Polarization

The radiation from the klystron horn is polarized, that is the electric field oscillates in a fixed direction in space, perpendicular to the long dimension of the horn antenna. With the horn sitting on a table, this means that the electric field will be vertical. The receiving antenna and diode are also only sensitive to radiation with an electric field oscillating in a vertical plane, perpendicular to the long dimension of its horn. When the two are pointing at one another, they are matched for polarization. If you pick up the detector, however, and rotate it $90^\circ$ so that it is on its side but still pointing at the klystron horn, the signal should decrease dramatically because the radiation emitted by the klystron has the wrong polarization for the receiver. Try it.

Now suppose you were to rotate the receiver only $45^\circ$. How much does the signal change?

A conducting screen with parallel slots or wires closer than the wavelength of the microwaves will act to polarize the beam too. Set the detector so that it is pointing toward the source and adjust the attenuator for a signal that is nearly full scale on the meter. Insert the polarizer between the detector and the source and rotate the screen so that the wires are at first vertical and then horizontal. What are the orientations of the polarizer that produce the maximum and minimum signals? Does this make sense to you? Explain.

Measure the maximum signal and rotate the polarizer so that its wires make an angle of $\approx 45^\circ$ with the vertical. Measure the signal again. What fraction of the signal is detected in this case? With the polarizer still at $45^\circ$, rotate the detector horn and determine the polarization of the microwaves passing through the polarizer. Explain these observations. (Hint: Read about the Law of Malus.)