[RFI] Keys, noise, resonance

[KD7JYK DM09]

> If memory serves me correct, playing around with an ultrasonic
> receiver years ago I noticed that car keys gave off quite a bit of
> 'noise' (ringing and jangling) when shaken

It took me years to figure out that some keys, in a certain order on a 
ring, would trigger some EAS (electronic article surveillance) systems, 
as well as trigger some demagnetizers at checkouts.  A real PITA.

Kurt