Does any one know what is inside these tuners as I would like to build
one for my Jeep here in Australia

SGC is probably the best-known brand of these tuners (although I
understand that they did not create the basic design). If you go to
http://www.sgcworld.com/PubInfoPage.html you can download the SGC-237
manual, which has a complete schematic.

The impedance-matching network in these tuners is a pi network, with
two switchable sets of capacitors making up the input and output shunt
legs, and a set of switchable inductors making the series (top) leg of
the pi. The capacitive and inductive legs are each composed of a
group of components whose values are generally in something akin to a
power-of-two relationship (e.g. 25, 50, 100, 200 pF), with individual
relays to connect or disconnect each individual component. The relays
are controlled by a microprocessor. By switching in/out the
individual components, the microcontroller can set each of the three
legs of the pi network to a reactance which can be varied over a
fairly wide range.

The incoming RF signal from the radio goes through current / voltage /
phase detector circuits that then feed into the microcontroller.

During a tuning operation, the microcontroller can use the
voltage/current relationship, and the signal phase, to determine
whether the (transformed) feedpoint impedance is above or below 50
ohms, and whether its residual reactance is inductive or capacitive.
The microcontroller then adjusts one or more of the three legs of the
matching network, re-measures, and tries again.

This sort of tuner seems to have several quirks or limitations.
Programming the search algorithm to find the right match (or *a*
correct match - often, many are possible) apparently isn't easy. The
SGC tuner I have seems to be easily "confused" if the radio varies its
power during the tuning process (e.g. as part of a high-SWR power
foldback).

Frankly, I wouldn't recommend a project like this as a one-off
hobbyist project, even for a real enthusiast. I can see it becoming
an incredible time-and-effort-sink.

A somewhat similar tuner, which would be much easier to build, would
be one which implements a "pi" or "T" tuning network using
motor-driven air-variable capacitors and a relay-switched multi-tap
inductor (or perhaps a motor-driven roller inductor). You wouldn't
get the very rapid bandswitching of an automated ATU, but would
probably have lower losses, much less physical complexity, and a
better chance of getting (and keeping) it working.

A "tuner plus whip" setup for a vehicle is often very electrically
inefficient, at least on the lower-frequency HF bands - you can end up
with most of your transmitter power being lost as heat in the tuner,
due to the very high circulating currents flowing between the pi-match
inductance and the very-capacitive impedance of the whip. My
understanding is that a whip/tuner setup is often a couple of
S-units poorer than a decent screwdriver antenna, and even worse than
that compared to a well-designed-and-built single-band "bugcatcher".

On the "plus" side, though, you do get a lot of physical ruggedness,
since the whip itself is sturdy and easily replaceable if it has a
serious disagreement with a tree branch.


--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!