How do I design an automatic antenna tuner with no moving parts?
 

How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor cotacts?

In message , Andrey
writes
How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor cotacts?



To make any tuner variable, you have to vary something (and this can
only be an L or a C).
If you don't vary them physically with moving parts, you have to vary
them electrically.
You can change C using Varicap diodes (where you vary a DC voltage.
You can change L by using inductors with cores which you can partially
saturate by varying a DC current.
Neither technique is really applicable to transmission.
Ian.


--

On Sat, 19 Feb 2005 21:43:57 -0800, "Andrey"
wrote:

How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor cotacts?

1.) Since you did not specify the power level, use varactor diodes and
inductors on a suitable core material and vary the core permeability
with an external magnet field, controlled by a variable DC-current.

2.) Alternatively, get rid of the antenna tuner by using resonant and
matched broadband antennas.

Paul OH3LWR

Here is a link to the original 1996 QST article on homebrewing a tuner
called the AT-11. It is now made by LDG Electronics. It uses relays to
switch inductive and capacitive elements in an L-network tuner
configuration.

http://www.3764.net/qst/1996-1999/au...tuner_at11.pdf

The article presents the complete schematic and theory of operation, but
there is no microprocessor source code included. The author does say that
you can but a programmed chip from him, but I don't know if that's still
available. Maybe someone in this group can suggest where to get source code
or a hex file of the object code.


Joe
W3JDR



"Paul Keinanen" wrote in message
...
On Sat, 19 Feb 2005 21:43:57 -0800, "Andrey"
wrote:

How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor
cotacts?

1.) Since you did not specify the power level, use varactor diodes and
inductors on a suitable core material and vary the core permeability
with an external magnet field, controlled by a variable DC-current.

2.) Alternatively, get rid of the antenna tuner by using resonant and
matched broadband antennas.

Paul OH3LWR

"Andrey" wrote in message
...
How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor
cotacts?

I'm not so sure that is really all that off the wall.

Most autotuners are pretty mindless ... you only need to get down to a
"reasonable" SWR, so you can use a finite number of LC circuits. Most
tuners count frequency (easy) and use that to look up where they were the
last time they were near that frequency, so they randomly try everything at
first, but then use their experience to get there quicker the next time. A
few actually look at the phase angle to figure out which way to go, but they
are in a minority and it isn't all that clear that they do much better than
just guessing.

Typically, relays are used in commercial autotuners. Latching relays are
simple, cheap, and require very little power. But I can see no reason why
one couldn't use PIN diodes if the "no moving parts" was a requirement. It
would ratchet up the price a bit, and probably also the power consumption,
so I can see why it wouldn't be a common commercial approach, but if the no
moving parts limitation is a hard requirement I can't see why it wouldn't
work.

I would also think about the "why" on the no moving parts. Something like
PIN diodes is going to mean constant power consumption, which in turn means
constant generated heat. It isn't obvious to me that this will result in
better reliability than latching relays, which are pretty darned good these
days as far as reliability is concerned. It will also end up (I think)
increasing the parts count quite a bit, which again will hurt the
reliability, to say nothing of the cost.

...

As others have noted, it's certainly possible to do with the only moving
parts being relays -- there are several commercial tuners on the market
in this category.

But it'll be awfully tough to replace the relays, if your intention is
to transmit. Relay contacts have very high impedance (low capacitance)
when off, and very low resistance when on. They'll handle very large
voltages and currents. And the impedance, both on and off, doesn't
change with applied signal level. There isn't any other component that
has all these characteristics, and some or all are required in a wide
range tuner. You might, by very careful analysis of the circuitry under
various load conditions, come up with some sort of switch (e.g., PIN
diode, VMOS FET, etc.) which can handle the voltages and currents at
each switch point, while having acceptable on resistance, off
capacitance, and linearity (non-variation with signal level). But it
won't be a simple task. And you'll probably still have to sacrifice some
efficiency and tuning range.

In a receive-only tuner, you still have the problem of getting adequate
on and off impedances, but you are relieved of the voltage and current
requirements.

Relays are just simply very hard to beat for some applications -- and
this is one of them.

Of course, you can try varying the L and/or C by a number of methods,
such as others suggested -- using saturable reactors for L and varicaps
for C. Again, though, you run into voltage and current limitations. And
these devices tend to be nonlinear -- that is, the L or C depends also
on instantaneous signal level. So when they encounter a large signal,
they create distortion which produces harmonics. This would be a fine
approach for a receive-only tuner, but not a good one if you want to use
it for transmitting.

So, that's how you do it. Good luck!

Roy Lewallen, W7EL

Andrey wrote:
How do I design an automatic antenna tuner with no moving parts?

This is a good question.

How to do it - without relays, motor driven capacitors and inductor cotacts?