This description of the 13 element 20 meter Yagi on a 100 meter boom is copied
from:
http://jplarc.ampr.org/calling/1996/feb/feb96.html#yagi
A photo of Yagi is located at:
http://jplarc.ampr.org/calling/1996/feb/bigyagi2.jpg
Modern Giant Yagi
Excerpted from "Antennas and Propagation," June 1991
What may look like six separate HF yagi antennas pointing the same direction is
actually one large 13 element 20-meter beam! This giant antenna was designed,
constructed, and demonstrated, by Jack Hachten, W6TSW, JPLer Bud Ansley, W6VPH,
and JPL Amateur Radio Club member Dan Bathker, K6BLG. (That is W6VPH on the top
of the far tower).
Thirteen widely-spaced elements had been disposed on six towers in a
fixed-azimuth application. Overall antenna dimensions were 10 by 100 meters, by
25 meters above the ground: a structure roughly 0.5 x 5.0 x 1.0 wavelengths in
the WARC-allocated 20-meter amateur band. The elements were arranged for a
considered balance among forward gain, sidelobe level, impedance level, and
bandwidth, and structural wind survival and construction economies, with an EM
emphasis on forward gain.
Recently-available MININEC-based multiple optimization software has very
conveniently and efficiently automated several of the EM-related design steps,
requiring brute-force iterations. In fact, without such (PC-implemented)
method-of-moments analysis and weighted, multi-parameter optimization such a
project would not likely be initiated. The wire and optimization codes have
clearly enabled such a large undertaking to be approached with high confidence,
with assurance that multiple objectives will be sensibly realized, and with
trivial (or none at all) final adjustment. Still, indispensable human judgment,
experience, and strategy remain necessary ingredients, despite over 15,000
machine-aided EM design iterations, in this instance.
The 100 meter, divided-boom, antenna operated at a center frequency of 14.150
MHz . The design provided a feed-point resistance (30 ohms), with a bandwidth
somewhat more than 2 percent. In free space, the predicted directivity was 15.8
dBi. The predicted directivity was fully 21.5 dBi, at a favorably-low elevation
angle, when arrayed over low conductivity (in fact, good dielectric) ground.
Each element was built with heavy-wall aluminum tubing, starting in the center
with 32mm diameter, stepped twice, and ending with 19mm diameter at the tips.
Each of the six 75mm-diameter boom segments measured 9 meters in length.
To assure EM field purity in the six-tower environment, the topmost tower guys
were dielectric. A conductor-free zone of a half-wavelength (minimum) radius
was thus provided for the intended horizontal polarization.
The antenna operated from Southern California, at a boresight of 15 degrees
East of true North, on a great-circle heading to cover selected portions of
Europe and Asia. The azimuth beamwidth is slightly less than 30 degrees to the
minus 3dB points. Following first turn on in February, 1991, it was evident
that the performance in both transmission and reception equaled or exceeded all
expectations
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