Alan B Pearce wrote:

> this sounds horrendously complex, ... when I disassembled a store tag.
> ... seemed to be just a tuned circuit,

  Two types seem to be common here.  The bulky disc which pins on to
either clothing, or appliances via a steel cable, is a simple tuned
circuit.  The bulk of the casing is required to prevent objects coming
close enough to significantly detune it.  A hand-held rectangle of
aluminium foil would completely disable it when wrapped over.  FWIW.

  The "label" type is a punched foil spiral inductor with a component
printed over from one end to the other.  I have in the past suspected
this component was non-linear (i.e., diode) and generated harmonic
energy, but I now suspect it is a simple capacitor and this is merely
another tuned circuit detected by the same equipment as the first.  I
always wondered whether the shop that used both tags used the same
equipment in both areas of surveillance.

  Presumably the detector is a varactor-swept GDO looking for a peak,
but I suppose it could be a time-domain system just as easily (pulse
echo).  The interesting question is whether either detection system
would have any advantage over the other.

  The magnetically-switched devices used in books, I presume to use
detuning by saturation of the magnetisable coating, rather than a
mechanical toggle.  The pin-on discs however retain the pin by a collet
requiring a very strong magnet and suitably-shaped pole pieces to pull
the collet open.

  Wag that I am, I've been meaning to embed the tuned circuit into my
pocketbook with a tiny switch to enable/ disable.  Kinda fun to trip the
thing every time one walks in, but never out.
--
  Cheers,
        Paul B.