Well, I have had two requests for updates in one week. Guess I better say
something.
Early last summer I last posted on this subject asking for any ideas for
algorithms for autotuning. I received many replies, all very helpful.
Since then I have written the necessary code for autotuning and have been
using the tuner every day as my standard station tuner. My K2, my old
Valiant/SX28A, and homebrew gear all feed through it.
I have found it necessary to be able to place the tuning capacitor at the
input of the tuner (immediately after the balun) in some cases where I am
matching a low feedline impedance. Note that the balun I am using at the
input to the tuner is a 1:4 balun and so the tuner is matching the feedline
to a nominal 200 ohm source impedance. The capacitor switching is done with
a conventional two pole double throw RS relay and the optimum position of
this relay for each frequency is stored in permanent memory along with L-C
parameters and is switched automatically whenever changing operating
frequency.
I also added a second antenna output. This is an unbalanced output and it
is tuned also and in my station it feeds coax leading to an R7 vertical.
Switching between the two antennas is done via a pushbutton on the front
panel and the LCD displays which antenna is connected. This makes it
convenient to compare antennas with a touch of one switch .. and the
appropriate matching L-C is switched automatically depending on the antenna
chosen.
The only hardware problem I have had is the failure of one of my reed
relays. This occurred early just after putting the tuner into service. The
relay was one of those connected across an inductor and it was not 'making'
the circuit when closed. I am hoping it was a case of infant mortality and
not the start of a trend. I have had about 6 months of trouble free service
since then, usually at the 100 watt level. I have not attempted higher
power yet.
I still plan to repackage the tuner into a K2 case. That will be after the
holidays.
I am taking my time evaluating the tuner before starting to drill holes etc
in a nice new Elecraft K2 case. I want to be sure I really know where I
want every thing to be and how I want it to work because any modifications
later will be difficult and could make a mess of things cosmetically.
Don K7FJ
