Posted by
Stuart Rohre on
Mar 24, 2007; 9:12pm
URL: http://elecraft.85.s1.nabble.com/Vetical-Antennas-tp444325p444328.html
Both the half wave and 5/8 wave antennas are complete resonant structures
without the need for radials. See L. B. Cebik's web site, www.cebik.com for
his discussion modeling half wave verticals, and little was gained by
modeling radials under them.
The reflections you are concerned about are in the range beyond the Fresnel
Zone, some 5 or more wavelengths out from the vertical. Short 1/4 wave
verticals need radials to complete the circuit and return RF current to the
feedpoint.
You can feed with a link, and possibly not need the cable choke. But, the
cable choke is good insurance in any case. The whole issue is one of
balance and if the feeder leads off vertically under the vertical, there is
minimal pickup of RF by its outer conductor. Of course, if it is at right
angles to the vertical directly at the feed point, there is more chance of
current reaching the outer conductor inducing unbalance.
An efficient ground for a 1/2 wave vertical is any whose impedance is say
1/10 of the feedpoint impedance of the vertical conductor, which is
typically around 3000 ohms, depending on how close it is to RF earth at the
base of antenna. So, a ground of 300 ohms would work in this case, while it
would way inefficient for a quarter wave vertical, which needs a return
conductors impedance of less than 3.6 ohms or so. (36 ohms base impedance
on theoretical quarter wave vertical, base fed.) You want the ground to be
a much more attractive path for RF than the other impedances in the system.
73,
Stuart
K5KVH
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