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ESC 400 H
ENGINEERING ELECTROMAGNETIC FIELDS
LABORATORY MANUAL
ESc 400 H Engineering Electromagnetic Fields
Laboratory Manual
Three Centimeter Microwave Apparatus:
1. Introduction
The Three Centimeter Microwave Apparatus (36811) is used to demonstrate such microwaveoptics phenomena as:
· directive radiation
· wave polarizations (including linear direct, cross, and elliptical)
· free-space propagation
· reflection
· prism refraction
· resonance
· multi-path and double-slit (Young's) interference
· fiber optics
· standing waves
In addition, the apparatus can be used with a radio or microphone and a speaker or earphones to
demonstrate audio communications on a microwave link.
The included accessories (see Section 3D, "Accessories," for a complete listing) facilitate the
Michelson interferometer and the Fabry-Perot interferometer demonstrations. Because the
apparatus uses a 3cm wavelength, all the experiments can be performed within the limited space
of a laboratory benchtop; because the power generated by the transmitter is only a few
milliwatts, no radiation hazard is produced. This is in contrast to the typical power levels of
several hundred watts produced by microwave ovens.
This apparatus includes two features that allow for demonstrating modulated communications:
· an audio modulator in the transmitter
· an audio amplifier in the receiver that is capable of driving a low-impedance speaker
The receiver's meter does not require power to operate, so external power to the receiver is
needed only when an audio output is required -- as in the Audio Transmission Experiment (see
Experiment 5F).
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2. Specifications
Transmitter
Input Power
Voltage
110/125 VAC (rms)
Frequency
50/60 Hz
Power
5 W max.
Audio Input
Voltage, into BNC jack, 2k 30 mV max.
Microwave Ouput
Frequency
10.52 GHz
Wavelength
2.85 cm
Power
10 mW max.
Overall Dimensions of box (with horn and stand)
Width
10.2 cm (4 inches)
Length
24.8 cm (9.75 inches)
Height
13.95 cm (5.5 inches)
Receiver
Input Power
for audio experiments only
same as transmitter
Audio Output
Power, into 4mm (1/8-inch) audio jack 0.10 W min.
Overall Dimensions of box (with horn and stand)
Width
10.2 cm (4 inches)
Length
24.8 cm (9.75 inches)
Height
13.95 cm (5.5 inches)
3. Description
3A. General
The complete apparatus consists of a microwave transmitter and receiver, and the accessories
listed in Section 3D, "Accessories."
3B. Transmitter
3B.1 General:. The transmitter consists of four sub-assemblies:
1) a case with a Gunn diode mount and audio modulator
2) a flared directive-gain horn
3) a 9V wall-outlet power unit
4) a stand for the unit (with mounting screw)
The Gunn diode in its mount produces vertical linear polarized monochromatic energy, at a
single frequency, which is radiated by the flared horn.
3B.2 Case with Gunn Mount and Modulator: The Gunn diode is very small and is mounted in a
wave-guide rectangular cavity. This diode is a gallium arsenide junction device with two
contacts. It acts like a non-linear resistor when a DC voltage is applied and develops current
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fluctuations with strong components in the microwave frequency band. The cavity serves as a
vertical coax resonator at a wavelength of about 2.85cm. The cavity has a coupling hole in its
front face which feeds the horn. The audio section includes an op-amp with a voltage gain of
100X and a current driver at 9V. The top of the case contains the power switch and power
indicating light.
3B.3 Horn: The flared horn provides a directive gain of about 110. As noted above, it must be
positioned on the output flange for vertical polarized radiation.
3B.4 Power Supply:. The power unit plugs into any 115VAC outlet and its output jack plugs into
the matching jack on the side face of the transmitter case.
3C. Receiver
3C.1 General: The receiver consists of four sub-assemblies:
1) a case containing a detector, meter, level control, and amplifier
2) a flared directive-gain horn
3) a 9V wall-outlet power unit which is needed only for audio experiments 4) a
stand for the unit (with mounting screw)
3C.2 Case: The case includes a waveguide-mounted point contact diode. The level control
affects the meter reading at all times and it affects the audio output if power is supplied by the
9V power unit. The light works only when power is being supplied by the 9V power unit.
3C.3 Horn. The flared horn provides a directive gain of about 11 OX. As noted above, it must be
positioned on the output flange for vertical polarized radiation.
Note: The meter circuit may be damaged by an over-strong signal, so do not leave the gain knob
at a setting high enough to drive the meter off-scale.
3C.4 Power supply:. The power unit plugs into any 115VAC outlet and its output jack plugs into
the matching jack on the side face of the transmitter case.
3D. The Accessories
The following accessories are supplied with the Three Centimeter Microwave Apparatus
(36811):
Qty.
Name of Accessory1
Pattern Turntable (with fixed arm for transmitter)
2
Plug-In Side Arms (for interferometer experiments)
1
Plug-In Rotatable Arm (for pattern experiments)
2
Reflector Carriages
1
Turntable Card Holder
1
Tumtable Prism Holder
1
Refraction Prism Container
1
Polarized Grating (P.C. board with strips)
1
Single Slit Object Card (P.C. board with one slot)
1
Double Slit Object Card (P.C. board with two slots)
2
Small Grid Screen Cards (P.C. boards with 1.02cm, 0.40-inch, pitch)
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1
1
1
Large Grid Screen Card (P.C. board with 1.14cm, 0.45-inch, pitch)
20.3cm (8 inches) Polybead Tube
Polybead Zip Bag
4. Setup
Note: All the experiments In the next section (Section 5, "Experiments”) use this basic setup.
1) Assemble the turntable on top of a non-metallic lab bench or table with a smooth
surface at least 107cm (3.5 feet) in radius, so the rotatable arm will move easily. Turn
the azimuth disk, if needed, to align its 180° mark with the fixed arm. Plug the
rotatable arm into the center post below the turntable disk. Make sure the receiver
case is attached with the knob and meter on top.
2) Clip the transmitter on the back end of the fixed arm and clip the receiver on the back
end of the rotatable arm so the horns face each other and their faces are separated by
at least 30cm. Make sure both horns are attached with the wide dimension of the
wave guides horizontal.
