[Elecraft] KAT 500 SWR FAULTS [INTERMITTENT]

[Jim Brown]

I agree with Dave. The problem is very likely somewhere in your antenna 
system, including the feedlines, matching stuff, etc. FWIW, I've been 
using KAT500s with KPA500s since they were in beta, and we've used a 
pair of them in W6GJB's contesting trailer for almost that long, and 
we've never had the sort of issues you describe.

A LOT of antenna system problems like what you're describing are caused 
by JUNK connectors. If it doesn't say Amphenol or is a MIL-surplus 
connector stamped with a part number, it's junk. These no-name 
connectors have all sorts of issues -- dissimilar metals, out of 
tolerance dimensions, crummy dielectrics. I've been a ham since 1955, 
but was off the air for long periods running my own small engineering 
biz. When I got back on the air in 2003, I bought a bunch of adapters, 
like inter-series, Tees, elbows, barrels. Over the next ten years, they 
caused me a lot of problems that were tricky to find. Several were 
exactly the sort of things you're describing.

A more fundamental question is, why are you not using resonant dipoles 
in those trees? A 20-15-10 fan is easy to build and needs only 34 ft 
horizontal. An 80-40 fan also loads on 15M. There's also a very nice 
design using a pair of loading coils that resonates on two harmonically 
related bands, like 80 and 40 or 40 and 20. The loading coils 
approximately at what would be the end of the shorter dipole, with a 
fairly short length of wire beyond the coils, so would fit in about 80 
ft. If you decide you want to try one of those 2-band antennas, I'll 
work up a design for you.

There's another very important reason for using resonant half-waves like 
this -- receive noise. With an antenna that's matched to a coaxial 
feedline, we can add a serious common mode choke at the feedpoint to 
kill the noise that's picked up on the feedline and coupled to the 
antenna and then to the receiver. Where most of us live, there's a lot 
of noise that limits what we can work.

73, Jim K9YC

On 7/19/2025 11:52 AM, David Gilbert via Elecraft wrote:
> You are making some very flawed assumptions here.  The very fact that 
> both KAT500's are showing the exact same thing should have told you that 
> they are not the likely problem.  Having "new coax" doesn't mean a thing 
> since there are several other possibilities ... connectors, the antenna 
> itself (including the ladder line or the connection to it), or even your 
> new coax.  All of those could be fine at low power and yet be 
> problematic at higher power.  In fact, if the problem showed up shortly 
> after you installed the new coax I'd bet $100 that the amps aren't the 
> source.