[Re: TopBand: Radials]

[Eric Gustafson]

OOPS! forgot about not using CC: field through reflector. Sorry.

Eric

------- Start of forwarded message -------
Return-Path: <n7cl>
Date: Mon, 29 Dec 1997 18:03:16 -0700
From: Eric Gustafson <n7cl@sparx.mmsi.com>
To: n9dx@michiana.org
Cc: topband@contesting.com
In-Reply-To: <3.0.32.19970228191327.006bd7d8@michiana.org> (message from Larry
	Higgins on Fri, 28 Feb 1997 19:13:34 -0500)
Subject: Re: TopBand: Radials
Reply-To: n7cl@mmsi.com


>Date: Fri, 28 Feb 1997 19:13:34 -0500
>From: Larry Higgins <n9dx@michiana.org>
>
>Here are a couple of questions which might stimulate some
>worthwhile discussion.  Depending on the answers, they might
>also suggest some useful radial system design info.
>
>1.  It is well established that a whole bunch of radials reduces
>ground losses.  However, often the available space precludes a
>radial system which is less than optimum. If there is a
>particular direction either to favor or to disfavor, is there a
>directional shift in the radiation pattern toward the radials?
>For example, suppose we want to favor northern paths, and the
>available space is narrow north to south.  We might put our
>vertical as close as possible to the south property line in
>order to have more radials to the north.  Would this result in a
>directional pattern?  Obviously the lack of radials to the south
>means higher system losses, but if we can shift those losses to
>the least desired direction we can minimize the effect of the
>losses.
>

Larry,

I recently (within the last year sometime) posted a fairly long
discussion of ground system losses here on the reflector.  I
won't repost it here but I will send you that piece plus a couple
of the useful responses to it if you are interested and didn't
catch the previous discussion.

The short answer to your point (1) is yes... And no.  Unless you
are willing to improve the ground conductivity in the preferred
direction in the region from 0.75 to 3 wavelengths out from the
antenna base (assumes a 1/4 wave or shorter base fed vertical),
then there won't be much if any directivity "enhancement" in that
direction.  If this large a radius were improved, it would show
up as increased gain and lower takeoff angle over the improved
ground area.

Also (unfortunately), losses to mutual coupling to lossy earth in
the near field region close in to the base of the antenna will
show up as loss affecting all directions pretty much equally.  So
even the region of improved pattern will be suffering from losses
incurred near the base of the radiator.


>2.  There has been much discussion about ground power losses
>with any kind of radial system: above ground, surface, or
>buried.  Is there a way to calculate ground losses for any given
>radial system, even approximately, other than by installing the
>systems and then measuring the feedpoint impedance?  Obviously
>it would be nice to have some idea of what to expect before the
>installation work is done.
>
>3.  Kinda related to question 2: What kind of real ground
>results in the best, or worst, efficiency?  If your counterpoise
>is sea water it certainly has low loss.  But likewise, if the
>ground under your vertical is a nonconductor, it will also have
>no loss because there will be no current in it.  Somewhere
>between these extremes (my backyard, for instance) there is a
>worst case.  Can we find and avoid this worst case?
>

In my experience, the kind of real ground that results in the
best effeciency is ground that is shielded from coupling into the
antenna by a sufficiently dense ground screen in the region near
the base of the antenna.

In the near field of the antenna even salt water isn't really all
that great.  But it is pretty wonderful as a far field reflector!
And it is a lot better than dirt even in the near field.


>73
>
>Larry, N9DX  
>


73,  Eric  N7CL

------- End of forwarded message -------

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