3) Plug the power unit cord into the jack on the microwave unit. Then, plug the power
unit into a 115VAC outlet. If the experiment requires the audio amplifier, repeat with
the other power unit for the receiver.
4) Turn on the transmitter and adjust the receiver's gain control for a mid-scale meter
reading.
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5. Experiments
5A. Wave Polarization
5A.1 Purpose. To demonstrate the polarization characteristics of linearly polarized
electromagnetic plane waves.
5A.2 Theory. Electromagnetic energy throughout the frequency spectrum up to and including
microwaves has wavelike characteristics. Electromagnetic energy at higher frequencies in the
form of light has both wave and particle characteristics due to quantum effects.
The wave-like characteristics of the propagating electromagnetic fields include both oscillating
electric (E) fields and magnetic (H) fields. The polarization direction of an electromagnetic wave
is defined by the direction of its (E) field.
The most common polarization mode is the one in which the electric field propagates (travels
along) and stays within only one transverse plane, as shown in Figure 1.
Figure 1 (a) Linearly polarized wave with vertical E field
(b) E & H fields for approaching vertical polarized wave
(c) E & H fields around a dipole antenna
If both the (E) and (H) fields are perpendicular in the transverse planes and do not change
alignment along the wave, the wave is called linearly polarized. Another type of wave
propagation is by circular polarization with constant-magnitude (E) and (H) fields rotating about
the axis of travel (in a screw fashion). Both linear and circular polarization are limiting extremes
of the more complex general case of elliptical polarization. Elliptical polarization can be resolved
into special pairs of orthogonal-polarized waves such as right-circular and left-circular or
vertical-linear and horizontal-linear time-shifted a quarter cycle.
The Three Centimeter Microwave Apparatus operates in TE10 (dominant waveguide) mode. The
letters "TE" indicate that the (E) field is transverse to the direction of propagation. The subscripts
1 and 0 denote the number of half-cycle variations of the (E) field intensity along the wide and
narrow inner dimensions of the guide. A pictorial representation of the TE10 field pattern is
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shown in Figure 2.
Figure 2 Representation of the fields in a TE10 guide mode
5A.3 Procedure
Direct Polarization
1) Follow the directions in Section 4 under "Setup" .
2) Place the polarization grating on the card holder. Place the card holder with the
grating on the center pivot post with the grating's surface parallel to and 2.5cm
(about 1 inch) in front of the receiver horn's aperture. Make sure the grating's
conductor strips are horizontal. This setup is shown in Figure 3. The signal level
should change only slightly.
Figure 3 Top view of polarization setup
3) Remove the grating from the card holder and rotate it 90 o about the axis of
propagation (conductors now vertical).
Replace the card holder. And note down the sgnal level.
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Question: Explain why the signal level is decreased ?
Cross Polarization
1) Loosen the knob on the rear of the receiver assembly and rotate the assembly 90 °
for horizontal polarization. Tighten the knob. The receiver is now cross polarized
to the transmitter.
2) Follow the directions in Section 4,under "Setup". The received signal will be weak
and can be made nearly zero -- even at maximum gain -- if the receiver rotation is
adjusted carefully for a deep null on the meter.
3) Hold the grating 2.5cm (about 1 inch) in front of and parallel to the receiver horn
aperture and rotate it on axis as before. The signal is nearly zero when the
conductor strips are vertical or horizontal, but it increases to a maximum value
about half as large as for direct linear polarization when the conductors are at a 45°
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tilt angle from vertical.
4) Return the receiver horn and detector assembly to its original position for vertical
polarization.
Question: Explain why the signal level is increased when the conductors are at 450.
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5B. Reflections, Standing Waves, and Speed of Light
5B. 1 Purpose. To demonstrate various plane wave reflections, accompanying standing wave
phenomena for single-frequency (monochromatic) waves, and a crude determination of the speed
of light.
5B. 2 Theory. When electromagnetic waves impinge on metal or dielectric surfaces they produce
redirected and generally reshaped waves at all those surfaces in accordance with field theory
equations. If the surface is metallic and flat with dimensions which are at least ten times the
wavelength, the process is called planar reflection. In this case the directions of the new waves
are related to the directions of the incident waves by the Laws of Reflection, of which the
common one is: The angle of reflection equals the angle of incidence. If the surface is smooth to
within 1/8 wavelength, a specular (shiny) reflection occurs. If the surface is not smooth within
this limit, diffuse scattering occurs and wave polarization usually is altered.
The complete and detailed reflection process is based on a simple requirement: The E field
component parallel to the metallic surface must be zero since the metal is a conductor and
"shortcircuits" this field component. For flat metallic mirrors, it is easy to prove the above law of
reflection mathematically. The law will be demonstrated in the following experiment.
The reflected wave travels with the same speed as the incident wave (assuming it is propagating
through the same dielectric medium) and the two interact to produce a standing wave in the
space forward of the reflector. When the wave has NORMAL incidence (angle of incidence = 0)
the standing wave has a spacing between nulls of a half-wavelength, and this case allows
accurate wavelength measurements. Another precise way is described in Section 5F, "Reflection
by Ionosphere" in the product manual.
This wavelength , is related to the velocity of propagation C and the signal frequency f by the
propagation equation: C=/f. The transmitter's frequency is factory-set to about 10.525GHz
(10,525 MHz). The velocity of propagation can be calculated from the measured wavelength.
The propagation constant in air is about 3108 meters per second, virtually the same as the
propagation constant in vacuum (C).
5B.3 Procedure
1) Follow the directions in Section 4, "Setup".
2) Rotate the receiver on the rotatable arm to 90°. Place a small grid screen in the card
holder and rotate so the red pointer is at 45°. See Figure 4.
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Figure 4 Top view of reflection setup
3) Slowly rotate the rotatable arm with receiver around the 90° value and note the angle
for maximum signal.
4) Repeat for other angles of the screen and receiver.
Note: The rotation of the mirror causes the reflected waves to rotate through twice
the mirror rotation angle. This arrangement in the compound scanning systems of
optical and radar systems reduces the angular excursion and enables rapid-scan
features.
Question: Explain how well the lows of reflection applies?
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Direct Standing (Multi-path) Waves and Wavelength Measurements
1) Follow the directions in Section 4, "Setup". Remove the receiver from the rotatable
arm and place it alongside the transmitter with both horns facing the azimuth disk on
the fixed arm and with the inner sides of the two horns almost touching. See Figure 5.
