Dave Platt wrote:
In article ,
Rick (W-A-one-R-K-T) wrote:


Are there any construction articles around anywhere that describe an
auto-tuner similar to the LDG and SGC offerings?

I need an auto-tuner that I can control from a laptop, and set frequencies
as needed (mostly for ALE). I can do the digital design and the software
in my sleep, for an embedded microprocessor that can control all of that,
but the heavy-lifting part of the antenna tuner (all the toroids and caps
and relays) is a bit beyond my capabilities.

If there are any construction articles that I can adapt to my needs that
would be a very big help.


There was a two-part (I think) article in QST a couple of years ago
which showed a high-power external ATU with onboard microprocessor
control.

Its design was rather different from the LDG/SGC/Sunair/what-have-you
microprocessor-controlled switched-cap/inductor pi/L tuner. It
uses air-variable capacitors and a roller inductor (in a fairly
typical T-network).

This was an interesting approach (and I believe parts of it are sold as
a kit or assembled device, too). It would be a good approach for a
one-off, but if are going to do more than one of them, implying that you
need to get all the parts new (as opposed to scrounging surplus), it
gets pretty expensive pretty quick. (fiddling around to work out
mechanical details to use what's in your junk box and at the surplus
store is fine, when you build one.. gets pretty tedious if you want to
build 8)

It also might not tune fast enough for some ALE type applications or
have sufficient resolution.

Consider a switched relay scheme with 8 relays for L and 8 for C..
you've got 256 possible values. With a standard air variable C driven
by a motor, you'd need some sort of reduction gearing to get the ability
to set the plates to better than 1 degree accuracy. You could use a
stepper or a dc motor with an encoder, but you're still limited in the
speed at which you can go from one setting to another (I'd guess a
couple seconds... fast compared to cranking knobs yourself, slow
compared to 10-20 milliseconds for a relay). The roller inductor has
reduction gearing essentially built in, but, again, you're not going to
be spinning it at 3600 RPM.. Probably more like 60 or 120 RPM (2 knob
turns per second is pretty quick). Vacuum variables are more like the
roller inductor (many turns to span the capacitance range). Jennings
does life testing with 50-75% stroke at 200-300 rpm... at that speed, a
cap that takes 10 turns from end to end could do 20-30 strokes per
minute, or a complete stroke in 3 seconds or so.


Eliminating the "try lots of combinations to achieve the lowest SWR"
part of the system will greatly simplify the RF part of the
implementation... you won't need to implement the voltage and current
detectors, phase comparators, etc. needed to permit the digital logic
to measure SWR, etc. The on-board digital logic might be something as
simple as a micro which can [1] run a set of three stepper or gearhead
motors to rotate the cap and inductor shafts, and [2] read out DC
voltages from a set of potentiometers connected to the shafts.


An ordinary pot hooked to the shaft probably won't do as a feedback
device. a) resolution issues, b) life... Typical panel pots have lives
in the million moves range, which seems like a lot, until you do
scanning 24/7 with a move every couple seconds.. say 20,000 moves/day
will burn through a million moves in less than 2 months. Their
resistance vs position will also change over life (you don't notice this
on a volume control). There are feedback pots designed for this kind of
duty that have much longer lives and have high quality resistive
elements and wipers, but they aren't cheap. You'd probably want to use
a shaft encoder (and incremental shaft encoders with 1000 pulses/rev are
easy to come by).


Any way you look at it, there's a fair amount of electromechanical
engineering that goes into making a reliable system like this (which is
why the commercial units are fairly expensive..It's not just because
they're connected to multi $100K plasma etchers.). In the classic
antenna tuner case, it's a bit easier, because you don't need high
quality initial position, just get close enough, and home in on the
optimum setting for the match.


A simple serial-port or USB protocol would then allow the PC to
determine the current settings of the reactances, and control the
motors to adjust them as needed for a new frequency. A second serial
or USB port would be used to communicate with whatever your rig is, to
either read out the frequencies being tuned, or to switch the rig to a
different frequency.

A similar system could be done with switched banks of inductors and
caps, in the usual power-of-two size hierarchy. In this case, the
micro would be just ordered to turn on or off the various relays used
to switch toroids and caps in and out of the series and shunt circuits.


Which is essentially what something like the LDG AT200PC is..