There must be at least two variable components on every antenna tuner.
This is because every impedance has two components, R and jX.

With a magloop one variable is the loop tuning capacitor. The other
variable is the diameter of the small coupling loop.

It so happens that the setting of the coupling loop diameter does not
change very much from one band to another. Users who never have to
touch it don't realise how lucky they are compared with other types of
tuner which have at least two knobs.

The main loop and coupling loop are like the primary and secondary
windings on an impedance matching transformer. The turns ratio cannot
be changed but the diameter of the coupling loop can be. The greater
its diameter the greater the coupling.

It so happens that for loops of typical dimensions the ratio of the
two loop diameters is approximately 5-to-1. Optimum setting does
change a little from one band to another as can be seen using program
MAGLOOP4 from website below.

The coupling loop is often screened inside a short length of coax line
because the designer saw it done that way in a magazine. Or he just
copied somebody else's without having any reason.

If there's any noise to be picked up it will be 25 times stronger on
the main loop and nobody bothers to screen THAT. What might be picked
up on the very small coupling loop is quite insignificant in
comparison.

And it's much easier to adjust the coupling between the two loops if
one of them is just a self-supporting length of wire no thicker in
principle than the diameter of the coaxial inner conductor.

Instead of reducing its diameter it can be rotated a little or bent
out of the plane of the main loop to have a similar effect. Or
squashed flatter to reduce the area enclosed.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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