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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