1. "The Problem" is almost never caused by "the worst thing that could
happen."
2. Humans will nearly always assume it's "the worst thing that could
happen."
While learning to fly an airplane many years ago, IP told me, "By far,
the reason for landing with the gear up is because you abandoned the
checklist and forgot to put the wheels down. It's almost never the
airplane's fault."
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
Tom N4LSJ wrote on 1/4/2023 6:31 AM:
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
<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
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