You, ON4UN, and I are in complete agreement, Eric.
I just finished putting out 120 radials; average 160'long.
That last .5 DB is a long, hard job with little return for the
labor involved. When it is a labor of love, it is not really
so bad!
(((73)))
Phil, K5PC
>
>
> 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>
To: <towertalk@contesting.com>
> >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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>
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