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Blue Dawg wrote:
"What do you think?" I gather Blue Dawg has a 1/2-wave dipole made from (2) 1/4-wave coil-loaded whips. That works. The coils are necessary because the whips are too short without them. At 27.185 MHz, WL=11.035 meters=36.20 ft. Kraus proposes a "modern version" of the Yagi-Uda antenna on page 258 of his 2002, 3rd edition of "Antennas". The reflector is spaced 1/4-WL behind the radiator. It could be closer for more gain but this would make dimensions of the array more critical, and the improvement in pattern is probably not worth it. The reflector is 1.03 x the radiator length to get the required inductive phase lag required of a reflector. Length of the reflector is 0.475 WL. Length of the radiator is 0.46 WL. Lengths of elements shrink as they get fatter and Kraus assumes thin wires. They are also shorter with loading coils. Bandwidth improves with fat elements and worsens with loading coils. The elements are shorter than 1/2-WL due to "end effects". The length of a thin non-loaded radiator for 11.35-mtr operation is 5.22 m = 17.13 ft. The length of a thin non-loaded reflector for 11.35-mtr operation is 5.39 m = 17.7 ft. Should you trim the antenna to optimize it? Probably not unless performance is at least 1 dB less than expected. Best regards, Richard Harrison, KB5WZI |
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