Reg Edwards wrote:

It can now be seen why the conventional ATU cannot be used with
magloops. What is needed is something which searches for minimum loop
impedance, or zero phase angle, as the tuning capacitor is varied. It
then stops.

Any ideas?

There was an article in RadCom some years ago describing an auto-tuning
Top Band vertical, which used a simple inline phase detector and a
little DC tuning motor. The phase detector is nothing complicated - it's
very like a toroid-type SWR bridge rearranged components - but there
was a lot of practical information about what's necessary for RF
shielding and decoupling.

The whole thing should work equally well for a magloop, since the
electronics don't care what kind of antenna it actually is.

The article was by Tim Forrester, G4WIM, and was reprinted in G4LQI's
'HF Antenna Collection' book... my copy of which is unfortunately in
another country.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

There was an article in RadCom some years ago describing an
auto-tuning
Top Band vertical, which used a simple inline phase detector and a
little DC tuning motor.

==================================
Ian,

A magloop is an altogether different kettle of fish to a top-band
vertical. For a start, the Q of a magloop is in the order of 1000.
For a top-band vertical it may be about 50. For most antennas it is
about 10.

What I would like to know is has anybody ever made an automatic tuner
which works with a magloop. Or has manufactured one for sale?

By the way, thanks for the Teslar papers although I am unable to run
the programs.
----
Reg.

Reg Edwards wrote:

There was an article in RadCom some years ago describing an
auto-tuning
Top Band vertical, which used a simple inline phase detector and a
little DC tuning motor.

==================================
Ian,

A magloop is an altogether different kettle of fish to a top-band
vertical. For a start, the Q of a magloop is in the order of 1000.
For a top-band vertical it may be about 50. For most antennas it is
about 10.

Not fundamentally different - it only means the magloop tuning will be
more sensitive. The servo will still try to drive the system to
resonance at zero phase angle.

The overall gain around the servo loop will be the product of the
antenna Q, the mechanical gear ratios and the voltage gain in the
electronics. The last of these can be adjusted with a single pot. If you
have a higher antenna Q, you simply need less voltage gain.

What I would like to know is has anybody ever made an automatic tuner
which works with a magloop. Or has manufactured one for sale?

Don't know, never looked.

By the way, thanks for the Teslar papers although I am unable to run
the programs.

(I only downloaded the program for coupled inductors, but haven't
studied it yet. It ran OK, after having extracted all of its files into
a real directory; it won't run from inside the .zip 'directory'.)


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Not fundamentally different - it only means the magloop tuning will
be
more sensitive. The servo will still try to drive the system to
resonance at zero phase angle.
=================================

Ian, what slightly worries me is -

(1) The resistive component of antenna input impedance, as measured at
the input of the small coupling loop, when the main loop is even only
slightly off-resonant, is altogether different from 50 ohms but is not
included in the bridge balancing process. The diameter of the coupling
loop is fixed. Yet magnitude and phase adjustments react upon each
other as is experienced by a human operator with two variable
controls.

(2) The coupling between the two loops is very loose. We are trying to
adjust the main loop exactly to resonance via a means which is very
insensitive to its resonant condition. Direct voltage and current
sampling connections to the main loop itself are impossible.

(3) We can imagine a situation where the impedance phase-angle is zero
at the measuremnt point, and the green LEDs light up, but which does
not correspond to exact resonance in the main loop. And exact
resonance matters with a magloop.

(4) Because the system is trying to reduce a phase angle to zero in
the presence of two unknowns, instability can result. We can imagine
the system continuously hopping about trying to find the zero.

As you can see, I have difficulty in describing what I think happens
circuitwise. But I shall be convinced only when somebody produces
something which WORKS reliably without human intervention.

It may be possible but where is it?
----
Regards, Reg, G4FGQ

Reg, G4FGQ wrote:
"It may be possible but where is it?"

In the shortwave broadcast plant I worked in 50 years ago we had a 3.5
KW AM Raytheon "Autotune" transmitter we used to talk back to our
program relay transmitting station in another country. We called it our
"order-wire " transmitter. It or its twin were sometimes used for
broadcasting but it was low in power for that job.

This autotune transmitter had a rotary telephone dial on its panel for
programming its mode, operating drequency, etc. You could instruct it to
listen to instructions, then dial in A-3 for AM, followed by the
frequency you wanted it to operate on, such as 15,925, hit the go
button, then stand back and watch the knobs spin as it tuned itself up
completely. including putting the desired power into a dummy load. A
ready lamp informed you it was good to go on the air at the push of a
button. It worked like a charm.

