[TowerTalk] Phased element arrays

[L. B. Cebik]

Thought I might add a little history to give a context to the discussion
of phased-element arrays.  The ZL-Special and the HB9CV arose in the 50s
and 60s.  Initial claims were better performance than 3-element Yagis.
The claims were not always incorrect, given the state of Yagi design in
those days.  However, the gain numbers got to be excessive, since they
used the Yagi theoretical numbers and added to them, rather than using the
actual performance folks were getting from those "primitive" Yagis.

However, 2-elements with proper phasing (which varies with the spacing and
lengths of the elements) can give about 6.5 dBi free-space gain and a good
20 dB to go with it with pretty fair band width.  In contrast, a 2-element
driver-reflector will give about 6.2 dBi gain and 10-12 dB F-B with good
band width, while a driver-director Yagi can give 6.5 dBi and 20 dB, but
over a narrow bandwidth if 0.07-0.07 wl spaced.  Each type of antenna can
be rearranged for max gain--about 7 dBi with poor (6-7 dB) F-B and very
low impedance.

For the ZL-Special and HB9CV type phased array, there are no secrets to
the method of phasing.  Transmission line runs from the feed point to each
element to set up current magnitudes and phases on those elements that are
what is needed for a particular pattern.  Since mid-feed, as used in the
HB9CV, results in a mismatch between the element and phasing line, he
added a gamma match to each element--or a Tee match in the alternative
version that used an independent 2-wire phase line rather than using the
boom as 1 of the two lines.  The requisite voltage, current, and impedance
conditions can all be calculated--and modeled via NEC.  Morover, feeding
in the center or at the end (what distinguishes the HB9CV from the ZL
Special) does not exhaust the possibilities.  Off-center feed is possible
and expands the range of impedances that can be used for phase
lines--something that has also been done with phased vertical arrays.

Expanded phased-arrays using combinations of phased and parasitic elements
arose in the 70s and 80s.  2-element drivers, are described in the Orr
book, and multi-element drivers (with only a reflector and director) were
described by Rhodes in QST.  Again, claims of superior-than-Yagi
performance were made and may have been true for the generation of Yagis
available in that era.  (We are speaking of monoband Yagi designs.)
However, none of these published designs has the gain of a Yagi on the
same boom length when the Yagi is designed via current software, whether
that software is expressly used for Yagis or is more general, such as NEC.
Yagi gain runs at least a half dB higher than the best of published
designs, and it is possible to design such Yagis with satisfactory SWR
curves across a band of interest--with a good front-to-back ratio as well
(20 dB is the usual standard).  If the Rhodes designs show any advantage
today compared to the Y2K crop of computer-designed Yagi monobanders, it
is only in a superior F-B ratio that a log-cell driver gives in monoband
use.

Some of these notes are covered in considerably more detail in the current
series on log-cell drivers appearing in NCJ.  Part 2 will look at the
2-eleent phasing question and Part 3 will look at some published designs
in reasonable depth.

These notes do not speak to any commercially offered antenna where
complete physical and electrical specifications are not given.  Without
complete specifications, one cannot sort by appropriate analysis something
that may be a proprietary "secret" from simple claims designed to sell
something.  Hence, nothing here should be taken as speaking to any
commercial offering, but only to such published designs as the ZL Special,
HB9CV, Orr book phased driver beam, and the Rhodes log-cell Yagi.

However, one should be cautious in looking at 20-40 year old articles and
their claims of performance.  What authors were comparing their arrays
with may have had the same name as the class of antennas we today call
Yagis, but the available Yagi samples back then bear little performance
relationship to the ones commonly used today in monoband service.

Hope this helps give a little perspective.

-73-

LB, W4RNL



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