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MicroWaves as Electromagnetic Waves

The term microwave is applied to those electromagnetic waves whose wavelengths are of the order of a few centimetres. Since the speed of light is about 3 x 1010 cm/sec, for a wavelength of 3 cm the frequency is about 1010 Hz. Since the transit time for electrons to go from the cathode to the plate in a normal vacuum tube is longer than that required for microwaves, a very special device is required.

This device is called a klystron tube. An electrode from the klystron tube (funny looking metal device on top of the transmitter) protrudes into a resonant cavity. Through this duct, or waveguide, the microwaves are transmitted. A horn is attached to the waveguide to match the impedance of the klystron to the impedance of the air, thereby allowing the microwaves to be emitted without too much reflection or spreading out.

Electromagnetic energy, throughout the frequency spectrum up to and including both microwaves and visible light, has wavelike characteristics. These characteristics are produced by the electromagnetic field, which includes both dynamic (oscillating) electric (E) and magnetic (B) fields. The direction of polarization of an electromagnetic wave is conventionally described as the direction of the (E) field.

Figure 1:
  1. Linearly polarized E and B fields 
  2. How E and B are perpendicular approaching antenna as a wavefront 
  3. How E and B are configured close to antenna.
Note that E ^B

The most common polarization mode is one in which the electric field travels within only one transverse plane, as shown in Fig. 1. If the components of the electric and the magnetic fields in the transverse plane do not change alignment along the wave, the wave is linearly polarized. Another common type of microwave polarization is called circular polarization. In this type of polarization, the electric field rotates helically (in screw fashion) about the axis of travel, with constant magnitude as it propagates along this axis. Both of these polarizations are limiting extremes of the general case; elliptical polarization.

The electromagnetic energy radiating from the klystron's antenna in the transmitter is linearly polarized. When the klystron tube and its antenna are vertical the electric field oscillates in the vertical plane as shown in Figure 1, and the wave is said to be vertically linearly polarized.

The vibrational mode of the energy propagated in a waveguide is not a single plane wave. This wave bounces off the guide walls in such a manner that the incident and reflected waves give rise to a transverse standing wave and a longitudinal traveling wave. The physical and dimensional characteristics of the waveguide determine the polarization made propagated.

As per the common terminology used in describing microwaves, our apparatus operates in the TE1,0 or dominant mode. The letters TE indicate that the electric field is transverse to the direction of propagation. The subscripts 1 and 0 indicate the number of half-cycle variations of the electric field intensity along the side and narrow inner dimensions of the guide respectively.

OBJECTIVES:

To observe at least qualitatively the aspects of electromagnetic waves through the study of microwaves. These include:
  1. the inverse square law
  2. polarization
  3. attenuation

PROCEDURE:

  1. Set up the apparatus as described by your instructor.

    2.  
  2. Turn on the power and allow the klystron to warm up for 2 minutes.

    3.  
  3. The repeller controls a repelling voltage to the flow of electrons to set up a resonance required to produce microwaves. Adjust it for maximum signal and then adjust the receiver apparatus to full scale deflection if possible.

    4.  
  4. Move the receiver unit away from the transmitter in a straight line. Record the distance and readings. Note that there is a slot on the bottom of both the transmitter and receiver so they can be placed on a metre stick or a 2 - metre stick. Plot distance and intensity on linear graph paper. Verify the inverse square law. In doing this part there may be some reflections from other objects. Remember, according to the Intensity Law,        I µ 1 / R2

    5. Be sure to explain any anomolies or exceptions to the inverse square law noted.
     
  5. Hold the grating in front of the unit so that the parallel grid lines on the sheet are aligned horizontally and note the readings. Your instructor will demonstrate this for you.

    6.  
  6. Repeat the previous step, but this time the parallel grid lines should be aligned vertically. What is the difference? You should be checking for polarization phenomena. Is there evidence of this? Which wave are you receiving and how is it polarized?

    7.  
  7. Turn the receiver on its side and repeat 5 and 6. Describe what you see.

    8.  
  8. Insulating materials usually have an appreciable loss to the passage of microwaves. This is called dielectric loss. It is due to the attenuation of the signals by the conversion of energy into heat. Low loss materials include teflon and polystyrene whereas high loss materials include carbon and powdered iron.

    9.  

     

    There are two parameters which describe the relative transparency or absorption ability of a material: the dielectric constant, e, and dissipation factor, D. We will find out later that the index of refraction, n, is the square root of the dielectric constant. You will do two tests to quantitatively measure the loss. You will check for absorbtion by water and secondly by pages of a book which is a dielectric material.
     

  9. Adjust the signal for maximum gain.

    10.  
  10. Hold a dry paper towel in front of the receiver horn so it is in front of the waveguide. Readjust for maximum intensity.

    11.  
  11. Saturate the towel with water. Make sure the towel is larger than the horn so that all of the signal that gets into the receiver travels through the water soaked paper towel. Note the loss. Record the readings. Explain.

    12.  
  12. Reflection: Set up the apparatus as shown below. Adjust the horns for maximum signal intensity with the reflector sheet about two feet in front of the horns. Adjust the angles for max signal and mark the position of the plate on the table with chalk.


    13. Figure 2: Top View while calibrating (Step 12).

  13. Place a book with several hundred pages in front as shown in Figure 4.

    14.  

     
     


    Figure 4: Side View while measuring.

  14. Plot the reading versus page number. The plot should be nearly a straight line showing that the attenuation varies directly with the thickness of the material. It is possible that there is a moderate signal increase when doing this. It is more or less due to the spacing of the sheet between the horns causing a matching effect in the reflection of the waves. It is just like the thin dielectric material that is deposited on coated lenses.

    15.  
  15. Design your own procedure to determine a simple relationship between the angle of reflection and the incident wave. This is totally up to you. What way might you get a wave to reflect into the wave guide?

    16.  
  16. List limitations.

    17.  
  17. Explain fully all results.

    18.  
  18. Summarize.

    19.  
  19. Conclusions and Recommendations. Overall, you should have some sense of how microwaves act in our every day environment.
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