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Old July 8th 05, 04:51 PM
Richard Harrison
 
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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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