🚀 go-pugleaf

In article <875@oakhill.UUCP>, charlie@oakhill.UUCP (Charlie Thompson) writes:
> OK Larry, I'll make a guess at how the LC resonant circuit with
> stripline antenna works......
> 
> The circuit is activated by gaussing and degaussing a magnetically
> biased inductor.  When the inductor is magnetized (gaussed) the material
> that the inductor is formed from becomes *saturated* (i.e. its
> small signal inductance is reduced) if the inductance is reduced
> then the tuning for the tank circuit goes up (like out of resonance
> for the giant grid dip meter that forms the detector).  When a person
> carries the book through the RF field he does not *resonate*...provided
> the book has been legally deactivated.  To re-arm the book, when it
> is checked in, the little inductor is de-gaussed using a decaying
> AC sinewave just like your color TV uses.  The inductor will then
> be un-saturated and will allow LC resonance at the detection frequency.
> So how'd I do Larry?  Gee if nobody thought of this maybe I should
> patent it (fat chance now that's it's in netland!).

	You hit the nail right on the head!

	In one system that I have seen, the stripline antenna has four or so
(the actual number is not that important) pieces of metal along its length.
These pieces of metal have a fairly high magnetic remanance.
	When the sensor is "active" (i.e., capable of being detected), there
is little or no magnetism in the above pieces of metal.  As a result, the
stripline antenna and capacitor present an LC resonant circuit in the UHF
frequency range which can be detected by its RF power absorption.
	When the sensor is "inactive" (i.e., when the book is checked out),
a magnetizer upon which the book is placed causes a magnetic field to remain
in the pieces of metal.  Two mechanisms now come into effect (not being privy
to the sensor design calculations, I don't know which effect is predominant):

(1)	The magnetic field of the stripline antenna-inductor is saturated
	due to the permanent magnet, and its effective Q is severely reduced.

(2)	The UHF signal across the stripline antenna undergoes several
	phase-shifts as caused by the magnets, thereby severely reducing
	the amount of RF energy that can be absorbed by the effective LC
	network.

	The power absorption of the LC network is now so reduced by the
above methods that it is no longer detectable.
	I don't have the precise design specs of the system, but I would
guess the magnetic remanance to be measured in hours, or a few days at most.
	A book when returned is run through a demagnetizer to assure the
detector element is "active".
	The nice aspect of this system, is that the magnetic remanance is
sufficiently short in life such that ALL books return to an "active" state
in a few days.
	This system is decently immune to fraud.  The magnetizer uses a
capacitive discharge into an electromagnet, resulting in a pretty high
momentary magnetic field; a permanent magnet will not work to deactivate the
sensor.  Of course, one could rip out the sensor, but some libraries glue
and imbed the sensor in such a fashion as to make it non-trivial to remove.
The only assured method of defeating a sensor is to wrap the book tightly
in metal foil - but that's a little obvious while strolling out the library
door!

	Peter Wu, who sent me mail, also correctly guessed the "secret".

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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