Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
Laboratory 6: APPLIED ELECTROMAGNETIC
OBJECTIVES
- Introduce the students to fundamental concepts of remote sensing applications using
microwave frequencies.
- Familiarization with Basic principles of Scatterometer.
- Observe the behavior of reflected electromagnetic waves on different types of
terrains.
- Observe the behavior of reflected electromagnetic waves in the cables.
MATERIAL AND EQUIPMENT
- Two X-band Horn Antenna: NARDA- 640
- Coaxial Cables
- Waveguide/Coaxial transitions, waveguide sections
- Network Analyzer 8917ES
- Oscilloscope Agilent 54622A, 100 MHz
- Plastic containers, sand, gravel, water.
- Coaxial cables
- Load of 50Ω, Short and Open circuit.
- T-magic and adapters.
Introduction
Electromagnetic waves are used as means of transmitting energy and information,
typical examples of Electromagnetics waves are: radio waves (RF), television waves,
radar waves, X-rays, infrared, cellular telephones, microwaves, and visible light.
All electromagnetic waves behave according to Maxwell equations, which define
how the waves travel in different media such as air, ocean water, clouds, etc. All media
can be classified as belonging to one of these groups: free space, lossless dielectrics,
lossy dielectric and good conductors. Depending on the media, the attenuation and phase
change of the EM wave can be characterized with the attenuation (α) and phase constant
(β) respectively. When a wave traveling in air encounters another medium, a fraction of
the energy carried by the wave is transmitted while another fraction is reflected at the
boundary. The parameter that quantifies the amount of energy transmitted and reflected
are the transmission (τ) and reflection (Γ) coefficient respectively. These coefficients are
related to the electrical parameters of the two media as:
Γ = (η2 - η1)/ (η2 + η1) and τ = 2η2 / (η2 + η1) where η = µ / ε ; ε and µ are the
permeability and permittivity of the medium, respectively. For the equation above,
normal incidence and lossless media (∞=0) has been assumed.
The reflection caused by the boundary can be studied to obtain information about the
media encountered by the EM wave. Depending on the kind of surface and composition
of the media, the reflection of the EM wave will react accordingly. For example, the
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
response of a flat surface will differ from that of a rough surface. Wet conditions in a
terrain will have a different response from the same media when is dry.
Other factors such as polarization, angle
of incidence, and frequency of operation
also affects the EM reflection from a
surface. This is the basic principle of
many remote sensing applications, such
as detection of ice salinity using radar
systems, tracking of oil spills in the
ocean, identification of terrain for
agricultural applications, etc. See Figure
1.
Fig 1. NASA TRMM-Microwave radar studying the clouds within a hurricane.
In this particular project, a network analyzer configured as a scatterometer will be used to
observe the reflection from different types of terrain under several conditions. A
scatterometer is type of radar that sends electromagnetic waves and measures the amount
of energy scattered back from the object under test. The scatterometer is always looking
at a normal angle of incidence; the instrument will be used to observe the shape of the
pulse received due to reflections from the surfaces. Change in amplitudes and distances
can also be observed with the network analyzer. This demonstration is intended for
freshman and pre-college students.
PROCEDURE
Initial Calibration of Network Analyzer
Before performing any measurements with the Network Analyzer, the instrument has
to be calibrated depending on the measurement to be done. The HP8917ES Network
Analyzer has the option of measuring the S parameters in the frequency or time domains.
S-parameters will provide information about the transmission and reflection of a signal
incident on a device under test. In this laboratory we are interested in looking at the
inverse transmission parameter, S21 as a function of time. The following steps describe
the necessary calibration process of the equipment:
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Turn on the Network Analyzer and wait for the automatic self-test to finish.
Choose the parameter that is to be measured and look up in the options of Response
the option Meas and then Trans: REV S21 (A/R).
- Adjust the frequency range from 8.2 to 12.4 GHz. To do this look under the options
Stimulus the keys Start and Stop, and enter the initial and final frequencies.
Start > 8.2 GHz
Stop > 12.4 GHz
This is the operational range of the antennas.
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
Measuring Equipment
AntennaT
SMA Coaxial
Reference Plane
Transitions
Figure 2 - Network Analyzer with Waveguide transitions.
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There are two SMA cables connected to port #1 and port #2 of the network analyzer.
Using the adapters from N-type to SMA that you have available connect the cables to
the waveguide transitions and screw them tight together as figure 2 shows.
Go to the “Display menu” and press “Data-to-memory” to store the displayed data in
the memory of the network analyzer. Now select under the “Display menu” the
option “Data/Mem” to display the current data normalized with respect to the data
stored in memory.
