College Physics (4th Edition)

Since the electric field oscillates vertically as the wave propagates, the magnetic field oscillates horizontally east-west as the wave propagates north from the transmitter. To detect the magnetic field of the wave, we need an area for the changing magnetic flux to pass through. As the magnitude of magnetic flux is $A~B~cos~\theta$, ideally we would like the angle between the area and the oscillating magnetic field to be zero. Thus, the receiver used to detect the magnetic field should be oriented so the area of the receiver faces east-west, so the magnetic field passes through it with an angle of zero.