At a prior QTH I had a 90' Rohn 25 with 16' of mast above and stacked 4el
W2PV design yagis. The 1/4 wave resonance was 1585 KHz if I remember and the
1" CATV hardline used for a gamma was space 3'. Using an Omega match the 2:1
BW was about 30KHz so I modified the amp to load higher in the band without
having to resort to switching relays or motor drive. Voltages were very high
at 1200W which required wide spaced bread slicers of 9KV from hamfests and
15KV fixed 857 series caps as padders so I could use low C variables.
It worked good enough to win a 160M CW contest, CQ if I remember in 87 or
88.
Carl
KM1H
----- Original Message -----
From: "W2RU - Bud Hippisley" <W2RU@frontiernet.net>
To: "John Harden" <jhdmd@bellsouth.net>
Cc: <topband@contesting.com>
Sent: Thursday, December 15, 2011 7:27 AM
Subject: Re: Topband: Shunt fed tower
Many shunt-fed, loaded towers on 160 exhibit narrow bandwidth and are
difficult to match with a single series capacitor for one simple reason:
The gamma rod (shunt wire) is TOO CLOSE to the tower.
A few years ago, after struggling with Omega matches in conjunction with
MANY trips up my tower, I modeled my system with EZNEC. For me, the "sweet
spot" was to position the gamma rod SEVEN (7) FEET from the tower!
For my tower (92 feet of Rohn 45, 8 feet of mast above it, shorty 40 at 97
feet and 4-el. 20-m monobander at 92 feet), the tap point is 57 feet up.
My minimum SWR (in a 50-ohm system) at my center frequency is around 1.4:1,
but my 2.0:1 SWR bandwidth increased (with no change in my skimpy radial
field) to over 75 kHz as a result of my modeling efforts.
Having struggled with Omega matches for years before that, the present setup
is a joy.
One way to get in the ballpark without doing any serious modeling is to
think about the gamma matches you've probably seen (and maybe even used) on
your 20-meter beams. Very roughly, since 160 meters is 1/8 the frequency of
20 meters, all things being equal, the gamma rod spacing on 160 should be
eight times what it is on 20. If your 20-meter gamma rod is 7 or 8 inches
from your driven element, that's equivalent to 5 or 6 feet on 160. Of
course, a grounded, shunt-fed, top-loaded tower isn't exactly the same as a
full-size half-wavelength Yagi driven element, but the comparison is at
least a good starting point.
Construction: My local ACE hardware store stocks 8-foot lengths of angle
aluminum, which is what I used for my horizontal tap rod. Their
heaviest-duty stock is more than strong enough to support itself plus the
top of my gamma rod. I don't support the weight of the entire rod -- which
consists of stepped diameters of plumbing tubing -- that way — I simply
"steady" the top portion while making electrical connection to the tower at
the tap point. (The nearest Lowe's has even heavier aluminum stock, but if
you're using wire instead of heavy tubing, the ACE stock is plenty strong
enough.) The bottom of my gamma rod sits on a single piece of 2x8
pressure-treated lumber from the scrap bin. I use a couple of scrap lengths
of 1x2 furring strips between one face of the tower and the gamma rod to
maintain spacing along the length of the rod. It ain't pretty, but it
works...I apologize to no one about my signal on 160!
Bud, W2RU
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