This may work out to be an optimization problem where you need to find the
gradient of the LC surface and descend down that gradient.
Imagine the SWR as the z axis of a basin. This basin is your attractor
with the best SWR at the bottom of the basin. The x axis is your choice
for L and the y axis is your choice of C in this tuner setup. The
simplest method is to find the gradient by choosing a few points at random
to begin to describe the character of the basin. Hopefully it is a simple
basin with no false minima. There are ways of determining how to walk
around them but that is a bit tougher than I want to explain right now.
Since the function of this basin is not known a priori you will need to
make the random choices I mentioned earlier. This can be used to perform
piecewise differentiation to determine the gradient of the steepest path.
Think of the gradient as the fall line a skier takes to go down the slope
in the fastest manner. If you can determine the gradient you can adjust
the L and C of your tuner in the direction indicated until you start to
increase the SWR. At that point you simply back up to the one before and
you're done.
It is almost my bedtime so write me a note if you think you want to try
this and I'll write out the mathematics of it in algorithmic form instead
of my little thought experiment. It truly can solve your problem in a
very short number of iterations. If you like you can determine the
gradient at every step to achieve the optimal solution. This will take a
little longer but your answer will be the best one you can get.
Sincerely,
Kevin Rock.
On Tue, 13 Sep 2005 20:16:52 -0700, Don <[EMAIL PROTECTED]> wrote:
A few months ago I maintained a brief correspondence with some on this
list with an interest in a homebrew balanced autotuner. The project is
nearing the point where I will be installing it into a K2 case. At the
moment, though, I am having trouble coming up with a software algorithm
that finds minimum swr and this message is a request for suggestions.
I have found that it is not as easy as I had imagined. I have been
using several methods to automatically find minimum swr and some work
pretty well most of the time but none work well *all* of the time. The
problem is that on some bands minimum swr occurs over a very narrow
range of C and L. With either C or L are set only a little off of these
optimum values any change in C or L will have *no* effect on measured
swr.
I have used complex algorithms that vary step size, vary C and L values
intelligently with either positive or negative changes in value. I have
used simple algorithms that stupidly go through an entire range of
possibilities of all values of L for every C and so forth (ugh!) ... and
have to believe there is someone out there in Elecraft-land a lot more
clever than I who can suggest ways to intelligently find best
combinations of L and C quickly using measured swr as the criteria.
It really does not matter a great deal if I never perfect the autotuning
feature of the tuner. It is easy to manually tune the tuner, store L
and C for every band segment in nonvolatile memory, and then let the
tuner automatically switch to the proper values when it senses the
frequency at the input. But I can't call it an autotuner if it doesn't
tune well automatically can I? Perfectionist that I am I would like to
make it autotune like it is supposed to.
Any ideas?
Don K7FJ
http://www.qsl.net/k7fj/
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