I received a lot of request for copies of any info I received
concerning retuning the TH-7 to lower frequencies. I'll just
summarize and include the reply from Roger Cox at Hy-Gain
as an appendix.
The TH-7 was designed to be a broadband antenna with one
setting per band to handle everything. The early models just
showed one dimension for each of the two driven elements.
Pattern measurements across the bands indicated that the antenna
itself had good performance across all of each band, but the
VSWR was not as low at the band edges as would have been nice
for an amp like the Alpha 87A which has a sensitive built-in VSWR
protection circuit.
Other dimensions were developed by individuals to lower the
resonate frequencies to provide for better VSWR in the CW
segment as well as the DX phone segment at the expense of
the highest end of the bands.
Later, Hy-Gain developed new dimension called "DX-settings" to
improve the lower end of each band. In a discussion with
Roger Cox at Hy-Gain he has suggested an additional setting to
emphasize CW at an even greater sacrifice in VSWR at the top
of the bands.
I have summarized the figures below and will try the CW
setting on my TH-6/TH-2 conversion to a TH-7. If these
check out well, then I will incorporate the changes into
my four other TH-7s to confirm the repeatability.
The only chages that need to be made are to the two driven
elements. The following changes represent an evolution
of settings.
section original suggested Hy-Gain's Hy-Gain's (Cox)
number settings by some DX settings CW settings
RDE-2 31" 32" 33" 34"
FDE-2 40" 41" 42" 43"
I'll report on the performance of these settings once I have
taken some data.
73 John W0UN
Appendix----------------
To: <towertalk@contesting.com>
Date: Mon, 25 Aug 1997 10:57:06 -0400
From: "Roger A. Cox WB0DGF" <hygain@compuserve.com>
Subject: TH-7 CW settings
To: John Brosnahan <broz@csn.net>
Content-Disposition: inline
John,
We never developed a CW setting for the TH7DX. From the beginning,
it was supposed to be a broad-band, one-setting antenna. Several
years ago we came up with the DX-setting as a way to satisfy those
who wanted better SWR at the low end of each band. We checked the
patterns at that time, and there was no change. The patterns and
gain are pretty good over each entire band. If you would like to
improve the VSWR a little more for CW operation, I would recommend
setting RDE-2 = 34" and FDE-2=43". RDE-2length is 36", so there is
still 2" insertion. You could possibly go another inch on each
dimension, but I would replace RDE-2 first. Recommended insertion
minimum is (3 X DIA).
73 Roger WB0DGF
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