Figure 5 Top view of multi-path setup
2) Set the rotatable arm at 0° (in line with the fixed arm) and put a reflector carriage on
this arm about 10cm past the disk and with its top slot facing the transmitter and
receiver. Insert a small grid screen in the carriage.
3) Slide the carriage and screen slowly back and forth, looking for a signal maximum. At
one of these points slowly rotate the rotating arm with reflector and carriage for the
highest maximum.
4) Slide the reflector and carriage slowly back and forth, thus varying the phase of the
reflected wave compared to the leakage signal between the adjacent horn sides.
Note the oscillatory variation of the signal level This is an example of two-path
propagation, and the null depth depends upon the inequality of the levels of the two
signals which have very different path lengths.
5) Carefully measure the distance between successive nulls as the reflector is moved as in
Step 4. The distance between all nulls should be a half-wavelength (about 1.4cm),
depending upon the transmitter's exact frequency.
Note: This is a rather accurate measurement if the reflector is at least 50cm away. The
accuracy of the measurement can be increased by an averaging process: The overall
distance between several successive nulls is measured and divided by the number of
nulls passed through after the starting null.
Oblique Standing Waves
1) Set up as in Figure 6 with a side arm plugged into the front clip under the azimuth
disk.
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Figure 6 Top view of oblique standing-wave setup
2) Remove the reflector and carriage from the rotatable arm (as in Step 2 above) and
place on the front side arm with the screen about 5cm from the azimuth disk pivot
post.
3) Slowly move the receiver back and forth on the rotatable arm and observe the cyclical
variation of the received signal, an example of a standing wave in space due to two
signal paths.
4) Measure the distance between successive nulls to determine the phasewave length.
Note: This phase wavelength is longer than the free-space wavelength measured
above because the two waves are traveling at oblique angles in the vicinity of the
receiver horn's aperture.
5C. Antenna and Aperture Radiation Patterns
5C.1 Purpose. To demonstrate the techniques for measuring directivity patterns of radiating
apertures and to demonstrate the large directivity factor of the 10.2cm (4-inch) horn.
5C.2 Discussion of Antennas. An antenna is a passive "field converter." In the case of a receiving
antenna, an incident free-space-propagating wave is converted into a guided wave which
propagates in a transmission line such as the rectangular wave guide in the receiver's detector
mount. Most antennas do this with some directivity. The larger the antenna's aperture in terms of
wavelength, the more directive is its pattern, and the greater its gain and signal pickup. All
common antennas and radiating apertures (such as slits) are reciprocal -- i.e., they have the same
gain, pattern, polarization, and radiating characteristics (except voltage breakdown) whether they
are transmitting or receiving. This characteristic allows a single antenna to serve in a radar
system where a short burst of transmitter power is radiated and then time-delayed weak echoes
are received by the same antenna.
Antennas are made in many shapes and sizes, depending upon the desired directivity pattern,
gain, low side lobes, power, and polarization characteristics for their system usage. At one
extreme, broadcast receiver antennas have a broad "doughnut" pattern which is called omnidirectional in the horizontal plane if the axis of the doughnut is vertical such as the dipole shown
in Figure 1(c).
Ordinary whip antennas on cars and some portable radios are of this type. At the other extreme
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are the radio astronomy and radar tracking antennas with array apertures or reflectors several
hundred wavelengths wide and beam-widths less than 0.1°.
5C. 3 Characteristics of the 36811 Horn. The characteristics of these horns are:
Aperture:
Beam Width:
Gain:
10cm10cm (approx. 16 square inches)
About 22° at the half-power angles
About 21 dB in power, compared to a theoretical non-directional isotrope
reference. (An incandescent lamp has random polarization and a
somewhat isotropic pattern.)
Polarization:
Linear with the E field aligned with the smaller inside dimension of the
horn's flange.
Radiation Efficiency: Nearly 100%
Question: Measure the power for different receiving angles and plot the radiation pattern.
Angle
Power
Angle
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Power
Angle
Power
Angle
Power
14
110°
100°
90°
80°
70°
120°
60°
130°
50°
140°
40°
150°
30°
160°
20°
170°
10°
180°
0°
0
10
20
30
190°
350°
200°
340°
210°
330°
220°
320°
230°
310°
240°
300°
250°
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260°
270°
280°
290°
15
5C.4 Theory of Spherical Wavelets. In the late eighteenth century, Christian Huygens postulated
the wavelet theory of electromagnetic wave propagation. This theory states that each point on a
wavefront acts as the source of an elemental, spherically-expanding wavelet, and all of the
wavelets add in proper phase to form the succeeding wavefront of the propagating wave. This
theory explains a variety of transverse-polarized waves, including plane and spherical waves.
When a small aperture in a metal screen is illuminated, the wavelets in this aperture cause a
spherically expanding wave. If a slit, narrow in one dimension, is illuminated by a plane wave, a
cylindrically expanding wave results whose axis lies in the long dimension of the slit.
5C 5. Procedure
Single Slit Diffraction
1) Follow the directions in Section 4, "Setup" but with the transmitter moved forward on the
fixed arm so its horn aperture is 2.5cm (about I inch) from the tumtable disk pivot, and
with the receiver fully back on the rotatable arm. See Figure 7.
Figure 7 Top view of slit diffraction setup
2) Place the single slit card on the turntable card holder and move the card parallel to the
horn aperture with the slit's long axis vertically aligned.
Note the decreased signal
3) Carefully move the transmitter toward and away from the card until you get a maximum
signal.
4) Slowly rotate the receiver unit and rotatable arm off axis as in the previous procedure for
the open horn. The signal level will decrease off-axis since the slit has some directivity,
but the pattern lobe will be much broader than that measured for the open horn. Plot the
receiver meter reading vs. angle off-axis every 1° on polar graph paper for the power
pattern.
