Electronic – How does AM radio passive signal boost antenna work

antenna

When I was a child I saw advertisements for an AM signal booster which used no batteries or electricity and was simply placed near an AM receiver and a knob turned to tune it. The ads said things like "turns a tiny transistor [radio] into a tiger!"

The pictures showed a flat round box about one foot high with the knob in the center. I saw one built with a long piece of wire on an X-shaped frame about a foot high, connected to a tuning capacitor from an old radio. How can this device boost signal strength with no energy input?

Best Answer

The booster comprises a high-Q tuned circuit in which the coil has a large loop area.

The large area allows it to capture more signal power than the radio's built-in antenna can, and the fact that it resonates with the signal allows a large current to build up in the coil.

Since the radio is primarily sensitive to the magnetic component of the radio signal anyway, it couples readily to the current flowing in the booster, producing a stronger output than it can on its own.

Also, the high selectivity of the booster helps to eliminate adjacent channel interference, further improving the perceived quality of the signal.