Download Was there a measurable power received when

Department of Electrical Engineering
Electromagnetics II
Laboratory 4: Microwave Optics
I. OBJECTIVES
The objective of this experiment is to demonstrate the concept of magnification (gain) achieved
by application of microwave optics.
EQUIPMENT REQUIRED
1. E5071C Network Analyzer
2. Variable Flap Attenuator
3. Antenna Horn radiators (two)
4. Slotted Line
5. Waveguide Connectors
6. 900 Elbow Waveguide
7. 10” Parabola Reflector
8. 8” Parabola Reflector
9. Various waveguide stands
10. RF Power Meter
II. INTRODUCTION
The laws of optics apply to microwave radiation as well as to light since they are both forms of
electromagnetic waves. The path followed by a light wave from one point to another is an optical
line-of-sight. The path followed by a microwave signal is also a line-of-sight between the two
points. The transmitted wave for either portion of the electromagnetic spectrum can be focused,
reflected, refracted, polarized, filtered, attenuated, diffused, or absorbed. (In fact, the optics of
electromagnetic waves is concerned with the properties of wave motion that are common to the
entire electromagnetic spectrum from audio to gamma rays.) Some of the corresponding terms
and mechanisms used in the two electromagnetic regions are:
OPTICS
light source (lamp)
mirror
parabolic reflector
focusing lens
color filter
magnifier
MICROWAVES
oscillator
reflector plate
parabola
lens (dielectric or "artificial dielectric II)
bandpass filter
amplifier
Most people are familiar with the law of reflection as it applies to light waves; that is, when light
strikes a reflecting surface at some angle of incidence it is reflected in a direction that makes the
angle of reflection equal to the angle of incidence. This is shown in Figure 14 and holds true for
microwave wave-fronts as well. In this experiment we will demonstrate the law of reflection.
Figure 1 Law of Reflection
Both light waves and microwaves travel through a vacuum with a speed nearly equal to 330
yards per microsecond. In more common units, they travel nearly 186; 000 miles per second or
3x108 m/s. In almost any other medium the velocity of the waves differ from their velocity in a
vacuum, and signals of different frequencies travel with different velocities. Visible light, for
example, travels through water about three -fourths as fast as through a vacuum, and microwaves
travel through water only one-ninth as fast as through a vacuum. (Microwaves are greatly
attenuated, or diminished, by water and for practical purposes it may be considered that they do
not travel through water at all.) Water is partially transparent to electromagnetic waves, partially
reflective or conductive, and partially absorptive. Its ratio of transparency to reflectiveness, or
conductivity to absorption, varies with frequency. Water is more transparent to light than to
microwaves.
If we place an object that is not completely reflective to the electromagnetic wave between a
transmitting and receiving antenna, (or in a waveguide) some of the propagated energy will be
reflected, and some will pass through the object. The relative ratios will depend upon the
dielectric constant, the reflectivity, and the loss tangent of the material. Metals, which are good
conductors, are usually highly reflective. Ceramics, glass, and plastics are generally good
dielectrics and are fairly transparent though causing phase shifts. Semiconductors and other
materials have resistive components and act as attenuators as well as reflective obstacles. This
will also be illustrated in this experiment.
One of the most attractive properties of microwaves, from the antenna designer’s viewpoint, is
that they follow many of the rules of optics. Microwaves can be focused into a narrow beam by
various lens or reflector arrangements (see Figure 16). Both the lens and reflector must be
illuminated by a primary radiator in the same way that an optical lens or reflector must be
illuminated by a light source. In a microwave system this primary illumination is most often
accomplished by an electromagnetic horn (a flared end on a waveguide) or an open waveguide.
Figure 2 Microwave Focusing by Lens or Reflector
A microwave lens is a device for collecting and focusing divergent radiation into a parallel beam
by refraction (bending) in much the same way that an optical lens focuses a light beam.
Microwave lenses may take a variety of shapes, including disks, cylinders, and spheres. Lenses
are often manufactured of dielectric materials such as polystyrene, but may also be constructed
of sets of metal plates (the so-called "artificial dielectric” lens). The main advantage of a lens is
that it is fed from the back. Therefore, there is no front feed structure with its attendant
mechanical problems and aperture blocking. A high "front-to-back ratio" (the ratio of power in
the beam to the power scattered to the rear) can be achieved with a lens because the rear feed
radiates energy in the same direction as the lens. However, the lens inevitably has reflections at
the surfaces and losses in the dielectric material, which result in an insertion loss of perhaps 1 to
3 db. Thus, the gain of the lens is a little lower than that of an equivalent reflector. The lens also
has larger side lobes which are undesirable.
Electromagnetic wave beams can be formed by reflection as well as by refraction. A shaped
reflector is used in the majority of applications where a high gain antenna is required. The most
widely used reflector shape is the parabola. Many parabolic reflector arrangements are possible,
but the most common is the paraboloid of revolution illuminated by a waveguide horn located at
the focus of the paraboloid.
