I am wondering if the air variable is flashing over. I am willing to bet that a vacuum variable would correct the problem. I use two (2) Vacuum variables in an Omega match.
73, John, W4NU Sent from my iPhone > On Aug 27, 2020, at 1:38 PM, Guy Olinger K2AV <[email protected]> wrote: > > Hi Dale, > > The RF voltage is always looking at the far end (stuff on the end away from > the tower base) to jump onto. > > *** Provide a deliberate, designed, long-term weather survivable metallic > path to the boom of the highest yagi and provide a low R&X path from the > boom to BOTH sides of at least one element at either END of the boom. This > will drastically reduce current via unintended paths. > > You will have to retune the match. > > This is much easier for a parasitic element, where the center of an > insulated element can be just strapped to the boom. But you should also > replace the balun. It will remain a source of RF loss, even if it doesn't > change value or burn up. > > RF current heating of a "balun" comes about because an engineer is only > designing the balun for the performance at 40 or 20 meters. The R&X of such > a device on 1.8 MHz can be quite low. I have measured one that worked FB on > a tribander that was only 173 ohms on 160. That will get very hot at power > and its effect on the tuning at the tower base can be enormous. And it is a > guaranteed source of RF loss that throws away dB's of your amp's power. RF > heating of a ferrite device gets to a point where the R&X characteristics > suddenly fall off the table. 80 meters likely just doesn't quite get to > that temperature because the R&X of the balun is higher on 80m > > You still may need to change that device for something with good stiff R&X > at 160, so it will not heat up and change. The 160 RF will still try to go > there. It just needs to be solidly blocked. See the Balun Designs 1116dx (x > is one of 6 or 7 hardware configurations, same innards). Or go to K9YC's > web site to make your own. Jim has some recent designs, excellent R&X for > 160 based on a new monster #31 ferrite core & RG400. > > Good luck, 73, and stay safe from that virus, > > Guy K2AV > >> On Thu, Aug 27, 2020 at 1:06 PM Dale Drake <[email protected]> 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 _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
