Interesting post, Tom. From the youtube videos that I saw a year or so ago
I thought that LDMOS were nearly indestructible. Transmitting into no
antenna, a broken antenna or a wrong antenna will eventually occur so
obviously commercial amps will need some very good protection methods. I
recall making antenna selection mistakes years ago with older design tube
amp with no protection and it did not miss a beat.
For expeditions, especially with the newer airline luggage fees, solid
state amps are ideal. I will probably eventually purchase the Elecraft
KPA-1500 to match my K3S. For a home station where weight and size are not
an issue, legal limit tube amps are much less money and perform quite
well. I hope that tubes remain available for those using them.
John KK9A
To: "'Ria Jairam'"
Subject: Re: [CQ-Contest] Solid State Amps in Multi-Multi Stations?
From: "Tom Georgens"
Date: Thu, 28 Dec 2017 10:54:03 -0800
As I said, I am a believer, but the tradeoffs are more real than you allow
While the LDMOS are more rugged than the MRF150 style parts, and a I have
a jar
full of 150s to show for it, they are still not as tolerant of faults as
tubes.
The 65:1 SWR metric is not under CW conditions. Likewise the open/short You
Tube videos are not CW either. 3:1 is likely too high for most fully
available SS AMPs, not just for the transistors but the filters and matching
networks as well. Beyond survivability, you will see big variations in
output
when you switch between antennas with 1:1 and 3:1 SWRs. You are correct
on the
issues of the filters. I have had more filter failures than RF module
failures, with the most common component failure being the caps. If you are
going to build single band amps, use heavy duty power caps. With today’s
available SS amps with switched filters, and smaller parts, 3:1 SWR would be
pushing it.
Protection circuitry is important, but exhaustingly testing the many fault
scenarios is a time consuming and expensive process where pure economics
preclude completeness. A vendor with significant volume outside of the ham
market will be more likely to fund this activity. However, nothing will be
foolproof given the inherent small geometries of the LDMOS parts. Also,
there
are surely failure modes that need to be protected with analog hardware since
microprocessor response times will be too slow.
Using multiple parts improves IMD, but lowers efficiency. The dynamic VDD
modulation from 4O3A sounds like an interesting way to keep the efficiency
up.
I am curious to see how effective this is.
Overall, my approach to this may be conservative, but this is what it took to
stabilize the amps in my environment. All antennas have SWR’s below 1:5:1.
This helps with reliability and minimizes differing power levels when
switching
antennas on the same band. Lots of airflow and temperature monitoring with
automated shutoff. All antennas selection is automated and interlocked with
PTT to keep from changing while transmitting. Manual antennas selection will
surely result in occasional errors. With all of that in place, antenna,
coax,
and switch failures can still occur. I protect the amp the best I can
hope for
the best. This is the risk I take for the operating efficiency they create.
I am excited by some of the value add being brought to market on top of the
traditional designs. 4O3A’s team is doing some interesting work and I expect
some good work from Elecraft. K3LR will team with top engineers and produce
top notch work that we can all benefit from. SS Amps are clearly the future,
but there is a lot of overpromising going on now and people with marginal
installations will experience failures and be disappointed.
Tom
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