I experienced wide spaced air variables flash over at moderate to high power in my shunt feed omega match. I changed to vac variables. That solved the problem with the additional benefit of easier tuning via the vac cap's verniers.
GL - Steve WB6RSE > On Aug 27, 2020, at 10:58 AM, John Harden, D.M.D. <[email protected]> wrote: > > 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 _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