The gain of an antenna depends on its size, measured in wavelengths. Gain increases as the
wavelength becomes shorter or the antenna becomes larger. Thus, a 1 meter diameter reflector at
6 GHz has the same gain as a 2 meter diameter reflector at 3 GHz.
Figure 3 Antenna Gain Nomograph
A six foot diameter antenna operating at 9 GHz would have approximately 44.7 dB of gain as
shown by the dashed line drawn on Figure 17. This gain is for an antenna 100% efficient, and
would be 41.7 dB for a typical parabolic antenna (50% efficient).
III. PROCEDURE Part1:
1. Configure the network analyzer for a start frequency of your 8GHz, and a stop frequency of
10GHz.
2. Perform a full 2 port calibration of the network analyzer as follows:
a. Press “Cal” softkey, and then select “Cal Kit” from the touchscreen menu.
b. Use the Navigation wheel to scroll down to the tab marked “cal kit 1” and press
the “Enter” softkey.
c. Select the “Calibrate” tab from the touchscreen menu and then select “2-Port
Cal”.
d. Press the “Reflection” menu item from the touchscreen.
e. Now place the required measurement standard on the requisite port and press the
touchscreen button designated by the particular standard. For example “Port1
Open” would mean place the open standard on the end of the cable attached to
port 1.
f. Now repeat for all standards.
g. Once all standards are done select the “Return” touchscreen item.
h. Connect both ports together using the Cal Kit thru adaptor, then select
“Transmission” from the touchscreen menu, and press “Port 1-2 Thru”.
i. Select “Return” and then “Done” from the touchscreen menu.
3. Connect up the waveguides using figure 15 below as a guide.
4. Connect port 1 and port 2 of the network analyzer to the two waveguides respectively.
5. Press the “Marker Fctn” soft key and turn “Statistics” on.
6. Rotate the flat reflector plate until a maximum mean indication is achieved on the network
analyzer. Record the angle at which the reflector plate is now set.
7. Dielectrics and Attenuation
a. Remove the reflector plate and protractor board. Set up the equipment and make
sure the antennas are in line with each other, with a distance of approximately 30
inches between transmitting and receiving antennas.
b. Press the “Display” soft key and then select the “Data -> Memory” menu item.
c. Select “Data Math” and then “Data/Memory” from the touchscreen menu.
d. The display should read close to 0 dB.
e. Place various dielectrics and objects (min 3) midway between the two antennas.
Record the received signal level (by noting the db change) through a sheet of
paper, a book, a human hand, a foam cushion, the reflector plate, and a piece of
wood.
Figure 4 Experimental Setup for Law of Reflection
IV. QUESTIONS
1. Does the angle of reflection correspond to the angle of incidence, as expected?
2. What was the effect of moving the transmitting section further away from the reflector plate?
Moving the receiving section?
3. Determine the percent transmission through the various dielectrics, db equal 10log(P1/P2) ;
where P1 is the transmitted power and P2 is the received power.
4. Was there a measurable power received when the metal reflector plate was between the
antennas? Explain.
III. PROCEDURE Part 2:
1. Set up the equipment as shown in Figure 18 with the transmitting antenna located at a
distance of 3 feet from the large parabolic reflector, and on the optical axis of the reflector.
2. Adjust the 900 bend so that the open end is on the optical axis of, and approximately 10"
away from, the parabola.
3. Configure the network analyzer for Continuous Wave @ 9GHz and move the receiving
waveguide section back and forth along the optical axis. The focal point of the parabola is
indicated by the maximum signal level. Record the magnitude of this signal level and also the
focal length (the distance from the center of the parabolic dish to the open end of the 90 0
bend).
4. Measure and record the total optical path length of the signal (the distance from the
transmitting antenna to the parabolic reflector plus the focal length measured above).
5. Remove the parabola, and place the receiving waveguide section at the distance away from
the transmitting antenna measured in Step 4. Be sure the open end of the 900 bend is facing
the transmitting antenna. Record the signal level.
6. Repeat Steps 3,4 and 5 using the 8" parabolic reflector.
Figure 5 Experimental Setup for Application of Microwave Optics
IV. QUESTIONS
Part 1:
1. Does the angle of reflection correspond to the angle of incidence, as expected?
2. What was the effect of moving the transmitting section further away from the reflector plate?
Moving the receiving section?
3. Determine the percent transmission through the various dielectrics, db equal 10log(P1/P2) ;
where P1 is the transmitted power and P2 is the received power.
4. Was there a measurable power received when the metal reflector plate was between the
antennas? Explain.
Part 2:
5. Why was the signal level received in Step 4 larger than that received in Step 5?
6. If we define the gain of the parabola as the db difference in level in Steps 4 and 5, what
relationship exists between the size and gain of parabolas?
7. What effect does the bend have on the signal transmission?