I got a KPA500 this summer and it was doing a lot of faulting. I had to
"correct" a few things that were just fine at 100 watts.
1: I discovered that a solder joint in the UNUN had gone bad, and fixed
that. That solved most of the faults.
2: With still the occasional fault, I looked again and discovered there
was a spot on one of the windings in my UNUN that had been scraped free
of enamel. I applied insulation.
3: After several months of great operation, my receiver had started to
"cut out" after transmitting. At first, I thought my rig was going
kaput, but it was an inline SO-239 barrell that had gotten heated up
because of no longer being a tight fit. Replacing that cured it.
Moral: It's almost always the antenna system, somewhere.
On 12/30/22 15:20, Jack Brindle via Elecraft wrote:
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 <[email protected]>
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
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