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Angle
Power
Angle
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Power
Angle
Power
Angle
Power
17
110°
100°
90°
80°
70°
120°
60°
130°
50°
140°
40°
150°
30°
160°
20°
170°
10°
180°
0°
0
10
20
30
190°
350°
200°
340°
210°
330°
220°
320°
230°
310°
240°
300°
250°
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260°
270°
280°
290°
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Double-Slit Interference Diffraction
In the early nineteenth century Thomas Young performed a classic optical experiment with two
closely spaced pin holes in a screen back-illuminated by a plane wave. This gave a multiple-lobe
pattern of the type always generated by interference between two separated wavelet sources. In
the following experiment a screen with two parallel vertical slits demonstrates this interference
in the horizontal plane. The peaks occur at angles where the waves from both slits arrive in
phase, including the "first" peak on axis. The nulls occur at angles where both waves arrive out
of phase.
1) Replace the single slit card described in the previous procedure with the double slit card.
Make sure this card is centered and parallel to the horn aperture with the slits' long axis
vertically aligned.
2) As in Step 3 above, move the transmitter toward and away from the card until a
maximum signal is achieved (about 0.6cm,1/4-inch, spacing).
3) Repeat Step 4 above. Peaks should occur at 0° and ±38° off axis. The minimums should
occur at about ±19° and ±55° with levels less than 20% of the 0° maximum.
Angle
Power
Angle
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Power
Angle
Power
Angle
Power
19
110°
100°
90°
80°
70°
120°
60°
130°
50°
140°
40°
150°
30°
160°
20°
170°
10°
180°
0°
0
10
20
30
190°
350°
200°
340°
210°
330°
220°
320°
230°
310°
240°
300°
250°
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260°
270°
280°
290°
20
Question: Explain why the multiple lobe pattern for double slit illumination?
5D. Michelson-Morley Interferometer
5D.1 Theory. An interferometer is an instrument, which uses the phase differences between two
portions of a signal, which traverse different paths of controllable length. Various interferometers
are used for: wave-length and atmosphere refractive index measurements; precise radar target
direction measurements; radio astronomical telescopes; gas flow measurements in wind tunnels;
spectral line studies; and gauges for standard-length calibrations.
Both of the following interferometer experiments will give fairly precise measurements of the
standing wavelength which can be used to calculate the speed of light (at 10.525 GHz).
These interferometers have several common features:
1) They require semi-coherent input signals (wave-fronts)
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2) There are at least two separate space-stable paths
3) One path length (and thus its phase delay) is adjustable
4) The separate signals should have nearly equal amplitudes
Michelson Interferometers are used for measuring a null-shift as the characteristics of one path
are changed. The Wheatstone Bridge is a low-frequency electrical circuit which also has both
separate signal paths where the phase and magnitude of one can be adjusted for a null.
The parallel-plate Fabry-Perot interferometer depends upon a series of partial reflections
between the end plates. If the signal is completely reflected at one end and strongly reflected the
other end plate but with a slight transmission (leakage) through this plate, the output signal from
this end peaks strongly at particular spacings between the plates. It then acts like a parallel-plate
resonator similar to the ruby rod in a laser.
The ruby rod is optically many wavelengths long and its ends are polished parallel, then coated
with a thin metal film so multiple reflections occur which couple the light beam strongly to ruby
atoms then stimulated to give laser action. The many internal-reflection round trips produce the
many in-phase leakage signals giving a narrow, collimated beam of light.
5D.2 Procedure
Michelson Interferometer
1) Follow the directions in Section 4, "Setup" with the extra accessories (see Figure
10).
Figure 10 Top view of setup for the Michelson Interferometer Experiment
2) Plug the two side arms into the fixed arm under its disk with the high-reading ends
of their scales outward. Adjust the rotatable arm and receiver to align with the
fixed arm (0° on the dial).
3) Place the large grid card on the turntable card holder and put this assembly on the
turntable pivot at an azimuth angle of 135°.
4) Attach the two small grid cards to the reflector carriages, and place one on the rear
side arm and the other on the rotatable arm with the carriages (carriages behind the
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screens to reduce reflections). Place the receiver on the front side arm. Check that
the setup approximates Figure 10 (so the center screen does not provide a directreflection path from the transmitter to the receiver).
5) Slide either carriage/grid back and forth to see the oscillatory variation on the
received signal. This two-path phenomenon gives phase interference as follows:
Path #1: A half-reflection off the center grid out to the reflecting grid on the rear
side arm and then back through the center grid by a half-transmission to the
receiver.
Path #2: A half-transmission through the center grid to the rear grid, and then back
with a half-reflection at the center grid.
6) Measure the distance between successive nulls, moving first one carriage/grid and
then the other. The null separations should be identical and equal to a halfwavelength in air.
7) Hold the polarized grating vertically with its conductors running horizontally in
front of and parallel to one carriage/grid. Note the shift needed in either
carriage/grid to re-null. This shift is due to the plane wave phase-delay in the
plastic base material.
8) Repeat Step 7 with sheets of other dielectrics such as window glass, plastic sheets,
etc.
Question: Find the velocity of EM Wave in free space.
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5F. Audio Transmission
5F.1 Theory. Microwave and fiber optics communication links have used several methods for
modulating the carrier audio/data information. These are: amplitude, frequency, pulse width,
pulse rate, and phase shift. This equipment uses amplitude modulation by means of an audio
amplifier and voltage driver for the Gunn diode. The amplitude-modulated microwave signal is
detected by the receiver, which amplifies the signal to feed a low- or high-impedance speaker
(transducer).
This audio signal is distorted by the square-law detection process and is rich in harmonics. An
input sinusoidal signal will sound quite musical since all the harmonics of the base signal are
heard as harmonics similar to the overtones in a single-voiced musical instrument. Speech and
music are distorted but quite intelligible.
5F.2 Procedure
1) Follow the directions in Section 4, "Setup" and plug a power supply into the
receiver.
2) Plug an audio source into the BNC jack on the side of the transmitter, and plug a
speaker or earphones into the phone jack on the side of the receiver.
3) Adjust the level of the input audio signal and the receiver’s level control for
listening comfort at the receiver.
Note: With optimum adjustment of the input signal a range of several feet should
be obtained as the level drops off with the square of the range.
Question: Explain what do you understand by Modulation
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Experiments Using the Arra System
Experiment #1: Components of The ARRA Microwave System
Introduction:
For the purpose of this laboratory, a microwave training system made by ARRA inc. is used. The
setup of this system is illustrated in the block diagram in figure 1.
