[TowerTalk] Scaled Antenna

[Ron]

Hi guys,

I have completed the first part of the scaled tower tests. The results
contained a big surprise, at least to me.

Since my tests were on the 17M band and I wanted everything to be as
realistic as possible, I began by constructing a scale model (actually two
of them) of Rohn 20 tower using 1/8" galvanized pipe spaced 1-1/8" on
center. This section was 6 feet tall. 3 feet of ¼" galvanized pipe was
attached to the top of the miniature tower and a top hat with a diameter of
7-1/2" was used to resonate the model at 18.12 MHz. ¼" fiberglass rod was
used to insulate the antenna from ground, with a space of 2".

I set the antenna up on a 190' x 170' area near my pond, which stays
somewhat damp (according to previous owner). This is where I plan to install
the 160M antennas. 60 radials, each 13 feet long, equally spaced at 6
degrees were installed at the test site. The antenna was fed with 70-ohm
coax through a shunt feed. An MFJ 259B was used to adjust the match. One
watt of power was fed through a step attenuator to the antenna under test
and readings were taken with a F/S meter. Antenna pattern was clean and
quite circular.

Next, I raised the antenna and used a counterpoise system. This is where the
surprise came. It only took 8 radials to provide the same F/S as the 60
ground-mounted radials! F/S with 60 radials was approximately 135% higher
than when the radials were ground mounted.

Is this a surprise to anyone but me? Has anyone else observed the same
results regarding a counterpoise?

Next step is phasing the two antennas.

Thanks,
Ron, KA9ALC






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