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 <[EMAIL PROTECTED]> 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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