Lab 3
EE 3407 Electromagnetics
Lab 6
Application of electromagnetics to remote sensing
Note that remote sensing may refer to many types of noncontact applications, from
sensing of the environment from space and remote sensing of the cosmos from Earth,
to noncontact sensing of physiological parameters and remote infrared measurements
to assess temperature in industrial settings. Two everyday applications are the
automobile speed detection by radar on the roadside and weather maps provided by
satellite radar.
Remote sensing may be characterized as passive if the instrument consists only of a
receiver, or active if both receiver and transmitter (illuminator) are included in the
instrument. All objects emit energy due to thermal excitation of molecules and atoms.
For objects characterized as “blackbodies”, emission is predicted according to Planck’s
Law, i.e., E (T ) 
8hc 2

5
1
hc
kT
, where Eλ is energy per unit wavelength, k is
e 1
Boltzmann’s constant, h is Planck’s constant, and T is absolute temperature (oK).
Objects other than blackbodies, referred as “gray bodies,” demonstrate similar
temperature dependence but with lower emission strength. Note that the peak emission
at body temperature occurs in the thermal infrared range, i.e., at a wavelength of
approximately 10 µm. Passive remote sensing instruments, also often referred to as
radiometers, may operate in the microwave range up through the visible light range.
One example is the infrared or near-infrared goggles used for night vision.
For this lab work, emphasis is on active remote sensing systems, in particular on
radar and related proximity/field disturbance techniques. The Vector Network Analyzer
(VNA) will once again be used, but this time as a probing transmitter and receiver that
assesses interaction of electromagnetic waves with objects in the environment.
Two modes of instrument operation are used in this lab; the instrument is operated
in a step-frequency mode in which the instrument sequentially collects phase and
amplitude data over a bandwidth of frequencies. Also a single frequency mode of
operation is used in which phase variations over time correspond to object motion in the
environment. In the former the amplitude and phase data collected over frequency is
converted to a synthesized time domain mode, via use of an inverse Fourier Transform;
this operating mode of the instrument is often referred to as the Time Domain
Reflectivity (TDR) mode.
For our case, TDR will be applied through 1 and 2 antenna setups so as to emulate
a radar. Note that when applied through cables the TDR mode is useful for locating
damage or breakage in cables.
The basic ideas of the VNA in the TDR mode can be compared to the operating
principle of a simple pulse radar. In this case, distance to the object (also known as the
target) is determined based on the total time delay of the pulse from the radar
transmitter to the object and back to the radar receiver. Since the pulse propagates
through air, which is electrically equivalent to the free-space at microwave frequencies,
the time delay associated with an object at distance R is t = 2R/c, where c is the speed
of light.
The ability of the radar to discriminate between two closely spaced targets depends
on the width of the pulses that are transmitted. The radar can only discriminate between
two objects if the return (at the receiver) from the closer object is completed before the
return from the farther object begins. That is, for a pulse of width the relationship with
inter-object distance is =2r/c, where r is the inter-object spacing, implying that
r  c to ensure that the objects can be identified as separate objects.
2
The instrument setup is illustrated in Fig. 1. Each of the two dual polarization
antennas is connected to the network analyzer through coax with SMA connectors.
Note of caution: When connecting and disconnecting these coaxial cables, do
not use tools to tighten the SMA connectors as this can damage the connectors.
Only use fingers to tighten the connectors.
Dual polarization antennas
Port 2
Network
Analyzer
Port 1
d
Figure 1: Instrument setup.
Question 1: Step frequency mode measurements
• Set the instrument to start frequency of 2 GHz and stop frequency of 3 GHz. Set
the number of points in the sweep to 401 points.
• Then turn the transform on (through the SYSTEM menu).
This places the
instrument in TDR, i.e., synthesized time domain.
• Position the display on the screen by adjusting the reference value, and adjust
the size of the display by appropriately setting the dB/div value. You may adjust
the total time sweep by using the stop and start keys. The stop value should be
about 80 ns.
• If you have any questions about setting up the instrument, be sure to ask the TA.
• Make
sure you are monitoring log magnitude display in the following
experiments.
1.1 Effect of distance
•
Set the instrument to measure s21, i.e., placing the instrument in 2-antenna
mode.
•
Place a large object at a range of approximately 1 m from the antenna(s). Find
the time associated with the return using the marker key.
•
Move the object to a distance of 2 m, and find the time associated with the new
position.
•
Explain the relationship between the time difference of these two returns and the
difference in distances.
1.2 Effect of number of antennas
•
Set the instrument to measure s11, i.e., placing the instrument in 1-antenna
mode.
•
Place a large object at a range of approximately 1 m from the antenna (note that
you need to decide which one), and find the time associated with the return using
the marker key.
•
Then move the object to a distance of 2 m, and find the time associated with the
new position.
•
Explain the differences you observe between the 1-antenna and 2-antenna
returns.
1.3 Effect of frequency range
•
Turn off the transform. Change the start frequency to 2.7 GHz and leave the stop
frequency at 3 GHz. Turn the transform back on.
•
Set the instrument to measure s21, i.e., placing the instrument in 2-antenna
mode.
•
Place a large object at a range of approximately 1 m from the antenna(s), and
find the time associated with the return using the marker key.
•
Move the object to a distance of 2 m, and find the time associated with the new
position.
•
Explain the differences you observe between the returns with 1 GHz bandwidth
and 300 MHz bandwidth, i.e., the differences related to frequency range
resolution.
1.4 Effect of number of points
•
Change the number of measurement points to 51.
•
Set the instrument to measure s21, i.e., placing the instrument in 2-antenna
mode.
•
Place a large object at a range of approximately 1 m from the antennas, and find
the time associated with the return using the marker key.
•
Then move the object to a distance of 2 m, and find the time associated with the
new position.
•
Change the number of measurement points to 401.
•
Place a large object at a range of approximately 1 m from the antennas, and find
the time associated with the return using the marker key.
•
Then move the object to a distance of 2 m, and find the time associated with the
new position.
•
Explain the differences you observe between the returns with 51 points and 401
points. Note the effect you will see relates to “undersampling”, and leads to a
limitation of the maximum distance at which measurements can be obtained, i.e.,
an effect referred to as maximum unambiguous range.
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