Topband: Gamma Match Tower Question

[Bruce Whitney]

Bill,
I sure wish Tom W8JI was still participating on this reflector because he is a true expert on many, many matters including this one. What a loss to all of us!
Anyway, it can make a big difference to the performance of a shunt fed tower to have "other" conductors hanging on the outside of it and connected to it in an unplanned way. You should look into the methods used to prevent tower resonance documented in various places including the W8JI website. One of those methods to "choke off" tower currents is to connect a conductor parallel to the tower and connected to it that is designed to become a parallel  circuit and thus a very high impedance at that frequency to RF current. 
Without going into it in more detail, my advice from someone that has operated a very effective omega matched tower on 160m for many years - is to reroute the feed cables and other rotor control and any other conductors inside the tower and make sure they are effectively grounded at the base of the tower. Then I would experiment with your shunt wire length (tap point) and spacing from the tower and check with a VNA for impedance readings that make sense for your matching network. If things don't seem right or you suspect the feed lines up the tower are having any significant effect, - which is unlikely - figure out how to measure current in them and do some bonding to minimize it. 
I'll send you a short PP I made a few years ago on my shunt tower system - off list. 
73, Bruce W8RA


Sent from my iPhone

> On Aug 9, 2016, at 4:01 PM, Bill Hein <bill.aa7xt at gmail.com> wrote:
> 
> I want to get back on Top Band this fall and am planning on Gamma Matching
> one of my free standing towers (and plant a few Beverages).
> 
> The tower I'm planning on using is an AN Wireless HD-70 (70 foot tall, wide
> base that tapers as it goes up) grounded tower topped with a 15 ft mast and
> a Force 12 Magnum 340 620 Yagi, the later should make a nice cap hat.
> 
> My question: I have two hardline coax cables running up the outside of the
> tower held in place by Andrew non-conductive clamps. Should I (a) move the
> cables inside of the tower, (b) ground the cables at the top of the tower
> (they are gounded at the base now), (c) do something else, (d) do both, or
> (e) leave it as is.
> 
> I am also planning on grounding all the parasitic elements on the big F12
> dual bander to the boom at the element center.
> 
> 73
> 
> BIll AA7XT
> Glade Park CO
> DM59pa
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband