How does AM radio passive signal boost antenna work?
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.