[majoconz]

By 'passive' do you mean 'unamplified' or 'untuned'. Tuned loops are easy to make, perform really well - but can hardly be call 'unobtrusive'. Here's one that I made in an effort to tune down to the NDB band - 200 to 400kHz - but did not achieve it - even with a 3 gang capacitor all in parallel. Each loop arm is a metre long. The inner two turns are a low impedance coupling loop to a receiver - all the other turns can be connected in various ratios to tune the required frequency. Works brilliantly on the AM MW band with very deep nulls.