[TowerTalk] GAP Titan

[Eric Gustafson]

Hi Phil,

I could not agree with you more.  But I think I can offer a more
useable guideline to use to determine "how much" screen is really
necessary when the antenna is mounted as close as possible to
earth.  Of course, it is true that more is better.  But at some
point, the ground under your antenna isn't the only concern in
your life.

Basically we are concerned about screening the near-field zone of
the antenna from being able to "see" the lossy earth in its
vicinity.  To do this for a physically short linear radiator
requires a screen with a radius approximately equal to the length
of the radiator.  I don't know how long the radiating portion of
the Gap is.  But for the sake of argument, lets say it is 25
feet.  And lets further say that the band we are concerned with
is 40 meters.

>From the above we already know we need a screen with a radius of
25 feet.  But how many radials are really required?  The answer
is (as usual) it depends.  What it depends on is how completely
you want the fields screened.  To get all but the last 0.1 dB of
the possible improvement, the screen needs to have enough radials
that the open ends are no farther than 0.015 wavelength from each
other.  So for operation on 40 meters, this translates to
tip-to-tip maximum distance of about 2 feet.  And on a 50 foot
diameter circle this means you need 78 to 80 (depending on how
you like to round off) radials.

Now, if you can stand that you only got all but the last 0.5 dB
of improvement, you can relax the requirement to 0.03 wavelengths
tip-to-tip and use only 38 - 40 radials.  Further relaxation of
the screen density requirements below this produces a fairly
rapid falloff in benefit.  But any number greater than four will
provide _some_ benefit.

How much improvement is to be expected?  Again, the answer is
that it depends.  It depends on a number of factors (including
but not limited to the specific characteristics of the dirt under
the antenna) none of which are usually under the control of the
owner of the antenna.  The typical range covers from 3 to as much
as 6 dB - depending.

73, Eric  N7CL


>From: "Phil Clements" <philk5pc@tyler.net>
>Date: Sun, 10 Oct 1999 22:21:20 -0500
>
>> Anyway, it appears from the Gov's experience, that radials
>> under 1/2 wave vertical antennas, with the lower end near
>> ground, can result in a much higher efficiency radiating
>> system as judged from field strength measurements way out in
>> the Western Pacific.
>
>
>Alas, Jim..I had no idea so many folks had drunk the Kool Aid at
>the Gap booth!
>
>When I started this thread, I ASSuMEd that the majority of TT
>subscribers have educated themselves with the fine literature
>available on the subject from ARRL and other sources. I got some
>really remarkable replys from the ignorant and/or brain washed,
>and the knowledgable just stood silently by until your post.
>
>One fellow even stated that the Gap antennas are not a vertical
>per se....I doubt if even the Gap advertizing goes this far!
>You and WWV are absolutely right, a half-wave vertical dipole
>requires even a better ground screen directly under the antenna
>and better soil conductivity in the Fresnal zone to realize any
>improvement over a quarter-wave vertical.
>
>The reason for the variation in performance with vertical
>antennas with little or no ground screen is the soil in the area
>for several wave lengths around the antenna. It is impractical
>in most cases to do anything about improving the the Fresnal
>zone other than moving to Dallas or Des Moines or an island
>surrounded by salt water. You can definately improve the soil
>under a vertical antenna by installing radials or a ground
>screen. This is what got the wive's tale of "verticals radiate
>equally poorly in all directions" started....with no or few
>radials, you are loosing energy into the lossy earth, big time!
>
>"C.J. Michaels, W7XC (SK) calculated the depth of penetration of
>RF current in ground of different properties. He defined the
>depth of penetration as the depth at which the current density
>is 37% of what it is at the surface. Under those conditions, for
>80 meters, a depth of penetration of an amazing 1.5 meters has
>been calculated for VERY good ground. For very poor ground
>(desert and beach, etc.) the depth reaches 12 meters!"  (Quote
>from ON4UN's "Low band DXing")
>
>All vertical antennas mounted near or on the ground including
>the Gap will warm the worms and dissipate power into the lossy
>earth unless the RF is reflected up by a proper ground screen.
>
>I think what some of the "Gappers" do not understand is that the
>so called radials or ground screen do not connect physically to
>the antenna.  One chap wrote that the worst thing you can do
>with a Gap is to put radials under it.  The radials or screen
>are constructed in a pattern, and just lay there on the ground,
>doing their thing. It is just like bringing in a new load of
>soil from the midwest! How long, how many, or how deep this
>radial system should be is very simple. Run as many radials as
>you can, as long as you can; you can bury them a few inches or
>just staple them into a scalped lawn and let them bury
>themselves.
>
>You doubting Thomases out there need only a field strength meter
>placed several wavelengths away from the antenna as you add wire
>under it. You can watch the needle climb as the RF quits going
>into the ground and into the air!
>
>Rebuttals are encouraged, but please use your own test results
>or literature other than Gap brochures to make your case.
>
>(((73)))
>Phil, K5PC

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