E-11: Electromagnetic Radiation

OBJECTIVES:

To study microwave electromagnetic radiation; to measure the wavelength, $\lambda$, in air and in a wave guide; to observe reflection, refraction, polarization, interference and diffraction.

NOTE: If this experiment precedes L-1 and L-8, first read appropriate material on diffraction, interference, polarization, etc.

APPARATUS:

Klystron oscillator kit with wave guide and manual; horns, detector and various auxiliary equipment.

INTRODUCTION:

Our source of microwaves is a klystron amplifier and oscillator which is a device where electrons can produce oscillations at frequencies large compared to those possible from lumped LC components. High velocity electrons pass through a gap in an electromagnetic cavity. Their electric fields excite resonate cavity oscillations whose electric fields then tend to bunch the electrons at the resonate frequency. (See Fig. 1). These velocity modulated bunches in passing through gaps in a similar resonator excite the second cavity.

Figure 1: Schematic of klystron amplifier.

If part of this generated power feeds back into the first cavity, microwave oscillations can result. A reflex klystron oscillator (Fig. 2) uses a negative repeller to return the original velocity modulated electrons back through the first resonating cavity.

Figure 2

SUGGESTED EXPERIMENTS:

1. Follow instruction manual to assemble and operate the equipment.
2. To measure $\lambda$ in air and in the wave guide, set up standing waves by placing a metal plate reflector about 30-40 cm in front of the transmitting horn.
3. Note reflection by a sheet of metal at various angles.
4. Place the metal plate with a 5 cm slot in front of the transmitting horn. Measure the diffraction pattern at a distance of about two meters from the slot. Compare with theory.
5. Make a pair of slots one cm wide and 6 cm apart in front of the transmitter. Measure the interference pattern two meters away.
6. Use the wire grill to study the polarization of the radiation.
   

   Describe and discuss what you observe.

7. Check the refraction of the radiation by a piece of plastic.
8. Study the Bragg reflection from arrays of bearing balls imbedded in plastic foam.
9. Study the transmission of the radiation through wood, plastic, metal, metal screen, sponge, wet sponge, etc.