I'll add my own experience, for what it's worth. My tower is 67 feet of
Rohn 25, originally with a D40 rotatable dipole at 75 feet, a TH7 at 70
feet, and an A3 on a side mount at about 50 feet. The original gamma arm
was 1/4" OD copper tubing about a foot from the tower, with the tap at
around 55 feet. A 500 pF 10 kV vacuum variable is in series. When
matched, the Q was very high, capacitance low, and peak voltage getting
close to 10 kV at 1.5 kW. I added a second similar gamma arm a foot
further from the tower in parallel with the first to reduce the
inductance. It didn't quite run the whole way, as I ran out of tubing.
In any case, the capacitance went up, voltage and Q went down, and the
tap didn't move much. It's been working that way for 30 years. There's
another gamma arm on another side of the tower for 160, switching the
feed with a relay.
You have to be careful of wire antenna supported by the tower if their
feedlines run up the tower. Initially, I had a 40 meter full wave on the
tower, inverted V fashion, supported and fed 1/4 wave from one end, with
the feedline running up the tower. The shunt feed coupled so much
current into that coax that it destroyed the balun. I finally had to
pull that feedline as far from the tower as possible. I did not want
that wire connected to the top of the tower, as it would have added way
too much top loading.
Last year, I replaced the TH7 and A3. While the parasitic elements of
those antennas were grounded to the booms, and the TH7 driven elements
were grounded through the matching network, the new antennas had
isolated elements. I was concerned that the shunt feed would couple lots
of current into the driven elements and damage the baluns, so I
installed what I call "shunt chokes". These have a high impedance across
the driven element feedpoints, but low common mode impedance to the
booms. As far as the shunt feed currents, the driven elements are
shorted to the booms. Just to be safe, I put one of those on the D40, as
well, even though I never had any trouble with it before. (I suspect all
the grounded elements on the TH7 diverted most of the current before it
got to the D40.) In addition, these chokes allow the driven elements to
contribute to top loading, which I need on 160. When all was said and
done, the shunt feed worked as before, without moving the taps much. I
can provide details of those "shunt chokes", if necessary.
Since my lot is only 60 by 120 feet, the radials are short and crooked.
Initially, I had only 13 of them, but the shunt fed tower worked very
well. Eventually, I added an extensive grid of some 1200 feet of wire.
It didn't change the SWR at all, so I doubt it affected performance at
all. In spite of the small radial field, the antenna seems to work very
well. The primary benefit of the 1200 feet of wire may be as a lightning
ground.
73,
Scott K9MA
On 11/25/2020 2:34 PM, Dennis W0JX via TowerTalk wrote:
Hi Dick:
I have had s shunt fed tower at five different QTH's since 1979. I have never
used anything other than a simple gamma match circuit utilizing one variable
capacitor. That variable capacitor has been in the range of 500 to 750 pf
depending upon the length of the tower and the conductors used for the gamma
wire.
At first my tower in MN was 60 feet high with a TH6 on top. The gamma wire was three #14
wires twisted together and the wire was attached to an aluminum tube at the very top of
the tower. As the tower grew in height and the antennas got larger, the "tap"
was moved down to find the 50 ohm sweet spot. I eventually replaced the wires with half
inch CATV aluminum coax.
In my latest installation, the tower is 80 feet tall with a TH-11. I'm still
using CATV coax as the gamma arm and it is about at the 48 foot level. The
tuning capacitor was a 400 pf variable (about 2500 volt rating) paralled with
two, 200 pf mica capacitors. Several years ago, I replaced the air variable and
mica caps with a 750 pf vacuum capacitor.
I suspect that with the tower you decribed for 80M, the tap would be somewhere
down below the top of the aluminum tower section. Unfortunately, that means
climbing and experimentally locating the 50 ohm tap point - unless you use the
two capacitor Omega match approach. Still I believe that this is a less
complicated approach than going through the work of insulating the base. A
second benefit is that the tower is grounded for static and lightning
protection.
73 Dennis W0JX
Milan, OH
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Scott K9MA
k9ma@sdellington.us
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