In the “scale” option look for “autoscale” and press the corresponding key. The
analyzer will automatically fit the data on the display. You can use this key anytime
you need to fit the displayed data in the screen.
By default, the data amplitude is displayed in dB. Make sure that your response is
now very close to 0 dB along all the frequency range. This means that the network
analyzer has performed a correction for the attenuation introduced by the cables, in
addition any distance is measured now with respect to the reference plane.
By default the equipment normally sets itself to work on the frequency domain. For
this experiment, it must be changed to the time domain. Look for the options of
Instrument State and select:
System > Transform menu > Transform On
Select the time range desired to be displayed in the monitor. Press the button
Stimulus and set the initial and final time.
Start > 1 nS
Stop >20 nS
Make sure the parameter that is to be measured is selected. Response the option
Meas and then Trans: REV S21 (A/R).
To better visualize the measurement, change the scale format from dB to linear. Look
up in Response the option Format > Lin Mag
Select the options Response > Scale Ref > Auto Scale, for a better view in the
display.
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
Scatterometer Configuration
Using waveguide sections and the two horn
antennas make the necessary waveguide
connections to place the antenna on the table
looking straight to the floor. The antennas must
be next to each other and close together to make
sure they are looking at the same area. The
antenna connected to port #1 is the transmitting
antenna while that connected to port #2 is the
receiving antenna.
Figure 3- Set up of equipment
Measuring Equipment
AntennaTx
Antenna
Floor
Figure 4 – Network Analyzer with horn antennas connected at each of the ports.
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Set up of the experiment according to figure 3 and 4.
Now look at the display and locate a pulse return. What do you think represents this
pulse? Explain. Using the marker record the maximum amplitude.
_______________________________________________________
_______________________________________________________
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Use the Marker option to read the distance in meters shown in the display of the
network analyzer? What represents this distance? Use a tape measure to verify.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
AntennaT
Antenna
Plastic Container with
sand, rock, gravel
Figure 5 – Container with terrain illuminated by antenna.
Place the container with sand below the antennas. Make sure the sand is centralized with
respect to the antennas. The sand should be dry and smooth, measure the maximum
amplitude of the new response. Is it different to the response from the floor? Explain.
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________________________________________________________________________
Move the sand around, make it smooth and repeat the process. Did you get the same
response? Why? Try to obtain to exact responses by repeating this process. Can you do
it?
________________________________________________________________________
________________________________________________________________________
Move the sand around, but this time, do not try to keep it smooth. Did you get the same
response? Why? What happened to the pulse response?
________________________________________________________________________
________________________________________________________________________
Place the metal plate provided on top of the sand, right below the antennas. What do you
expect to see in the display? Explain.
________________________________________________________________________
________________________________________________________________________
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
If you bury the plate in the antenna, will you still see a response corresponding to the
metal plate? Try different depths and explain what you observed. Is this frequency good
for ground penetrating radar applications?
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________________________________________________________________________
Place the container with rocks. Make sure it is centralized with respect to the antennas.
What do you expect to see in the display of the network analyzer when looking at the
rocks?
________________________________________________________________________
________________________________________________________________________
What happened to the pulse shape? Does it make sense? What is the maximum
amplitude now? Explain.
________________________________________________________________________
________________________________________________________________________
Polarization
Remove the container and look at the floor again. Now rotate the receptor (or
transmitter, whichever is easier) antenna by 90° and verify if there are any receptions in
the Network Analyzer display. Explain what happens with the received signal.
________________________________________________________________________
________________________________________________________________________
Repeat the procedure but with the smooth sand and the rocks. What did you observe? Is
the terrain sensitive to polarization? Why? Explain how polarization can help identify
terrain.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Final Question
Explain what will happen with the pulse shape that you see in the display if the
experiments are repeated at a 45 degrees incidence.
Transmission Line: Reflections in Coaxial Cables
We will configure the functions generator to produce a train pulses that will be
transmitted through a 100 ft coaxial cable. The transmitted waveform into the cable will
then be observed in the oscilloscope when there are a 50Ω load, an open circuit and short
circuit at the end of the cable. Thus, we will observe the reflections in a circuit when
there are different conditions.
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Universidad de Puerto Rico en Mayaguez
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Electrical and Computer Engineering Department
Toys and Tools Lab
First build the circuit as is shown in Figure 6.
