To: <topband@contesting.com>
>Date: Mon, 23 Mar 1998 18:45:01 -0500
>From: jbmitch@vt.edu (John Mitchell)
>
>At 10:51 AM 3/23/98 -0700, n7cl@mmsi.com wrote:
>
>>Everything works better over perfect ground. SNIP
>In fact, if you really have perfect ground, zero radials
>>is the optimum number.
>
>Well, no it isn't.
Well, yes it is. We (other than you) have been discussing ground
mounted 1/4 wave tall vertical antennas either with elevated or
with ground mounted radial systems of varying densities. If the
ground is perfect, neither radial system is necessary and zero is
the optimum number of radials. Even the modeling software gets
this right.
>Referring to 'SE's EZNEC results, there was about 1db gained
>from elevating the feedpoint and whatever radial system above
>the salt water. Elevated feedpoint counts for something,
>whether it is over brine or rock.
>
Yes, you postulated a feedpoint elevated far enough that you no
longer had a ground mounted vertical. It still wasn't elevated
far enough that without the perfect ground under it, there would
not have been losses that could have been reduced with more
screen density.
The 1 dB that you "gained" was not due to an effeciency change it
was due to a pattern change. We have been discussing relative
losses. Not which of an infinite variety of different antennas
might be "better". Or what is optimum for a particular site.
>>
>>>>Don't elevated radials give some near field advantage over
>>>>standard ground radials when the ground is very lossy?
>>
>>It depends on how many radials you are talking about. I think a
>>good case can be made that a system with four elevated radials
>>could suffer less total loss than a system with four radials
>>laying on the ground surface.
>
>I believe all this depends on what ground you are blessed with.
>Over poor ground, my experiments to date indicate four elevated,
>resonant radials will outperform at least a dozen on the ground.
I can't argue with that. I haven't done the experiment with that
few radials. What I have been saying is that your four elevated
radials won't outperform a system with "enough" radials to
sufficiently screen the radiating system from the earth under it.
With radials 1/4 wavelength long, "enough" is on the order of 107
or so. Or 60 to within 1/2 dB of that.
>Some argue that elevated radials that are fairly close to the
>ground are as tightly coupled as those lying on the ground but I
>believe there is a significant decoupling, comparatively
>speaking.
>Run a 160M dipole at 20 feet in the air, and then lay it on the
>ground and see how it works!
Or better yet, run it at 12 feet (one of the common heights we
have been discusing), _measure_ how well it works 5 wavelengths
away, and then do the same thing with it at 32 feet (on the high
side for the elevations we have been discussing), then do the
same thing with it at 64 feet. Let me know if higher isn't
better.
If higher IS better, since we haven't gone far enough to
materially change the pattern, the difference must be due to
losses increasing as you involve more of the near field zone with
the earth. So it is reasonable to assume that with the high
current portion of the antenna at 12 feet (as we have been
discussing) and the radials at 12 feet, there will be more loss
due to near field interaction with earth than ther would be if
either of the following were true:
1. The radial screen were dense enough to shield the fields off
the earth.
OR
2. The feedpoint and radials were elevated far enough to move
the near field zone of the antenna out of contact with the
earth.
I have been required to measure what "far enough" is several
times. Depending on the cutoff criteria used, it has been
between 1/4 wave and 3/8 wave each time. At 12 feet on top band,
we are talking about 0.02 wavelengths.
73, Eric N7CL
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