I am looking for troubleshooting suggestions.
I just assembled two new 15 meter antennas for my station. These are new
replacements. They are my own electrical design. They are 6 elements on
36' boom. The antennas are fed with a hairpin match (0.375 OD/4.125"
spacing) and a commercial coaxial balun. The design resonant frequency
was 21.2 MHz with a YO 1.0 VSWR match at 20". We assumed a 5" lead length
based on imperical recomendations of the manufacturer.
The antennas have been up and down the tower numerous times to about 20'.
The antennas have been carefully measured and elements are within 1/8"
between the two antennas. Both have new 7/4 wavelength flexible hardline
coax feeds (approx 65').
One antenna has a VSWR dip at 21.440 and the other is about 120 kHz lower.
Both come no where near the predicted 21.2 match freq. The driven
elements required lengthening about 3/4"-7/8" and rather than a flat VSWR
predicted by YO, the antennas can not get the VSWR below 1.4 at the bottom
of the band. The matches are 26" long on the 21.440 antenna and 28" on
the 21.320 antenna. I have tried as many manipulations as I can to alter
the match and DE length, but there is no obvious way to lower the antenna
resonance I can see.
I am looking for suggestions on what I might do next. Both antennas are
on the ground. I notice the VSWR tends to RISE as the antenna goes up. I
am reluctant to get the antenna at the top of the tower, only to find the
VSWR is not suitable. Can anyone offer suggestion on why this phenomenon
is occuring and/or what I can do next??
Thanks,
Dave
(KN2M)
David J. Rodman, MD
Assistant Clinical Professor
Department of Ophthalmology
State University of New York at Buffalo
FAX 716-859-4565
Office 716-854-1137
e-mail: rodman@acsu.buffalo.edu
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|