| |
K0CQ > TECH 09.02.04 20:42l 48 Lines 2777 Bytes #999 (0) @ WW
BID : 12615_W0AK
Read: DB0FHN GUEST OE7FMI
Subj: loop yagis.
Path: DB0FHN<DB0RGB<DB0FSG<DB0AAB<DB0ZKA<DB0KFB<DB0CZ<HB9EAS<HB9AK<K1UOL<
WB3DTG<N0RVX<W0AK
Sent: 040208/1737Z @:W0AK.#CIA.IA.USA.NOAM #:12615 [Des Moines] $:12615_W0AK
From: K0CQ@W0AK.#CIA.IA.USA.NOAM
To : TECH@WW
Many of the rod yagi designs have suffered from very tight tolerances to
construction dimensions. For example the 432 MHz K2RIW 19 element yagis
repeated well only if all dimensions were held to a 1/64" tolerance (0.0156"
or 0.4 mm). That same design scaled to 1296 would demand three times as tight
a tolerance.
Back in the 1960s 432 rod yagis didn't reproduce well. Even amateurs thought
to be skilled produced antenna copies that didn't work like the original. Then
the Central States VHF conferences started having antenna measuring "contests"
to compare various antennas, both commercial and ham made and the good ones
soon popped up. That 19 element K2RIW, while no longer the best yagi, is very
reproducible and is the standard of forward gain at most antenna measuring
tests. The testers at CSVHF conferences work at having a uniform test field
free of ground reflections.
Two publications have made rod yagis far more practical at UHF and 1296, the
first by Peter Pfizbecke from the US NBS. It was reproduced in ham radio
magazine I recall in July 1977. He included the effects of the boom diameter
and how the elements were mounted to or through the boom as well as element
diameter and gave a set of 5 yagis from under 1 wavelength to 4.2 wavelengths
boom that have proven to be very reproducible and scaleable. Then DL6WU
developed his yagis based on logarithmic impedances with more details on the
effects of boom and element on element length. The DL6WU designs seem
inherently more tolerant of construction tolerances which makes them far more
practical at 1296 and even higher.
The loop yagis on the other hand have not have the development of the rod
yagis, but all antenna contest tests have proven them to produce high gain for
the boom length and to allow construction precision that is practical up
through several GHz.
I saw a 432 Yagi, 2.2 wavelength made from NBS design data at one CSVHF
conference by a ham who builds absolutely beautiful equipment. Trouble was, he
used different sized boom and element rods and didn't apply the proper
corrections. Should have been a 12 dBd yagi measured 7.
I bought a loop yagi kit from Spectrum International 20 years ago or more and
tested it at a CSVHF conference where it yielded within a dB of claimed gain.
The driven element is fairly well isolated from the boom by the brass bolt its
built on so dissimilar metal corrosion isn't a big problem. A coat of epoxy
paint over the complete antenna could prevent corrosions for a long time. My
best DX with 2.3 watts out has been about 900 miles.
73, Jerry, K0CQ @ W0AK.#CIA.IA.USA
Read previous mail | Read next mail
| |
