical spacing on all bands. It showed good performance on 15 M
with poorer performance on 20 M where the spacing was too small and poorer
performance on 10 M where the spacing is too large.
Recall at the time that it was "common knowledge" that quads of the same
length had 2 dB better gain than Yagis. This was the result of a study by
Lindsay published in the 60s. The difference was that the study that
Lindsay did was at UHF frequencies where the Quad was more efficiently fed
than the Yagi.
In the same article Wayne also debunked the myth that quads are better than
Yagi's at the same height through a set of field strength measurements at
varying angles of radiation with a quad and Yagi on 6 M.
One amazing fact that emerged in the study was that the two element
tribander used as a reference, a Hy-Gain TH2 I believe, was very close to
the performance of several three element tribanders tested. The quad and 2
element Yagi both benefit from the fact that the radiation resistance is
higher at spacings that give good gains than the three and more element
Yagis are. This makes them easier to match with fewer resistive losses.
This is particularly important with trapped antennas. Newer tribander
designs such as the KT-34 with the trombone traps and the Force-12s using
closely spaced couplers are more effective than the older trapped
tribanders because the resistive losses are less. The price to pay for this
lower loss in these antennas is more elements.
Which brings us to the Quagi, a N6NB designed VHF antenna. There is no
better way for the average Ham to get started building VHF/UHF high gain
antennas than to build a Quagi. It is easily constructed with common hand
tools from materials available at your local lumberyard, hardware store and
Radio Shack. It is inexpensive. If built with care to plan it requires no
tuning or special test equipment. The performance is within a dB or two of
the theoretical maximum attainable from an antenna of that length. There
are much better designs available today, but none as easy to build. The
Quagi has withstood the test of time and now more than 20 years later, the
Quagi still holds its own for its length in antenna measurement contests.
The original Quagi design and later variants for 1296 MHz were published in
QST, not Ham Radio.
There is no contradiction between Wayne's Yagi/Quad studies and his choice
of a quad driven element/reflector for the Quagi. Wayne found in his
Yagi/Quad experiments that additional quad elements used as directors did
not improve performance as much as parasitic dipole elements did on Yagis.
N6NB found out that the improvement with quad elements on his VHF
experiments stopped at the first director element. Only on 1296 MHz did
Wayne find a quad with parasitic director elements that matched a Yagi of
the same length.
Wayne's success with the Quagi was due to feed problems that many VHF/UHF
antennas of the time had, particularly those that used gamma matches. KLM
addressed the issue with log periodic fed driven elements, and eventually
today the T match with large (relatively) elements has become a standard
efficient feed.
I am not sure that Yuri's findings on his Razorbeams are necessarily out of
bed with Wayne's on his Quagis. Yuri found no additional benefit from more
than one Quad director element in his Razorbeams, while Wayne found no
additional benefit from more than zero. Given that both Yuri and Wayne
optimized their antennas empirically, given the large number of variables
involved in optimization, and that the optimization algorithm may have been
different, it is clearly possible to come up with the two different
"optimum" designs. However the conclusions are similar; parasitic driven
Quad elements do not seem to help as much as parasitic dipole elements.
Wayne finds the number at which quad elements to be beneficial to be 0,
while Yuri finds it to be 1. For large values of 0 and small values of 1
these are similar results. Seriously, though these may be within the
experimental error of antennas optimized in this nature. Computer models
could shed some light on this as it is easier to cover all bases when
optimizing an antenna on a computer rather than on the range. Elements with
sharp angles, such as a quad has, can be difficult to model with a computer
though.
I hope that I have shed some light on this issue without generating too
much confusion. Sorry for the length. - Dr. Megacycle KK6MC/5
James R. Duffey KK6MC/5 DM65tc <jamesd1@flash.net>
30 Casa Loma Road
Cedar Crest NM 87008
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