Good points all.
Something else to note, even at 14V, running a K3 at 11-12 watts is a disaster.
Just because you're using "low power" doesn't mean that the amplifier isn't
distorting. IMD is also frequency sensitive. One frequency measurements tell you
nothing about IMD performance. At 30 watts output on 3.8 MHz my K3 IMD was 17dB
lower than at 14 MHz.
At 24 MHz the IMD at 10W was 29 dB worse than 30W! In all cases of frequency
or output power the IMD was worse at 10W output.
On 12/29/2015 5:00 PM, Bill Mader wrote:
Actually, IMD in the PA increases significantly as battery voltage drops!
For example, during Field Day a few years ago someone kicked off the
generator power to our SSB station which kept operating on the back-up
battery. Sometime later the CW captain stopped by asking if we had changed
anything. He could hear "splatter" from our 20m signal on his 20m rig.
This was not the case earlier in the day.
I checked everything and discovered the AC power issue. I turned the outlet
switch back to On and checked with the 20m CW operator. His reception had
immediately cleaned up! He no longer heard anything from 20 SSB the rest of
the (non-contest) event. BTW, we went on to take 4th place overall with
just six transmitters (2012).
When driving a KPA500, this is much less of an issue since the K3 runs
typically under 30 Watts for 500 Watts from the KPA500. NC0B has spoken
frequently, in less than glowing terms, about IMD issues in 12 VDC (nominal)
PA's.
I've seen a number of comments recently that some folks don't care if their
radios' TX IMD is high. That attitude is exemplified in many DX pileups. I
have raised my power supply's voltage in an effort to keep my TX IMD as low
as possible based on my reading and empirical evidence related above.
73, Bill, K8TE
Albuquerque DX Assn
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