Klystron
Oscillator
Klystron Power
Supply
Waveguide or
Antenna
Modulator
Microwave
Detector
Audio
Amplifier
Display
Device
Figure 1: Block Diagram of ARRA Microwave Training System
The functions of these components are as follows:
 Klystron Oscillator:
Generates microwave energy.
 Klystron Power Supply:
Provides the necessary voltages required to drive the oscillator.
 Modulator:
Modulates the microwave carrier with the information to be transmitted.
 Waveguide or Antenna:
Provides a mechanism for the microwave signal to propagate.
 Microwave Detector:
Detect and demodulate the microwave signal.
 Amplifier:
Increases the amplitude of the demodulated signal.
 Display Device:
Displays the information.
More details can be found from table 1.
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Table 1: Components of the Arra Microwave Training System
Component
Figure
Function
Klystron
Power Supply
Provides the necessary voltages required to
drive the Klystron oscillator
Square Wave
Modulator
Modulates the microwave carrier with the
information(square wave) to be transmitted
Klystron
Mount
Generates microwave energy
Variable
Attenuator
Reduces the energy of transmitted microwave
signal
Cross Guide
Coupler
Frequency
Meter
Couples microwave energy from one
waveguide to another
Used with the standing wave indicator for
measuring the frequency of the microwave
carrier signal
Matched
Termination
An ideal matched termination absorbs all the
incident microwave energy so that there is no
reflected signal
Slotted Line
A probe is inserted into the waveguide from
the open slot for measuring the microwave
signal amplitude distribution
Slide Screw
Tunner
Crystal
Detector
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Used for adjusting the impedance matching
Detects and demodulates the microwave
signal
27
Standing
Wave
Indicator
Amplifies the demodulated signal and display
the signal amplitude
Standard
Waveguide
Provides a mechanism for the microwave
signal to propagate
Antenna Horn
Provides a mechanism for the microwave
signal to radiate into the free space more
effectively
90O Twist
Used for rotating the polarization direction of
the microwave signal
90O Bend
Used for rotating the propagating direction of
the microwave signal
Parabola
Reflects the incident microwave signal in such
a way that the reflected signal energy is more
focused. (Energy density is maximum near the
focal point)
Polarization
Grating
Used for demonstrating the polarization of the
microwave signal
Reflector Plate
Reflects all the incident microwave energy
Turntable
Used for mapping the radiation pattern
Purpose:
To get familiarized with the components of the microwave training system.
Lab Assignment:
Identify the components of the microwave training system, understand the function of
each component. Refer to the figures and descriptions in the next three pages. Take
your time! How much you can learn from this lab depends on your understanding of the
Esc 400 H Engineering of Electromagnetic Fields Lab
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system components.
1--Klystron Power Supply
The klystron power supply generates and controls the operating voltages for the 2K25
klystron (see the following figure). Notice that there are three meters on the power
supply for displaying the beam voltage, beam current and reflector voltage. These
three parameters describe the performance of the klystron oscillator. There are three
control knobs, one for adjustment of the beam voltage and two for the reflector
voltage (coarse and fine tune). The purpose of adjusting beam and reflector voltage
is to obtain maximum output microwave power. Notice there is also an On/Off toggle
switch which controls the 120V AC power supplied to this instrument. The BNC
panel jack denoted as Modulation Input is the input terminal for supplying
modulation to the reflector voltage. The socket at the lower right corner of the panel
denoted as Power Out is for a male plug of the interconnecting cable to the klystron
tube. DONOT disconnect the cable from this power out socket. The DC voltage is
about 300V.
Figure 2: Front panel of the Klystron power
Supply
2-- Square Wave Modulator
This 1000Hz square wave modulator is supplied to apply modulation to the klystron
power supply and thus to the reflector voltage of the klystron. The Modulator
Frequency Control is for varying the square wave center frequency of 1000Hz plus
or minus approximately 100Hz. The 0~10 panel engraving is supplied to aid
resettability. The Modulator Volts controls the output voltage level of the modulator.
The BNC panel jack denoted as Mod Out is the output terminal for the 1000100Hz
square wave. The BNC jack Sync In is for external synchronization source (not used
in this laboratory). The On/Off switch controls the AC 120V power source.
The working principle of square wave modulator is similar to those of the AM radio.
The purpose of applying a audio frequency modulation is to make simpler the
detection circuit in the latter part of the system, because an audio amplifier can be
used. Since the modulation signal changes the reflector DC voltage applied to the
klystron, the reflector voltage of the Klystron Power Supply should be adjusted each
time the modulator voltage level is adjusted.
Esc 400 H Engineering of Electromagnetic Fields Lab
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Figure 3: Front panel of square wave modulator
3—Standing Wave Indicator
The standing wave indicator is also called VSWR amplifier, where VSWR means Voltage
Standing Wave Ratio. It is most often used with a crystal detector to detect, amplify and display
the electric field amplitude (or power density) at some point within the waveguide. The crystal
acts as a current generator, and when the RF signal is applied across it, the rectified current
output is proportional to the square of the applied voltage. This point is important in
understanding how to read the scales on the Standing Wave Indicator (more about this later).
Since the rectified current is usually very small, it needs to be amplified so that the information
can be correctly displayed. The Gain control is used to adjust the gain of the amplifier
continuously. The Range SWR or DB switch with 0~40dB engraving also controls the gain of
the amplifier, but not continuously.
Figure 4: Front panel of the standing wave
indicator
The meter face on the VSWR amplifier has two basic scales. One scale is for the reading of
voltage standing wave ratios, and the other is a relative power indication in terms of db. The
upper two curves (in black color) is the VSWR scale, while the third curve (in red color) is the
relative power db scale. Conversions from the VSWR scale to the db scale can be made by using
the formula db = 20 log VSWR.
