[saul_goodman]

Forgetting the differences between TX and RX, a loop antenna is a transformer and works on a different principle from what most muggles think of as an antenna. The "radio" part of a wave is one half E-field, the other half of the wave is the H-field or the magnetic component of the wave. What you think of as frequency is the EMF field resonating between the E and H fields. An E-field antenna is what most people think of as a "normal antenna" and it resonates with the E-field.

However you can just as easily resonate and capture the H-field or magnetic component of an EMF signal. A loop antenna is half of a transformer and it freely resonates with EMF that it comes in contact with. The tuning capacitor if your AM radio tunes the loop to resonate with the H-field. Loop antennas have different characteristics from E-field antennas, one of which is the ability to be much more compact for mediumwave and shortwave signals. This is why loop-bar antennas are used for AM radios and not much else in everyday life.

http://www.emagtech.com/wiki/index.php/File:EM_wave.png