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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
