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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Ground Plane for my CB antenna
Message-ID: <17660140@hpfcdj.HP.COM>
Date: 7 Jan 91 19:27:44 GMT
References: <B12yu1w163w@ozonebbs.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 40

>I believe that the statement about the radials being the same
>length as the vertical element is incorrect.  The ground radials
>should be 10% *longer* than the vertical element.  If you are
>building a 1/4-wave ground plane for 27 mHz, the length of the
>vertical element should be around 108".  That makes the radials
>come in around 119".  I'm no expert, but that is what I seem to
>remember reading about them.
>
>I also seem to remember something about making sure the radials
>are "drooping" at a 45-degree angle, because that makes the ground
>plane closer to the desired 50-ohm impedance.

This depends more than a little, I believe, on whether or not the radials
do indeed "droop."  A quarter-wave vertical antenna works just fine against
an *infinite* ground plane, if said plane is flat and orthogonal to the
radiating element.  So any length in excess of 1/4 wavelength should work,
and we see that ground-mounted verticals require no radials whatsoever, 
assuming that the local ground (in the sense of the earth itself) is good
enough.  However, once the "ground plane" starts to "droop away" from the
end of the vertical element, the thing starts to look less and less like the
theoretical 1/4-wave vertical, and more and more like a half-wave dipole.
Think of the radials drooping from 90 deg. to the vertical down to 0 deg. (in
line with the vertical element); as this "droop" progresses, the feedpoint
impedance will increase from the vertical's 37 ohms up to the dipole's 73,
and the length of the "bottom element" (the radials, which all come together
when you get to zero degrees) becomes more and more critical.  Obviously,
it damn well better be 1/4-wave when you get to zero and the thing becomes
a vertical dipole!

By the way, a ground-mounted vertical is an excellent long distance antenna
because of the "infinite" ground plane; the radiation pattern is that of
a dipole on end, meaning that the strongest radiation is a zero degrees from
horizontal - straight out to the horizon!  The higher the frequency, though,
the more difficult it is to make sure that a ground-mounted antenna isn't
swallowed up in local clutter.  


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.