[Elecraft] Homebrew Autotuner

[Kevin Rock]

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 <ehrlich at olypen.com> 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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