[Elecraft] Homebrew Autotuner

[Jack Brindle]

Interesting idea, but it has problems. Directional couplers are  
designed around a known set of values for its components, minimizing  
the number of  variables in the circuit. One of the things that is  
normally fixed is the load impedance which the coupler "sees."  
Accuracy of the measurements is directly dependent on this, as can be  
seen from the math that applies to the circuit. By connecting the  
coupler to a non-deterministic load, as you suggest, it becomes at  
the least difficult to verify the accuracy of any resulting  
calculations, with results most likely being invalid.

This being the case, the only time that data would then be valid  
would be when the antenna matches the design characteristic impedance  
of the directional coupler, a condition that the tuner itself is  
supposed to create. Thus the directional coupler really needs to be  
on the other, traditional side of the tuner.

Note that on this side you still have phase and amplitude information  
that can be used for calculations. It can be difficult, of course, to  
take these measurements since they need to be sampled at HF, although  
this can be done as is seen in the days antenna analyzers. You can  
then apply the LC transformation, which is known, to more accurately  
calculate the antenna impedance for the frequency.

Having said that, the design of the tuner will directly affect the  
algorithms needed in the firmware. As an example, the traditional L  
network will need a dramatically different algorithm than a tank  
circuit. The L network tends to have a single point of match, while a  
tank circuit-based tuner may have several. Finding the best one for  
your needs is the real trick, and wholly depends on your definition  
of "best." Examples of this are lowest SWR, broadest bandwidth at  
match, fastest time to match, etc...

In either case, this isn't a trivial problem to solve, but it  
definitely is an interesting one to pursue from a technical view  
point. It is definitely something that, while seemingly simple, can  
consume "perfectionist engineers" (we know who we are...) for years. ;-)


On Sep 14, 2005, at 7:20 AM, John Magliacane wrote:

> --- Don <ehrlich at olypen.com> wrote:
>
>
>> 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.
>> ....
>> Any ideas?
>>
>
> Hi Don.
>
> Most tuners contain directional couplers to indicate VSWR, but they
> are often sandwiched between the transmitter and the tuning network.
> The tuning network is then adjusted for zero reflected power using
> a trial-and-error approach.
>
> If the directional coupler were instead placed between your tuning
> network and the antenna jack (the OUTPUT of the tuner), you could
> use the amplitude and phase information returned from the coupler
> to determine the complex impedance of the load (antenna) you wish
> to match to 50 ohms.
>
> Next, you could calculate what conjugate impedance the tuner needs
> to place in series with antenna jack to produce the 50 ohm resistive
> output impedance you desire, and then select the appropriate L and C
> values in your tuner network to produce that impedance.
>
> Done!
>
>
> 73, de John, KD2BD
>
>
> Visit John on the Web at:
>
>     http://kd2bd.ham.org/
> .
> .
> .
> .
>
>
>
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- Jack Brindle, W6FB
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