[TowerTalk] Scaled Antenna

[Eric Gustafson]

Hi Ron,

I also have a few questions.  I'm asking them on the reflector
because it is information that anyone trying to evaluate your
results will need to know.

How was the feedline shield decoupled from the antenna system?

How long was the feedline from the transmitter to the antenna?

How was the transmitter powered?

How much RF power was used for the measurements?

How far away was the field strength measured?

What was the field strength measured with (instrument, RX-S-meter,
what type antenna, etc.)?

How was the field strength instrument feedline decoupled from the
field strength measurement antenna?

How long was the feedline for the field strength antenna?

How was the field strength measurement system powered?

How high was the system elevated (already asked in another post)?

What is the ground conductivity in your area (if measured or
known from local broadcast system certification measurements)?

73, Eric  N7CL


>Reply-To: "Ron" <majrabsr@rconnect.com>
>From: "Ron" <majrabsr@rconnect.com>
>
>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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