[TowerTalk] Shunt Feeding the 80M Vertical

[K9MA]

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 at sdellington.us