Dear Antenna Group: Here is an antenna question. December 24, 2007

On the cover of the Winter 2007 QCWA Journal one may see three pictures of
antennas used by Collins Radio. Two of the pictures feature a circular array
of six curtain antennas. In other words, the array is expected to effect six
beams - one every 60 degrees.

The curtain part of the array is straight forward. The six antennas that are
in front of each curtain are new to me. It is my hope that an explanation
will be forthcoming about how the front antennas "work."

Curtain description: Six, triangular, lattice towers of uniform cross
section are placed equally on the circumference of a circle and guyed in
four places. At least 52 (equally spaced in a vertical plane) horizontal
wires are stretched between adjacent towers and the horizontal wires appear
to be insulated from the towers at their ends. No noticeable sag is seen. A
crude estimate suggests the towers are something like 80 to 100 feet high.

Front antennas: 17 horizontal doublets, all of essentially the same length,
and all in a vertical plane that is parallel to the associated curtain
emanate from a tubular mast placed in front of each curtain. [It is well
known to place fat, horizontal dipoles in a broadside configuration in front
of a curtain consisting of horizontal wires. At least part of the Collins'
array does not look like anything I have seen before.] What I see has mirror
symmetry with respect to a doublet that is in the middle of the mast. I
assume that the antenna is fed with a balanced transmission line connected
to the middle doublet. Above and below the middle doublet is seen exactly
the same collection of doublets fed with a balanced transmission line..

Starting with the middle doublet and proceeding upward a distance that I
call 3X, one encounters a collection of seven, equally spaced doublets
(spaced X apart). The bottom of the group of seven is seen to be fed with a
balanced transmission line coming from the middle doublet. A balanced
transmission line is seen emanating from the top doublet of the group of
seven for a distance 3X to an isolated doublet at the top of the mast.

If one were to ignore the groups of seven (one above the middle doublet and
one below the middle doublet) one would have an antenna of three doublets in
broadside with the likelihood of 1:2:1 factors so as to minimize lobes. The
groups of seven could comprise two "elements" in broadside. Here is one
additional fact about each group of seven: while the top and bottom doublet
(of each group of seven) clearly have an attached balanced transmission line
(one comes from the middle doublet and the other goes to a distal, solo
doublet), the group of seven appear to be connected by a "butterfly" of what
may be mesh (because one can see through the butterfly).

The butterfly is simple, but not easy to describe. It is two isosceles
triangles with their bases vertical and outward from the mast, and with the
apex of each triangle appearing to touch the mast at the middle doublet of
the group of seven. I estimate the height (base to apex) of each half to be
about X. I assume that the mesh of the butterfly is connected to each of the
doublets of the group of seven.

So, who might be able to explain this?

73, Mac N8TT
--
J. McLaughlin; Michigan, USA
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