We've got a lash-up we're using in order to demonstrate lack of security in a particular technology (you'll have to read between the lines to determine the application in question).
In any event we're remotely detecting a coil being pulsed on-off at around 1200 Hz from a few feet away. It's a small coil without a lot of energy but the coil is being fed by a square wave so there are sharp edges for the changes in magnetic flux in the coil.
Currently we're using a ferrite rod with some turns of magnet wire for a reasonable directional antenna in series with an audio transformer wired backgrounds (to give us a gain of ~10) followed by a OP27 for a preamp and then followed by filters, a PLL and lots of other odds and ends to form the detector and recover the signal from a pretty noisy area of the spectrum (AC lines, etc).
For the purposes of our demo we'd like to show that these signals can be intercepted from a greater distance than a couple of feet (sort of a-la-tempest, though not that kind of application).
Any ideas on something more sensitive for the front-end? We'd like to move away from a technology we feel has significant risk down the road to something more secure (and management wants to see the risk before they abandon a significant investment).
Thanks, in advance, for any thoughts or input,