In the simple linear dipole antenna (like the typical antenna on a car or rabbit
ID: 1441000 • Letter: I
Question
In the simple linear dipole antenna (like the typical antenna on a car or rabbit ears on a TV set) the alternating electric field in the electromagnetic wave sent out by the broadcasting station moves the electrons back and forth along the antenna's length. But a radio or TV receiving antenna can also be made from a loop. Explain how this works.
As a hint, note the two orientations of the antennas in the picture. For the orientation which provides good reception, the plane of the loop is the same as the one that contains the broadcast antenna. In the diagram below, this is the plane of the page. For the orientation which provides poor reception, the plane of the loop is perpendicular to the page.
Explanation / Answer
Let's simplify the dipole to a wire with a split in the middle.
If the wire is oriented in the same direction as the transmitting antenna, you get the optimal coupling between the propagating electric field and the receiving antenna.
This is because the electric field lies along the direction of the alternating current (changing dipole moment) in the transmitting antenna.
Now since this is a propagating EM wave the magentic field is at right angles to the electric field. You can use a loop of wire to let the oscillating magnetic field induce a current and receive the signal that way.
However since the magnetic field is perpendicular to the electric field in an EM wave, and the magnetic field must pass through the area of the circle made by the loop, you have to turn the loop so that the plane made by the loop is parallel to the electric field.
That is, if the electric field is along the x-axis and the wave propagates in the z direction, the plane of the loop has to lie in the x-z plane.