The Properties of Microwaves 98

The Properties of Microwaves©98

Experiment 8

Objective: To observe and measure reflection, absorption, polarization, and interference of microwaves.

DISCUSSION:

Electromagnetic radiation occurs whenever charged bodies are caused to oscillate. If the oscillation is simple harmonic, the radiation is in the form of electromagnetic waves that have definite frequencies, wavelengths and amplitudes. The character of the radiation, as we perceive it, is determined by the frequency. For example, a frequency of 60 Hz is associated with the radiation from the wires carrying commercially generated alternating currents. Frequencies from a few tenths of a megahertz to several hundred megahertz are used in commercial radio and television. Frequencies of about 1014 Hz are associated with visible light.

The frequency to be used in this experiment is about 1010 Hz and produces waves whose wavelength is about 3 cm. This wavelength is characteristic of that class of electromagnetic radiation called microwaves. Like all frequencies of electromagnetic radiation, these waves may be reflected, absorbed, polarized, and experience interference. Each of these phenomena is to be examined in this laboratory:

1. Reflection (Fig. 1) c

When a plane wave is reflected from a surface, the angle i at u

which the incident wave Ii strikes the reflecting surface equals g

i  t the angle r at which the reflected wave Ir leaves the surface. As r c

f shown in Fig. 1, the angles of incidence and reflection are Ir e

R measured relative to the normal to the surface. Figure 1 2. Absorption (Fig. 2) Radiation incident upon a medium may be partially I Absorbing reflected (Ir), partially transmitted through the medium (It), i Medium and partially absorbed by the medium (Ia). The intensity of the absorbed radiation may be measured indirectly by measuring the incident radiation intensity I and subtracting i Ir from it the sum of the reflected and transmitted radiation It intensities. This is expressed mathematically in terms of Figure 2 the incident radiation Ir which has been measured directly. I  I  I  I (1) i r t a

3. Polarization (Fig. 3) In an electromagnetic wave, the fluctuating electric and magnetic field vectors lie in a plane perpendicular to the direction of propagation. Ordinarily, during the course of

8-1 several cycles of oscillation, the electric and magnetic fields point in any and all directions in this plane, with the electric vector and magnetic vector at any instant at right angles to one another. If all directions of the electric field except one are suppressed, then the wave is linearly polarized. Whether or not a wave is linearly polarized may be determined by allowing the wave to pass through a medium that absorbs or reflects all electric fields except those pointing in a given direction, creating a plane of polarization. In Fig. 3, the plane of polarization is the x-z plane, meaning that all the electric vectors of the wave lie in that plane. If a wave is linearly polarized and if the medium is oriented so that the wave passes unhindered through the medium, a rotation of the medium about an axis along the direction of

A: The electric part of an E&M wave. B: The magnetic part of an E&M wave. X-axis X-axis

Z-axis Z-axis

Y-axis Electric Vector Y-axis Magnetic Vector

C: A linearly polarized (in the x-z plane)E&M wave. X-axis

Z-axis

Y-axis

Figure 3: Linear polarization of an E&M wave. propagation, through an angle of 90°, prevents the passage of the wave.

4. Interference (Fig. 4) Whenever a sinusoidal wave traveling to the right is superimposed upon a sinusoidal wave traveling to the left, each with the same amplitude, frequency and speed, interference of the two waves occurs in such a way as to set up what are known as standing waves. A schematic representation of a standing wave is shown in Fig. 4. At a node, the vibrating medium (in this case, the electromagnetic field) remains fixed. Between the nodes are loops, at the center of which the medium oscillates sinusoidally with a maximum amplitude. The maximum amplitude points are called antinodes. The wavelength of the standing wave is twice the distance d from one node to the next.   2d (2)

8-2 To set up standing waves in this experiment, we place a reflector opposite the generator. The waves reflected from the reflector have the same frequency and wavelength, and nearly the same amplitude, as do the waves incident on the reflector. But because they move in opposite direction, they interfere with the incident waves so as to produce standing waves.

The detector is too large to place in the region of the standing wave. We therefore place a very small reflector, called a probe, in the region of the standing wave to reflect some of the intensity in the standing wave to the detector, which is aligned at a right angle to the line connecting the generator and probe. This probe is small enough so that it does not seriously upset the conditions which produce the standing wave.

If the probe is placed at a node of the standing wave, little or no intensity is reflected to the detector. If the probe is placed at an antinode, a maximum amount of intensity is reflected to the detector. Moving the probe therefore allows the measurement of the wavelength if the wave since this motion detects the positions of the nodes and antinodes of the standing wave.

For best results, the probe and the detector are held in a fixed position and the standing wave is adjusted by moving the reflector in or out in a precise manner. Because there is always a node at the reflector, the standing wave is shifted a distance equal to the distance through which the reflector is moved.

EXERCISES:

1. Reflection a. Arrange the apparatus as shown in Fig. 5, so that the angle subtended at the reflector by the generator and detector is 90°. The reflector consists of a Detector metallic sheet. Reflector b. Rotate the reflector to find that orientation at which the power received by the detector is a maximum. c. Determine whether the angle of incidence of the Generator wave is equal to the angle of reflection. d. Repeat this experiment for angles other than 90°. Figure 5: Reflection of microwaves.

2. Absorption a. Arrange the apparatus as shown in Fig. 6. In this arrangement, with no absorbers intervening, the detector receives as much energy as possible Generator Detector from the generator. The reading of the Place various absorbers here intensity Ii received by the detector from the generator may be changed by Figure 6: Absorption of microwaves.

8-3 adjusting the ‘gain control’ of the detector. Set this reading to 100 units. This value now corresponds to the incident intensity Ii as seen in Fig. 2. b. Insert an absorber with a plane surface (such as a book) between the generator and the absorber. Orient the book so that the waves reflected from its surface are directed at right angles to the incident waves, as in Fig. 5. Measure the reflected intensity Ir received by the detector. c. With the apparatus aligned as in Fig. 6, measure the transmitted intensity It received by the detector. d. Subtract the sum of the intensity reflected and transmitted from the incident intensity to find Ia, the intensity absorbed. e. Repeat this exercise for other items suggested by the instructor.

3. Polarization a. Arrange the apparatus as shown in Fig. 7, with the polarizing grid between the detector and the generator. Orient the Generator Detector polarizer so that the grid is horizontal. Polarization grid Measure the intensity received by the detector. Figure 7: Polarization of microwaves. b. Repeat part a, with the grid of the polarizer at 45° to the horizontal. c. Repeat part a, with the grid of the polarizer orientated vertically. d. The polarizer transmits only the component of the electric vector which is perpendicular to the grid bars. Why perpendicular and not parallel? e. What do you conclude about the polarization of the waves emitted by the generator?

4. Interference a. Align the apparatus as seen in Fig. 8., with the Screw Adjustment detector at a right angle to the line created by the generator, probe and reflector. Reflector b. Move the reflector in or out by rotating the screw until a maximum reading on the detector is observed. Record this position; there is now an antinode at the detector. Probe Detector c. Move the reflector in or out until a minimum reading on the detector is observed. Record this position; there is now a node at the detector. d. Using these positions and Eq. (2), find the Generator wavelength of the electromagnetic wave. Figure 8: Interference of microwaves.