Detecting EM Waves

Radio and TV waves are examples of electromagnetic waves. What kind of antenna do we need to detect EM waves? Usually an antenna detects either the oscillating electric fields or the oscillating magnetic fields. Remember that the energy in the wave is split equally between these two components of the wave.

Antennas are generally either conducting rods or conducting loops. Which would be used to detect the different fields?

  1. Conducting rods are good for detecting oscillating electric fields and conducting loops are good for detecting oscillating magnetic fields
  2. Conducting rods are good for detecting oscillating magnetic fields and conducting loops are good for detecting the oscillating electric fields
  3. You can use either type of antenna for either type of field - you just need to orient the antenna appropriately
















Choice 1 is correct

To detect the electric fields, use a conducting rod. The fields cause charges (generally electrons) to accelerate back and forth on the rod, creating a potential difference that oscillates at the frequency of the EM wave and with an amplitude proportional to the amplitude of the wave.

A particular plane EM wave is propagating in the x-direction with its electric field vectors parallel to the y-axis and its magnetic field vectors parallel to the z-axis. How should you orient your antenna (a conducting rod) to detect the electric field vectors?

  1. The antenna should be parallel to the x-axis
  2. The antenna should be parallel to the y-axis
  3. The antenna should be parallel to the z-axis















The electric field vectors are parallel to the y-axis so the antenna needs to be that way too.

Use a conducting loop to detect the oscillating magnetic fields. How should the loop be oriented to detect the magnetic fields in our example, where the field vectors are parallel to the z-axis?

  1. The plane of the loop should be parallel to the x-y plane
  2. The plane of the loop should be parallel to the x-z plane
  3. The plane of the loop should be parallel to the y-z plane















Orient the loop so the oscillating magnetic fields pass through the loop. If the magnetic field vectors are in the z-direction the plane of the loop should be parallel to the x-y plane. The changing magnetic flux induces an emf in the loop that matches the frequency and amplitude of the wave.