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Here's a half-wave com antenna design that is easy to
make and has a very broad bandwidth and reasonably small size. It was tested to
have a VSWR of 1.3:1 or less from 110 MHz to 146 MHz, and 1.10:1 from
122MHz to 130MHz. It was tested while fastened to a 1/4" fibreglass-coated
foam plate. It is made from two triangular arc pieces of thin
aluminum and a 24" long, 3'8" ID soft aluminum tube. The two triangles have
a chord at the outer ends of 13" with a 15 1/4" radius. Separate the inner
points by about 1/2". The antenna is fed through the tube which forms a bazooka
balun. The tube is attached but electrically isolated from one antenna half with
one end near the feed points. The coax shield is connected to that antenna
half feed point, and the center conductor to the other. The coax shield is also
connected to the other end of the tube. This is done by stripping the outer
dielectric from the coax and running a wire around the bared shield and the end
of the tube. It is also possible to use one or two 1/4" ID X 1" long ferrites
around the coax at the feed-point to form a quasi-balun. This would allow the
coax to leave the array in the preferred perpendicular direction from the
line of the array. The The antenna can be mounted onto the curved fuselage
in a vertical orientation. Because it is a half-wave dipole, it has its best
radiation pattern normal to the array toward the horizon. As with all
antennas, it is best to keep it at least one wavelength, 8' or more, from
any metal that is in a parralel orientation to the antenna to prevent
deep pattern nulls.
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