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Figure 5: Meter on the standing wave indicator
Now we talk about how to read these scales. Basically we are trying to read the ratio of two
numbers (voltages) r=V1/V2. To get the VSWR, V1 should be the maximum voltage (electric
field amplitude), and V2 should be the minimum voltage on a transmission line. First we move
the crystal detector to the position where maximum voltage V1 is obtained, adjust the amplifier
gain until the needle lines up with VSWR=1 or Power Ratio=0db. Then we keep the Gain
Control fixed and move the crystal detector to a position where minimum voltage V2 happens,
the number you read on the first curve is the SWR. If the needle is moved off scale to the left,
turn the Range Switch counterclockwise once and you will see that the needle now moves back
to the right side of the meter. You can read the SWR from the second curve, which ranges from 3
to 10. Add 10 to the number you read from the db scale, that is the power ratio in db. If the
needle doesn't move to the right of the meter after turning the Range Switch once, turn it once
more again, and you should add 20 to the reading on the db scale.
4—The Transmitter System
Figure 6: Components of the transmitter system
1--Klystron;
The turning screw is used to adjust the frequency of the microwave signal.
2--Variable Flap Attenuator;
This changes the electric field amplitude in later parts of the system.
3--Frequency Meter;
This is for measuring the frequency of the microwave signal.
4-- Slotted Line;
5--Probe (connected to crystal detector, which basically senses the electric field amplitude at the
point where the probe is located);
6-- Slide Screw Tuner (not used in this laboratory);
7--Rectangular Horn Antenna; 8--Waveguide Stand;
9-- Terminations (not used in this laboratory);
10-- Lengths of Coaxial cable; 11--Cross Guide Coupler (not used in this lab);
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5—The Receiver System
Figure 7:Turntable for mapping antenna
pattern (the receiver)
1--Polarization Grating; 2--Standard rectangular waveguide;
3--Crystal detector; 4--- Turntable
Experiment #2: Frequency Measurements
Introduction:
The measurement of frequency is an important basic requirement in microwave
systems. We will use two methods to determine the frequency:
1. Resonant Cavity Frequency Meter;
2. Measurement of Guided Wavelength.
The working principle of the frequency meter is as follows: The meter consists of a
resonant cavity section connected in series with the transmission line (waveguide).
When the cavity is not resonant, the microwave energy flows in the waveguide without
interruption. When the meter is tuned to the correct position, the cavity is at resonance
and it “steals” (couples) energy from the waveguide. So that the power transmitted in the
transmission line is reduced. By monitoring the field amplitude in the latter part of the
waveguide and turning the frequency meter at the same time, you can determine the
position of the frequency meter at which the resonance happens. The position of the
meter actually corresponds to the volume of the cavity, which can in turn be related to
the resonance frequency. There is a calibration chart attached to the frequency meter,
showing the relation between the position of the meter and the resonance frequency.
In the second method, the guided wavelength of the TE10 mode of the rectangular
waveguide is measured. The concept of waveguide modes will be introduced in the latter
part of this course. For the purpose of this experiment, the only thing needed to be
understood is that there is a fixed relation between the guided wavelength g and the
operating frequency f:
Esc 400 H Engineering of Electromagnetic Fields Lab
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c
1  ( 2a /  g ) 2
2a
Where c is the light velocity in vacuum, a is the length of the wider side of the
waveguide rectangle (a=0.9 inches for the current system).
f 
Purpose


To get familiar with setting the klystron for oscillation and generation of microwave
energy in the waveguide system;
To perform and observe two methods for frequency measurement.
System Setup:
klystron
Flap
Attenuator
Klystron
Power
Supply
Square wave
Modulator
Waveguide
Frequency
Meter
Slotted
Line
Matched
Termination
Standing Wave
Amplifier
Figure 8: System setup for frequency measurement
Procedure:
1. Connect the equipment as shown in figure 8. Use at least two bolts in each flange joint at
diagonal corners. Make sure all flanges are properly aligned and tightly connected. Do not use
excessive force in tightening. Check all the cable connections.
2. Set the flap attenuator for maximum attenuation (15dB).
3. Set the standing wave amplifier as follows:
A. Adjust gain control to mid-range.
B. Set range selector switch to 0.
C. Turn the power switch to ON.
4. Adjust the power supply controls as follows:
A. Turn the beam and reflector voltage controls completely counterclockwise (0V).
B. Check the cable from the klystron mount to the output plug on the klystron power supply.
C. Turn the power supply ON. Allow two minutes for warm-up.
D. Set the reflector voltage to about 150 volts.
E. Adjust beam voltage to about 275 volts. (the beam current should be about 20ma.)
F. Increase the reflector voltage slowly until a slight dip of the beam current is noticed. If no
dip occurs, increase beam voltage to 300 volts, and readjust reflector voltage until dip is
noticed.
5. Adjust the square wave modulator controls as follows:
A. Set modulator frequency control to 5. Set modulator volts control to 0.
Esc 400 H Engineering of Electromagnetic Fields Lab
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B. Turn the modulator ON.
C. Increase the MOD volts control to about 7.
6. Set the flap attenuator to 0 dB. Turn range selector on standing wave amplifier to 30. If no
deflection of the meter is apparent, set the gain control fully counterclockwise and set the
range switch to 40. If greater than full scale reading is obtained, adjust range switch clockwise
until reading is on scale. Now increase the gain control to obtain about half-scale deflection.
7. Adjust the modulator frequency and voltage control for maximum reading on the standing
wave amplifier. Increase the attenuation on the flap attenuator as much as necessary to keep
the amplifier range switch on the 30 position with the gain control in the mid-range. If power
is too high, turn range switch to 20 scale.
8. Vary the reflector voltage control over its entire range. Oscillation as indicated by readings on
the standing wave amplifier may occur at more than one voltage. The voltage selected for
further use should be the one providing the most stable output. Slightly adjust the modulator
voltages and frequency controls for maximum indication on the standing wave amplifier.
NOTE: By now, you have finished setting the optimum operating condition for the klystron
oscillator. Keep in mind that you need the klystron power output to be stable, otherwise many
measurement results would be in error.
9. Slowly adjust the micrometer on the frequency meter until a dip in power is indicated on the
VSWR amplifier meter.
10. Adjust the micrometer carefully until the minimum meter reading is attained.
11. Read the micrometer position and determine the frequency from the calibration chart. This
completes the first method for frequency measurement.
Meter Reading: ____________; Corresponding Frequency: ____________(GHz)
Meter Reading: ____________; Corresponding Frequency: ____________(GHz)
Meter Reading: ____________; Corresponding Frequency: ____________(GHz)
12. Disconnect the matched load and connect a shorting plate (a metal) to the end of the slotted
line.
