Hello Dale, I had this same problem. In my case I discovered that it was the "Doorknob" capacitors heating up and changing value. I had installed some fixed value doorknobs to "pad" the variable cap in the matching circuit. Changed to Vacuum Caps, no more problems.
73 Lloyd - N9LB -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Dale Drake Sent: Thursday, August 27, 2020 12:06 PM To: [email protected] Subject: Topband: Shunt Fed Tower SWR Troubles 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
