[Elecraft] K3/KPA500/KAT500 SWR problem

[Jack Brindle]

That’s actually not a valid test. Try it at 500 watts and see what happens. We get away with many things at low power that show up as real problems when the power is increased. Bad connections, coax and components that are not rated for higher power (baluns tend to be a big one here), and sometime antenna parts that are just too close to their surroundings. Watch for include tight bends in the coax (especially in warm climates).  As antennas age, things tend to change, components age and start to break down. 

Remember, with a non-resonant antenna you can have very high mismatches, shown as high SWR, which results in high voltages and currents in the feed line going back to the tuner. At high power this will age the feed line and components very fast, causing breakdowns. Instead of small coax, try using large stuff, of the RG-8 size (only better). It will still see the high voltages, but is better suited to handle them.

Perhaps a better solution is to use open-wire feedline from the antenna to a point very close to the shack (or inside), then use ashore coax feed and balun between the ATU and feedline. This should work much better, since the open-wire feed is better suited for handling high SWR with very low loss.

Jack Brindle, W6FB


On Dec 30, 2013, at 11:13 AM, Ray Coles <raycoles96 at gmail.com> wrote:

> Forgot to mention that I hauled my MFJ wattmeter/dummy load out to the bush and pushed up 100 watts into it down the feeder with 1:1 SWR, which convinced me that at least my UR8 feeder is OK. Also didn't mention I hooked up my FG01 antenna analyser up at the shack and all seemed normal, but of course the problem only comes over 40 watts (which equates to a pre balun 500V). THE PROBLEM REMAINS and I am still mystified!
> 73
> 
> Sent from my iPhone
> Ray Coles CEng M0XDL
> 10 Littlemoor Road,
> Weymouth DT3 6AA
> United Kingdom
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