[TowerTalk] Scaled Antenna
Ron
Ron" <majrabsr@rconnect.com
Fri, 21 Apr 2000 11:44:17 -0500
Hi Eric and all,
While I do not claim this to be the definitive work on this subject, I did
endeavor to be as accurate as possible, within the limits of available test
eqpt.
----- Original Message -----
From: Eric Gustafson <n7cl@mmsi.com>
To: <towertalk@contesting.com>; <majrabsr@rconnect.com>
Sent: 21 April, 2000 11:09 AM
Subject: Re: [TowerTalk] Scaled Antenna
>
>
> 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?
Feedline was connected to feedpoint through a W2AU 1:1 balun.
> How long was the feedline from the transmitter to the antenna?
Feedline approx 450 ft of 70ohm CATV hardline (1/2")
> How was the transmitter powered?
Kenwood TS-850 SAT in shack, powered from 12v supply.
> How much RF power was used for the measurements?
1 watt through step attenuator & Bird 43 AT antenna feedpoint.
> How far away was the field strength measured?
Approx 500 ft from antenna.
> What was the field strength measured with (instrument, RX-S-meter,
> what type antenna, etc.)?
HB F/S meter with 3dB attenuator. Found point where meter read full scale,
kicked in attenuator to put needle mid-scale, and walked a circle around the
antenna. Did not use a tape measure. Had no help available, but distance was
approx same all the way around.
> How was the field strength instrument feedline decoupled from the
> field strength measurement antenna?
18" vertical antenna attached directly to F/S meter.
> How long was the feedline for the field strength antenna?
As above.
> How was the field strength measurement system powered?
Self-powered by rectified RF.
> How high was the system elevated (already asked in another post)?
Counterpoise was elevated 6 feet above real ground, approx 1/8 wavelength.
> What is the ground conductivity in your area (if measured or
> known from local broadcast system certification measurements)?
Do not know specific ground conductivity. Location is SW MN and I suspect
typical for good farm land. ~18" of topsoil over yellow clay.
As I indicated, these are rough measurements to the best of my ability and
limits of the equipment on hand. If anyone can suggest improvemnets in
methodology, I am open to them.
Ron, KA9ALC
> 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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