Collins made autotune transmitters which are now military relics of
WW-2. I never toyed with one of those.

Best regards, Richard Harrison, KB5WZI

Reg Edwards wrote:
Not fundamentally different - it only means the magloop tuning will
be
more sensitive. The servo will still try to drive the system to
resonance at zero phase angle.
=================================

Ian, what slightly worries me is -

(1) The resistive component of antenna input impedance, as measured at
the input of the small coupling loop, when the main loop is even only
slightly off-resonant, is altogether different from 50 ohms but is not
included in the bridge balancing process. The diameter of the coupling
loop is fixed. Yet magnitude and phase adjustments react upon each
other as is experienced by a human operator with two variable
controls.

(2) The coupling between the two loops is very loose. We are trying to
adjust the main loop exactly to resonance via a means which is very
insensitive to its resonant condition. Direct voltage and current
sampling connections to the main loop itself are impossible.

(3) We can imagine a situation where the impedance phase-angle is zero
at the measuremnt point, and the green LEDs light up, but which does
not correspond to exact resonance in the main loop. And exact
resonance matters with a magloop.

(4) Because the system is trying to reduce a phase angle to zero in
the presence of two unknowns, instability can result. We can imagine
the system continuously hopping about trying to find the zero.

As you can see, I have difficulty in describing what I think happens
circuitwise. But I shall be convinced only when somebody produces
something which WORKS reliably without human intervention.

It may be possible but where is it?

I see your point... but what do you actually tune for, when you do it
manually?



--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White wrote
I see your point... but what do you actually tune for, when you do
it manually?

===============================
Frequency is selected by the receiver, not the transmitter.

The transmitter is OFF when I do it manually and I tune for maximum
noise in the receiver. How do YOU do it? smiley

If I can't do it when the transmitter is ON then neither can an
automatic ATU. It would have to be more clever than I am.
----
Regards, Reg, G4FGQ

Reg Edwards wrote:

Ian White wrote
I see your point... but what do you actually tune for, when you do
it manually?

===============================
Frequency is selected by the receiver, not the transmitter.

The transmitter is OFF when I do it manually and I tune for maximum
noise in the receiver. How do YOU do it? smiley

I freely admit, I've never touched the things... just trying to be
helpful :-)

But what do you think "maximum noise" means? You hope it's going to mean
maximum field strength when you come to transmit, but what does that
actually mean in terms of loop tuning conditions?


If I can't do it when the transmitter is ON then neither can an
automatic ATU. It would have to be more clever than I am.

If we're not clever enough to build an automatic ATU for a magloop, it's
a sign that there's something about magloops we still need to know...
not abandon the idea.

First of all, somebody needs to build a phase detector for an existing
manually tuned loop, and see what results it gives.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:
If I can't do it when the transmitter is ON then neither can an
automatic ATU. It would have to be more clever than I am.

It would be relatively easy to use the SWR meter driving
current from an MFJ-259 to control the ATU motor.
--
73, Cecil http://www.qsl.net/w5dxp

Reg, G4FGQ wrote:
"The transmitter is OFF when I do it manually and I tune for maximum
noise in the receiver."

As Reg said, reciprocity rules.

You can also satisfactotily tune an antenna to resonance by adjusting it
for maximum output. When I was in Tierra del Fuego we had Land Rovers
and boats equipped rith RCA single-sideband HF radios.

The Rovers had whip antennas atop fiberglass boxes containing loading
coils and motor-driven tap-changing switches. The coil base-loaded the
whip.

I routinely tuned the whips by switching my multimeter to the a-c range
(it used germanium rectifiers) and tuning for a maximum meter
indication. I chose meter-probe positions for a convenient scale
indication.

The above was accomplished with the transmitter ON and fed with a test
tone driving it to put out some power. The whip was mostly
non-directional so it made little difference where the r-f sensing
antenna was placed. The tuning procedure worked fine even with the
multimeter in the near-zone laying on the bonnet (hood) of the Rover.

The capacitor tuning a high-Q loop could also be tuned for maximum
output because that is your goal. The sampling antenna could be located
almost anywhere but probably not in a null of the loop.

Best regards, Richard Harrison, KB5WZI