13. Moving the slotted line from one end to the other, determine the position where a minimum
occurs.
14. Record the position on the centimeter scale.
Minimum Positions: a. ____________; b. ___________; c. ____________.
15. Move the probe in the same direction and determine the next minimum.
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16. Record this position also. Notice that the VSWR meter should have gone from a minimum
through a maximum and back to a minimum.
17. The guide wavelength corresponds to twice the distance between two adjacent minimums.
Guided Wavelength: _______________
(from position of minimums)
18. Repeat the above procedure, but this time note the positions of the maximums. The distance
between maximums should be the same as the distance between minimums.
Maximum Positions: a. ____________; b. ___________; c. ____________.
Guided Wavelength: _______________ (from positions of maximum)
19. Using the formula described in the introduction to calculate the actual frequency.
Frequency: ________________(GHz)
Question:
Sketch the electric field distribution along the slotted line which was setup by a shorting plate
on the end of the slotted line. Why is the statement in step (17) true?
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Question:
Compare the frequencies determined by the two different methods. They should agree within
a few percent of each other. If they do not, could you have done something wrong?
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Experiment #3: Measurement of VSWR (voltage standing wave ratio)
Introduction:
The VSWR is defined as the ratio of absolute values of the maximum and the minimum
voltage for a standing wave: VSWR=|V|max|V|min. The absolute value of the reflection
coefficient ||, the reflected power ratio and transmitted power ratio can all be expressed
in terms of the VSWR.
Pre-Lab Question:
Write down the relation between the VSWR and amplitude of reflection coefficient ||:
What is the VSWR for a standing wave setup by a perfectly conducting metal plate?
=______;
VSWR=________=________(dB)
What is the VSWR for a standing wave setup by a matched load?
=______;
VSWR=________=________(dB)
Purpose:
To use the standing wave indicator to measure the VSWR of the standing wave setup by
different loads.
Procedure:
1. Setup the system as in experiment #2, if the setup has been changed.
2. Set the optimum operating condition for the klystron oscillator, if you think the condition
should be adjusted.
3. With a metal plate, a mathed termination, or a specimen (dielectric filled waveguide section)
connected as the load at the end of the slotted line, repeat the following steps:
A. Move the probe carriage on the slotted line until a maximum meter indication is read on
the VSWR amplifier. Adjust the gain of the amplifier until the needle lines up with
VSWR=1 or 0dB.
B. Move the probe carriage in either direction until a minimum meter indication is noted and
record the reading. If the meter did not go nearly off-scale to the left, the number
recorded should be between 1 and 3. This is the VSWR of the waveguide system under
Esc 400 H Engineering of Electromagnetic Fields Lab
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test. You can also read the VSWR in dB format fro the third (red) scale. Stop here unless
the meter goes off-scale to the left.
VSWR=___________=_________(dB) for Shorting Plate
=____________=_________(dB) for Matched-load
=____________=_________(dB) for the Specimen
C. Slowly move the probe carriage again until the needle approximately lines up with 3 on
the top curve. At this point turn the needle switch counterclockwise and you will see that
the needle now moves back to the right side of the meter. [question: did you increase or
decrease the gain of the amplifier?]
D. Move the probe carriage again until a minimum meter reading is indicated. Now read the
number on the second curve. You will note that the numbers on this scale range from 3 to
10. If the needle did not moves off-scale to the left, the number you read on the second
curve is the VSWR of the system under test. Add 10 to the number you read from the
third (red) scale, this is the VSWR in dB format. If the needle still moves off-scale to the
left, go to next step.
E. VSWR readings between 10 and 30 can be made by using a procedure similar to C and
D, switching the range switch counterclockwise when the needle reaches 10. With the
help of the range switch, the upper two curves combined together would then represent
VSWR values between 10 and 30. To get the VSWR in dB format, 10 dB is added to the
reading on the third (red) curve for each turning of the range switch.
Question:
1. Will the VSWR change if the klystron generates less power? Explain.
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Experiment #4: Measurement of the Permittivity of materials
Introduction:
In this experiment, you will have a chance to determine the permittivity of several low
loss materials: Plexi-glass, PVC, Eccogel and Teflon. Rectangular waveguide sections
filled with these materials are provided. They will be used as the load for the microwave
system, with a metal backing so as to setup a standing wave in the slotted line (figure 9).
You have already seen that, with a metal plate as the load, the voltage reaches minimum
at some fixed position on the slotted line. As illustrated in figure 9, the voltage minimum
positions will be shifted once the materials backed by a metal plate are connected at the
end of the slotted line. The displacement d will depend on the permittivity r of the
material. Theerfore the strategy for this experiment is to measure the displacement of the
minimum location and solve equations for the material parameter.
To establish the relation between displacement d and the permittivity r, we consider a
similiar problem. Suppose a plane wave in free space is normally incident upon a metal
plate coated with a lossless dielectric slab of thickness t and permittivity r, we can sketch
the standing wave profile as in figure 9(a). Choose the coordinate system such that the
plane wave is propagating along +z and the slab is occupying 0<z<t, then the
electromagnetic field can be expressed as:
E x ( z )  Em [e  jm ( z t )  e jm ( z t ) ]   j 2 Em sin[  m ( z  t )] (0  z  t)
E x ( z )  Eo [e jo ( z d )  e  jo ( z d ) ]  j 2 Eo sin[ o ( z  d )]
H y ( z) 
2E 
1   jm ( z t )
E m [e
 e jm ( z t ) ]  m cos[ m ( z  t )]
m
m
( z  0)
(0  z  t )
2E 
1  jo ( z d )
Eo [ e
 e  jo ( z d ) ]  o cos[o ( z  d )]
( z  0)
o
o
where  is the characteristic impedance,  is the wave number, a subscript m denotes
values in the material region, a subscript 0 corresponds to values in the free space region.
Notice that we assume the dielectric slab is lossless so that the standing wave in the air
region has min(|Ex|) = 0 on the plane z = -d. The way we get the field expression for the
region z<0 is to imagine there is a metal plate on z = -d. Because the tangential electric
field is zero there, putting a metal plate there will not change the electromagnetic fields
(once the standing wave has been established). If you are still not so sure how the above
formulas are written, refer to the textbook section about reflection from a plane conductor
at normal incidence.
