In radio sets, the problem of not receiving signals in certain directions is felt in the medium wave band (530-1620 kHz). This is because for this band they use a coil wound on a ferrite rod as the antenna.
Ferrite rod is directional in its sensitivity. It picks up all signals that come in a direction perpendicular to its axis and rejects them if they come parallel to its axis. Therefore the reception is entirely based on the direction in which they reach the radio set.
Radio receivers use an inbuilt loop antenna for receiving radio frequency (RF) signals, which are nothing but electromagnetic signals, broadcast by transmitting stations. Some sets use telescopic antenna to receive frequency modulated (FM) signals.
Normally the dimensions of the loop are chosen to be comparable to the wavelength of the signals that will be received by the set. The dimensions of the loop are such that the current generated by the signal in the loop are of same magnitude and phase throughout the loop. If the phase of the loop is set at right angles (perpendicular) to the direction from which the vertically polarized waves are coming, then currents are induced in each of the vertical side antennas.
But these two antennas send currents around the loop in the opposite directions; hence, they cancel out each other as both the currents are equal in magnitude and opposite in phase. It is because during normal position of the loop antenna plane with respect to the incoming waves, both the sides are equidistant from the transmitter signal and there is no induced EMF. If the loop is rotated by 90 degrees, the plane of the loop will be along the direction of incoming radio waves, and so the voltages induced in the vertical side of the loop are not cancelled.
Because the distance involved between the vertical sides is equal to the width of the loop the voltage is maximum when the plane of the loop is along the direction of the incoming radio waves. External antennae help reduce this directional effect to a certain extent.