[TowerTalk] Scaled Antenna
Eric Gustafson
n7cl@mmsi.com
Sat, 29 Apr 2000 12:52:41 -0700
Hi Rob,
Thanks for satisfying my curiosity. It looks like you have
managed to avoid most of the instrumentation traps that people
fall into when trying to do this kind of measurement. The one
possible exception is depending on a W2DU balun to adequately
isolate the feedline from the antenna system. But at low power
and reasonably high frequency, this may not be an important
issue.
However as Tom (W8JI) has ponted out, there are a number of other
potential sources of error in the assumptions made regarding
scaling this kind of measurement. And even some associated with
not measuring within the main lobe of the pattern when comparing
ground mounted to elevated systems in the same band (no scaling
involved).
Your results do not seem surprising given your setup and that you
did raise the system to a significant height above ground. Your
1/8 wavelength base height would equate to about 68 feet in the
160 meter band.
The thing that I find the most interesting about your measurement
is what it appears to show about the effectiveness of the radial
screen in reducing near field losses. From your reported
measurement it would apear that even with a base height of 1/8
wavelength, and with a shortened radiator (should actually
require a smaller screen), there is still 1.3 dB of near field
ground losses that can be overcome by improving the shielding
effectiveness of the radial screen. This improvement was
obtained by going from 8 to only 60 radials. It is notable that
this was over ground that was probably at least "average" (and
probably better) based on your description of the location. At
some base height (probably in excess of 3/8 wavelength somewhere)
the number of radials used in the counterpoise would not be a big
factor.
73, Eric N7CL
>From: "Ron" <majrabsr@rconnect.com>
>Date: 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>
>Sent: 21 April, 2000 11:09 AM
>Subject: Re: [TowerTalk] Scaled Antenna
>
Snip... (a number of questions about how Ron made the
measurement)
>>
>> How was the feedline shield decoupled from the antenna system?
>
>Feedline was connected to feedpoint through a W2AU 1:1 balun.
>
>> 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.
>
>> >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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