H y ( z) 
Now since the tangential electric and magnetic fields are continuous on the boundary z=0,
the ratio of them is also continuous:
Ex
Hy
  jm tan[  m ( t )]  j0 tan[ 0 d ]
z 0
For the waveguide problem, the same equality can be obtained, with  denoting the
characteristic impedance of the TE10 mode and  meaning the z-direction wavenumber.
The TE10 mode impedance is given by:
Esc 400 H Engineering of Electromagnetic Fields Lab
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 TE 
j
j
so the equality can be written as:
 2d 
tan  m t   o


tan 
mt
2t
 o 
where o=2/o is the guided wavelength. This equation can be solved for the number m
t. The sign “” is added because the shift of the minimum can be d or o/2-d, depending
on how you choose the position of the minimum. Note that there are infinitely many
solutions to this equation, you have to choose the one which gives a reasonable value for
the material permittivity. (this means that you should have some prior knowledge on r).
To obtain an expression for r in terms of m (=m t / t), we use the relation that for the
rectangular waveguide filled with some material:

k m2   2 0  0 r   m2  ( )2 ;
a
 2m  ( / a) 2
4 2 f 2
where co is the light speed, f is the operating frequency, a is the wider length of the
rectangle of the waveguide (a=0.9 inches for the current system.)
Therefore:
 r  c 20
Probe
Source
Load
Slotted line section
material
t
|Vm(z)|
(a)
zm1
o
zm0
d
d
z
metal
plate
|Vo(z)|
(b)
z
z0
o/2
z1
Figure 9: Use slotted line and VSWR amplifier to determine permittivity of a nonmagnetic material.
(a) Standing Wave setup by material backed with metal plate;
(b) Standing Wave setup by a shorting metal plate.
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Purpose:
• To measure the permittivity of the given samples.
• To gain an understanding of measurement techniques.
Procedure
1. Setup the system similar to those in experiment #2 and #3. Adjust the klystron power supply
etc. for the optimimum operation condition of klystron oscillator.
2. Measure the position of two minimums associated with the metallic plate (z0, z1). Referring to
figure 9.
3. For each sample:
A. Measure the sample thickness (t)
B. Insert the sample between the slotted line and the metallic plate
C. Measure the positions of two minimums associated with the sample + metal plate (zm0,
zm1), referring to figure 9 for the relative position of these two points.
4. For each sample:
Compute 0 = 2(z0-z1), d = z0 - zm1
5.
For each sample's measurements:
Solve the following equation to find  m t
 2d 
tan  m t   o


tan 
mt
2t

 o 
where:
d = displacement of the minimum
o= guided wavelength of TE10 mode =2x0
Use the following formula to compute r:
 2  ( / a) 2
 r  c 20 m 2 2
4 f
where
a=0.9 inches
6. Compare these calculated values with the actual values (r)
Plexi-glass 2.65
PVC
2.18
Eccogel
2.71
Teflon
2.10
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Experiment #5: Determine the focal length of the parabolic reflector
Purpose:
measure the focal length of the parabolic reflector with two methods.
Procedure:
1. Set up the equipment as shown in figure 10.
2. Determine the position of focus by moving the waveguide and detector horizontally in
between the horn and parabola. Measure the distance between the open-end of the 90o bend
and the vertex of the parabolic reflector. This is the focal length of the parabola.
3. Measure the following two geometrical parameters of the parabolic reflector: x’ and 2y’.
Calculate the focal length from the parabolic equation y2=4px. (see figure 11).
Set up to find the focal point of the parabolic reflector
MODULATOR
Parabolic
reflector
POWER
Horn antenna
SUPPLY
KLYSTRON
FLAP
MOUNT
ATTENUATOR
Waveguide
Waveguide and
detector
Figure 10: System setup for finding the focal point of the
parabolic reflector
y
focus
2y’
X
p
x’
Esc 400 H Engineering of Electromagnetic Fields Lab
x
Figure 11: Geometry of the parabolic
reflector
43
Focal length : Measured=
Calculated =
Esc 400 H Engineering of Electromagnetic Fields Lab
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Experiment #8: Mapping the Power Distribution Inside a Microwave Oven
The domestic microwave oven is multimode cavity, i.e., many cavity modes are simultaneously
excited inside the cavity. The purpose of exciting many modes is to realize a uniform distribution
of power inside the volume and avoid the occurrence of hot spots which is not desirable for
uniform heating of food or other materials. The cavity dimensions are designed such that 2.45
GHz, the operating frequency of the oven is below the cut off frequency of the first several
modes of the cavity. Thus when power is launched from the magnetron through the launcher, all
modes with a cut off frequency of 2.45 GHz or below are excited. The total field at any point is a
linear superposition of these modes, akin to summing up several terms of a Fourier series
containing sin t, sin 2t, sin 3t, sin 4t, etc. The maxima and minima are all smeared out. Another
way to achieve uniform heating is by using a turn table. Some ovens use a mode stirrer, it is a
rotating plastic paddle at the opening of the launcher that stirs or mixes up the modes.
You can map the power distribution inside the cavity by heating a fixed volume of water with
known temperature. Place the cup sequentially on grid points on the floor of the oven, heating it
for the same period of time at each location and measuring the change in temperature of water.
Use the heat capacity of water to find the power absorbed in Joules. Place the cup at the same
grid points at two higher locations using empty upside down cups to support them and continue
volumetric mapping.
For a given temperature rise T, the heat Q is proportional to the mass m of the material to
which the heat is added. Therefore: Q=mC T
The constant of proportionality C is the specific heat of the substance.
water specific heat = 4.1784 J/g K at 30 deg C
density of water = 0.99565 g/cm3 at 30 deg C
Calculate Q for different places inside the microwave oven. This heat is generated from a
magnetron having microwave power of 800 Watts
Esc 400 H Engineering of Electromagnetic Fields Lab
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Question:
Suppose the conductivity of the water is a given constant over a temperature range (say 20~90
o
C), and the microwave oven generates uniform power density. With above experiment, would
you be able to determine the amplitude of the electric field? If yes, how? If not, why?
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