Antenna elements, including beam elements, can be shortened by so-called "linear loading". For a while, this was always done by winding the element into a spiral
(Watch out! Mention that often enough and someone who doesn't understand how antennas work will say that makes it and RF choke and everyone knows that RF chokes don't radiate. The problem isn't in the winding, it's in the person saying it. That is categorically NOT true). Various manufacturers have made other forms of "linear loading" popular, especially the technique of zig-zagging the element back and forth. I've never seen good data on exactly how the loading effects the resonant frequency. Obviously, it lowers it, but by how much seems a matter of 'cut and try'. DeMaw years ago published the suggestion that a 1/2 wave length of wire wound into a linear-loaded element at least 1/16 wavelength long would produce resonance approximately as a 1/4 wave element. That is 32 feet of wire on a form at least 5 feet or so in length would resonate as a 1/4 wave on 20 meters. But it's at best a rough estimate. Several ARRL antenna books have examples of "linearly-loaded" beams for 20 that have more precise suggestions for an actual design. The renowned L.B. Cerbik, W4RNL, devotes considerable space to the subject of small beams on-line at: http://tinyurl.com/2rf96 In general, shortening any antenna by any means increases losses because it decreases radiation resistance, hence increases RF currents and ohmic losses in the conductors. Yagis already exhibit lower radiation resistances compared to ordinary half-wave radiators due to the inductive coupling to the parasitic elements. Still, short Yagi antennas can exhibit considerable gain and performance, especially when they are mounted at least 1/2 wavelength above the ground. Ron AC7AC -----Original Message----- I'm thinking about the possibilities of building a 3-element, 20-meter monoband yagi...Can anyone tell me if the elements can be practically shortened if the wire was spiral wound around the pole elements instead of laid straight. How can one calculate the effect of a spiral winding and predict the shortened length? Would such a spiral design denigrate gain, or SWR bandwidth performance to the point it would not be advisable? Also, would matching antenna to feedline (RG-8) using an off-center driven element approach to match impedance be a practical solution? If not, what might be the best method to reduce weight? Any comments or recommendations on the best element spacing parameter to ensure 1.5 SWR from CW to SSB segments of 20 meters. I see .2 wavelength even and off-set element spacing designs in the literature. Can anyone recommend one or the other approaches?... Thanks for any advice you may have to give--it will be appreciated. Jack Jack H. Shrawder, KT6JS K2 3256 _______________________________________________ Elecraft mailing list Post to: [email protected] http://mailman.qth.net/mailman/listinfo/elecraft You must subscribe to post. Subscriber Info (Addr. Change, Unsub etc): http://mailman.qth.net/subscribers.htm Elecraft page: http://www.elecraft.com
