Amazingly, it is probably not the KPA500. We have been seeing quite a few inquiries about this. Analysis almost always shows an issue in the antenna system caused by high power.
There are a few things at play. When measuring SWR (or better, antenna S parameters) at low power, you will get the results of the low power measurements. These are usually quite good for determine antenna characteristics and generally tell you almost everything going on with the antenna. They do not tell you how the antenna handles high power. Antenna system components, including the feed line , connectors, wire and so forth, are designed to handle some level of power before they will fail in some way. As an example, spacing of the windings in loading coils matters. Too small, and the coil will arc at high power. The same is true with connectors, center insulators and the like. Many antennas have matching boxes at their feed point to properly match the feed line to the antenna. These have a rating, and many are amazingly low. One popular (and very good) commercial dipole antenna is rated for 1000 watts ICAS. The specifications later state that the antenna should not exceed 500 watts on CW. This is especially important when running digital modes such as FT8 which keep power applied to the antenna for long periods of time. As power is applied, the components will heat and will eventually fail. When they fail, probably not catastrophically, they will change the matching characteristics, and the amplifier will see a very high reflected power that will cause it to protect itself. When the user inspects the components (with power off), they will probably not see a problem - unless the component failed catastrophically, it will have cooled by then. The problem will repeat itself, of course. No measurement taken with an antenna analyzer or VNA will show the problem, because they work at very low power levels. When you select an antenna, the ratings must be taken into consideration. Manufacturers like to hide the power ratings (especially those with built-in matching systems) in order to get more sales. If you want to run high power, don’t use a miniature beam - they need lots of matching and are almost always designed to run at low power. The same is true for compromise wire antennas and those that require matching networks at their feed point. Also watch for compromise connectors and adapters. Right-angle UHF adapters are notorious for arcing. You can’t see it because it is inside the connector, but that arcing is probably the cause of a PA Dissipation fault in the amplifier. Make sure connections are tight - hand tight is not good enough. Use the best connectors you can get. The industry standard is Amphenol - they have very high quality standards. In short, make sure the antenna system you are using is designed to handle high power for doing periods of time. If not, it will fail, and when you inspect it you will wonder why. 73, Jack, W6FB > On Dec 30, 2022, at 12:46 PM, marv marvrossphoto.com <m...@marvrossphoto.com> > wrote: > > I have pretty much the same symptoms reported by the KPA1500 users here. The > higher the frequency the more likely to see a fault. 10M is the worst one. > The amp shows a 2:1 SWR and that is running it into a tuned KAT500, which > shows 1.2:1 and a Daiwa bridge showing 1.1:1 into the antenna. It's the same > on a dummy load. > > I don't understand Bob McGraw's logic as if it does require power, my Daiwa > is gonna see it. If I measure the feedline, lightning protection and antenna > with a network analyzer I will be doing it with mili Watts. > > Cheers, > > Marv > W0PSY > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to jackbrin...@me.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
