The balun heating hypothesis could be tested by disconnecting the balun. Dave, KH6AQ
On Thu, Aug 27, 2020 at 7:06 AM Dale Drake <daleaa...@gmail.com> wrote: > Here's my problem. I'm experiencing SWR problems > with 160M shunt feed on my tower. Using a gamma > match with HV air variable caps I am able adjust > the tap point to get a very nice SWR curve using > my AA-35 Zoom analyzer. The problem I having is > that when transmitting the SWR goes up with power > and if I key-down for very long the SWR rises > rapidly and takes off. > > My set up is 70ft Rohn 25 with about 22 feet of > mast above the top of the tower. At the top of > the mast is a Diamond 2M/440 vertical. At 15 feet > above the tower top is an XM-240 and 2 feet above > the tower top is a 4el SteppIR. The elements of > the SteppIR are fully retracted. All of the coax > and control lines are run from ground level inside > the tower up to the service loop. On the mast > above the tower the coax is tywrapped to the mast. > There are 60 ground radials with an average length > of 90ft. The reflector of the XM-240 is insulated > from the boom and the driven element is fed > through a Comtek 1:1 balun. All of the coax > shields are grounded at the tower base. > > I have an 80M gamma matched shunt on this tower > that plays FB with no weird SWR stuff going on. > > I suspect that what is causing my 160M trouble is > that RF is coupling through the 40M coax, through > the balun and into the driven element. Somehow the > coupling changes with power, which I don't > understand. I suspect that when I key-down and > the SWR takes off, the core in the balun is > getting hot and the magnetic properties are > shifting until it cools off and returns to normal. > > My plan to try to correct this problem is to > connect the center of the XM-240 reflector to the > boom and to mount a box at the XM-240 feed point > with 2 vacuum relays that I will use to connect > the driven element to the boom when I'm on160. > This will be a considerable effort and expense so > I'm looking for the group's input as to the > soundness of my plan or if there may be other > approaches that would be less difficult to try > first. > > Thanks, > > Dale, AA1QD > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
