[TowerTalk] Scaled Antenna

[Major Ron]

Hi Tom,

Thanks for your comments. I'm fairly new to 160  and you pointed out things
that I certainly HAD NOT considered. I guess that means that most of my
effort on the scaling has been exercise! What I was trying to accomplish was
getting some real world info BEFORE setting up the 60-ft towers & 30-ft
stingers with top hats.

I've spent most of today mowing 6' tall grass (NEVER been mowed before)
around the pond where I plan the 160 array. Got another 20x25 ft area done.
Oh to be 20 YO again! HI HI.

Also plan 2-wire Beverage running to 60 degrees, and single running at 100
degrees. Shack is at SW corner of 440 yds EW & 220 yds NS.  14 acres are
under contract cultivation for this year, so Beverage will grow this fall
after harvest.

Ron, KA9ALC
-----Original Message-----
From: Tom Rauch <w8ji@contesting.com>
To: towertalk@contesting.com <towertalk@contesting.com>; Ron
<majrabsr@rconnect.com>
Date: Saturday, April 22, 2000 04:47
Subject: Re: [TowerTalk] Scaled Antenna


>Hi Ron,
>
>First, congratulations on doing some real good work. My
>comments aren't made to discourage you, but rather to point out
>something that most people might not consider.
>
>> 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".
>
>The skin depth on 160 meters is several feet to perhaps 40 feet or
>more deep. On 18 MHz, it is much less. Not only does everything
>work to reduce ground losses on lower frequencies, the ground
>itself has very different characteristics at different depths on
>different frequencies.
>
>Because of that, earth losses are much higher on 18 MHz for a
>given soil conductivity. That's why none of us, except those living
>on saltwater, can work long distances on 18 MHz groundwave and
>all of us can work many many miles on 1.8 MHz groundwave.
>
>Unless the dirt behaves exactly the same (same path losses,
>same conductivity) on 18 Mhz as 1.8 MHz...the data will be flawed.
>
>> 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.
>
>OK.
>
>> 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.
>
>Since you moved the antenna above earth, how did you insure
>propagation to the FSM was identical? What prevented propagation
>losses that are extremely high along the earth on 18 MHz from
>decreasing as the system moved towards spacewave instead of
>groundwave along the earth?
>
>I would expect an increase, unless the distance to the FSM was
>such a very long distance that you didn't decrease path loss by
>moving the antenna.
>
>One thing that clearly points out a flaw in the test is the efficiency
>of a vertical with 60 1/4 wl radials is very near 100%...no matter
>what soil is below the radials. That's because 60 radials pretty
>much look like a solid copper sheet, and the antenna fields are
>almost totally isolated from the dirt below the antenna.
>
>Unfortunately moving the sixty radials higher caused FS to improve
>to 135% (a 1.3 dB change). That amount of change is absolutely
>impossible due to account for in efficiency changes, since the
>original efficiency is nearly 100%. We certainly can not have 135%
>efficiency, so either the measurement had errors or the propagation
>losses were reduced.
>
>> Is this a surprise to anyone but me? Has anyone else observed the same
>> results regarding a counterpoise?
>
>Not me.
>
>I measured some systems on 160 meters, and it took about 20 or
>so elevated radials to equal 60 radials on the ground. 60 elevated
>radials and 60 ground mounted radials were almost exactly the
>same, well within instrument error.
>
>Since mounting 20-30 elevated radials is a real pain...unsightly...a
>maintenance headache...works on one band mostly....and offers no
>lightning protection I just bit the bullet and buried a bunch of radials.
>
>Since you can't scale the dirt to the much lower 160 meter loss
>values in the test, it doesn't mean much.
>
>By the way, this is why elevated groundplanes work very very well
>on 20 through ten meters...and elevating a vertical above earth
>hardly makes any change at all on 160 or 80 meters.
>73, Tom W8JI
>w8ji@contesting.com
>


--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm