Steve:
Would you consider shunt-feeding the tower using a Gamma matching
section? I would suggest placing a pair of shunt wires off the tower face
about two feet, using PVC T's to hold the wires off the tower. Tap the tower
maybe at 1/3 height AGL and tune out the reactance at the base with a series
transmitting variable cap of about 500 pF or so. Don't forget to connect the
coax's shield to the tower and its ground radials.
If this works for you, you could drive the cap with a DC motor and
appripriate gearing to get you the coverage you seek throughout the band.
73 de
Gene Smar AD3F
From: AB5MM <ab5mm@9plus.net>
Date: 2007/09/27 Thu PM 07:51:38 CDT
To: towertalk@contesting.com
Subject: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
Gentlemen,
I would like to extend a hardy "Thanks" to each of you that responded to
my plea for help with matching the 120'-6" 160m Rohn 25 Vertical. It's
still not working like I think it should, so let me tell you what we've
managed to accomplish to date.
1. We now have 40ea. 125' long ground radials installed. Another 20 will
be added this weekend for a total of 60. I think I'm going to say that's
enough for now. NOTE: Going from 22 to 40 ground radials made little
difference!
2. Both the MFJ-249 and MFJ-259B are virtually useless at these
frequencies. Too much rf from the BC band. This alone would have saved
us hours in trying various matching networks, not to mention the number
of "Pepcid Complete" ingested.
3. In sweeping the tower from 1.600 MHz to 2.500 MHz, we found it's
natural (no matching other than the 75' of LDF4-50A) resonance at 1.788
MHz. The lowest swr observed at this frequency was 2.6/1. At 1.800 it is
3.0/1 and slowly rising to 4.0/1 at 2.000 Mhz. Conventional wisdom tells
us that the thing is too long (tall). This still brings up the
question, "How can this be the case when the vertical is only 120 ft.-6
in. tall to begin with?" What are we missing here guys? The center of
the band (1.900 mhz) is what we've been shooting for all along.
4. A high pass "T" network brings the match down flat, but it's Q is
high enough to only realize a practical band width of ~33kc.
5. A low pass "L" network works as well and exhibits a little larger
band width of ~60 kc. Our goal is ~100 kc wide... is this reasonable?
We really don't know what to do or try next, if anything. Should we be
satisfied with what we have?
Press on, nothing is as persistent as persistence itself.
Tnx & 73,
Steve AB5MM
Lloyd K5ZO
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