David Cutter wrote:
>
> Is there a "sweet spot" to which one can adjust the power output for
> minimum IMD, if so, how would you do it easily? I presume you would need
> another rx or spectrum analyser. And I bet it changes with antenna
> reflections, ie from one end of the band to the other.
>
> I dare say a lot of K3s are followed by linears which add even more IMD,
> so, it might be tricky to optimise for the station as a whole and not just
> at minimum drive from the K3; perhaps a power pad is needed, but who would
> bother ?
>
> It is hoped that the Elecraft linear might take this into account.
>
>
TX IMD just goes on getting worse as you increase the power. It starts
getting worse much more quickly as you approach and then exceed the maximum
rated power. So your "sweet spot" would be "as little power as possible." :)
The issue with the K3 is that the transmitter does not seem to be as clean
as is possible with the "state of the sart", even given the restrictions
imposed by having a 13.8VDC supply. I am only able to make PSK31 IMD
measurements which are not directly comparable with two tone measurements
usually used for SSB transmitters, but I assume that they both reflect upon
the TX amplifier linearity, or lack of it.
My K3 goes from -30dB at 2W to -22dB at 12W.
My K2 goes from -29dB at 2W to -27dB at 14W (both measured on 30m).
Therefore it seems to me that my K2 has a cleaner transmitter (at least as
far as IMD goes) than my K3, which I find surprising since the K2 was built
my me nearly 10 years ago using only the barest minimum of test equipment,
whereas the K3 low power PA module was aligned and tested by Elecraft with a
lab full of equipment.
It seems to be the way on this reflector that anyone who makes any criticism
of anything Elecraft makes comes in for a lot of abuse and ridicule. But I
wonder whether those who claim the TX IMD of the K3 is poor actually have a
valid point, but their K3s are just not as well set up as those belonging to
people who don't see anything wrong with them?
It seems unlikely to me that Elecraft would deliberately choose a worse
design of PA for the K3 than that used successfully for years in the K2. Yet
the figures I just gave seem to suggest that considerably better performance
is possible. Obviously, my test results are for the QRP version. But if the
signal going in to the 10W PA is not as good as it could be, the output is
certainly not going to be any better.
-----
Julian, G4ILO. K2 #392 K3 #222.
http://www.g4ilo.com/ G4ILO's Shack http://www.ham-directory.com/ Ham
Directory http://www.g4ilo.com/kcomm.html KComm for Elecraft K2 and K3
--
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