TopBand: Shunt matching tower for 160M

Phil Clements
Tue, 2 Dec 1997 17:50:36 -0600

At 02:56 PM 12/2/97 -0700, you wrote:


>I plan to  shunt feed my 90' Rohn 45G tower on 160M. Since it will be
>(soon!) populated by insulated-element F12 antennas, top-loading
>contributed by the other antennas will be nil. I plan to run a 90' heavy
>gauge wire from the top of the tower over (and maybe slightly down) to a
>fir tree a ways from the tower. This should be long enough to ensure the
>tower/wire combo is resonant somewhere below 1.8-2.0 MHZ (inductive).
>Matching will be by shunt feeding with a vacuum-varible in series, hardline
>from matching cap to shack.

My tower is 90 feet tall..a Triex LM470D crank-up with a Hygain TH-11 at
the 75 foot level with a 20 foot mast above with a corona ball at the tip.
Try yours without the wire to the tree first. You will probably find that
the boom of the yagi will be enough loading to get you below 1.8 mhz. My
system is resonant around 1.4 mhz. I have no guys , but I assume you have
insulated yours at the tower connections. All these shunt feedings are
different due to a number of factors, but I suggest you run the shunt
element as high as you can. Mine is at 34 feet; made from RG8-U with the
center conductor and braid shorted together. Since you do not have a
crank up tower, I would suggest you use aluminum tubing. It is more
rigid than coax, and won't move in the wind. Space it about 12-15 inches
out from the tower. I use an Omega match (two vacuum variables; 500pf @
15 kv each) because I am stuck with the 34 foot gamma attach point with the
crank-up. The shunt capacitor allows me to electrically move the drop wire for
a match. It takes about 400pf of gamma cap. and 50pf of shunt cap to do the
job. SWR is flat about 50 khz either side of the frequency it is tuned to.
My caps are motorizwd, and are mounted under a plastic trash can at the base
of the tower. Ferrite beads are placed over the feedline where it attaches
to the matchbox.

>I was looking at the prices in the Surplus Sales of NE catalog, and the
>price of vacuum varibles took my breath away. But I am concerned with
>weatherizing/condensation problems attendant with using an air-varible out
>at the base. Also arcing (not running HP yet, but that is in the future

I started out with a 2000 volt vacuum variable and it burned up! You need
5000 volt size or bigger. The larger ones are sometimes cheaper than the
smaller ones used in amps. I know Bob at Surplus Sales of NE. He gets his
caps where I get hamfests. The going rate for Vacuum variables
is $85 for glass and $125 for ceramic units. Motorized units already in a
box are readily available for slightly more money. Put an ad in
and you will get your caps pronto.

>Also, ballpark figures for a range of capacitance needed for this

I have never needed over 500pf here. You can always pad with doorknob caps.
if you need more.

I think I have covered most of your questions, Steve. As I said, each
is different. Soil conductivity here is the best in the country and I get
by on a
small city lot with very few and very short radials. Does it work? 121
and 31 zones confirmed...GL; see you in the pileups on top band.

Phil, K5PC

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