Adjust the waveform generator so that it will supply a rectangular signal of 5MHz
and a duty cycle of 20 percent. For this:
o Set the frequency to 5MHz
o Set a rectangular signal with an amplitude of 5Vpp
o Modulate the signal pressing Shift, FSK.
o Press Shift, Duty Cycle and arrange a 20% cycle.
o Press offset and set a 2.5 VDC voltage.
To observe the signal in the oscilloscope press Auto-Scale.
Figure 6
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Once the equipments are calibrated and the coaxial cable has a load of 50Ω at the
end, you should be able to observe the following signal:
This is the signal observed at the input terminal of the transmission line when its
characteristic impedance is matched to a 50Ω load.
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Retire the 50Ω load of the output terminal of the transmission line to obtain an open
circuit. Now, you should be able to observe a change in the waveform at the input
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
terminal of the cable. This happens by the reflections caused by a line terminate at an
open circuit. Draw the shape of the input signal and identify the reflected pulses.
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Place a short circuit at the output terminal of the transmission line. You should be
able to observe a change in the waveform at the input terminal of the cable due to the
reflections. Draw the shape of the input signal and identify the reflected pulses.
Time Delay and cable length Measurement
When a signal is transmitted through a cable, it experiences a time delay until arriving at
the output terminal. The signal has a propagation velocity into the cable and takes it a
certain time to arrive to the out. This delay is proportional to transmission line length.
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Place a T junction at the output terminal of the coaxial cable with the 50Ω load, the
free port must be connected to the channel 2 of the oscilloscope.
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Enable the channel 2 to observe the output signal. You should be able to observe that
two pulses shifted. The shift or time delay depends of the transmission line length.
Draw the shape of the input and output signal and identify the reflected pulses.
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With the oscilloscope measure the input Voltage _____ and the output Voltage
_____. When a system is paired the impedance is the same, which means that it won’t
have any loss. So the input voltage should be equal to the output voltage at the end.
Laboratory # 6. - Applied Electromagnetic
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
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Using the time delay between the signals we can measure the length of the cable.
Measure the time delay that exist between two peaks consecutives of the signal
t=_______; to do this use the cursors and place one at each peak. The ∆x equals the
time that it takes the pulse to travel from the entrance to the end of the cable.
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With this measurement and the propagation velocity we can determine the cable
length. Notice that the percentage velocity for this coaxial cable is 66%. This
means that the velocity is 66% of light velocity.
Distance = Velocity x Time
Distance = .66(3.0 x 10^8) x Time (m/s)
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Based on the previous information what is the cable length? L=_______ mts.
Compute the error percentage e=________
Transmission lines terminated at Open and Short Circuit
When the load is removed the circuit the output impedance is infinite. The wave is
propagated through the coaxial cable with a characteristic impedance of 50Ω, and when it
reaches the open circuit there is a discontinuity for the wave and reflections are produced.
Then we can observe another wave with the same voltage ( V + ) and equal sign.
Reflection Coefficients
Γ=
Ζ L − Ζ0 1 − Ζ0 / Ζ L
=
=1
Ζ L + Ζ0 1 + Ζ0 / Ζ L
V−
Γ= +
V
V − = ΓV + = V +
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Build the circuit as shown in Figure 6:
Observe in the oscilloscope the signals of the channels 1 and 2. Measure the output
voltage _______. Explain the amplitude of the output signal in terms of the reflected
wave and incident wave?
_______________________________________________________________________
________________________________________________________________________
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The second pulse that can be observed from channel 1 represents the pulse that was
reflected by the open circuit and gives information to measure the cable length from
input terminal to the open circuit (cable length). Measure the shift in between both
curves, i.e. the time it will take the wave to travel from the generator to the load or
from the load to the generator, t= _____. With this information you obtain the cable
length ______.
Laboratory # 6. - Applied Electromagnetic
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Universidad de Puerto Rico en Mayaguez
Electrical and Computer Engineering Department
Toys and Tools Lab
Figure 6
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Substitute the open circuit by a short circuit at the output terminal of the transmission
line. In order to observe the signals in the oscilloscope you should synchronize the
edge trigger with the channel 1. For this, press the bottom Edge and select the
channel 1.
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Observe in the oscilloscope the signals of the channels 1 and 2. Draw the shape of
the input and output signal and identify the reflected pulses.
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Measure the exit voltage._____ Explain the amplitude of the output signal in terms of
the reflected wave and incident wave?
________________________________________________________________________
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The second pulse that can be observed from channel 1 represents the pulse that was
reflected by the short circuit and gives information to measure the cable length from
the input terminal to the short circuit (cable length). Measure the shift in between
both pulses, t= _____. With this information you obtain the cable length ______.
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