An interesting piece of technology which has potential applications here is the Theremin. These are musical instruments which are played via the performer waving their hand around its two antennas. One antenna is responsible for frequency, and the other is for volume. It is also possible to get smaller single antenna versions, where volume is controlled by a knob. Theremins work by generating a variable audio tone using the heterodyne principle, in response to the performers hand movements vs. the positioning of the antenna. Heterodyning is the process of mixing two frequencies together to produce a new one. One frequency used is generated by a radio frequency oscillator (set below 500kHz to reduce interference), and the other stems from the frequency generated by the operator. The performers hand acts as the grounded plate (as the performer is grounded) of a variable capacitor in an inductance-capacitance circuit. This circuit makes up part of the second oscillator, which generates the user-determined frequency. It appears that this could be very useful for the detection of nearby devices, if not for one major problem: UAVs are not grounded. However, it seems that the principles behind this technology could still be of use here.
Lateral-line inspired UAV detection: Part 4