"Through-wall radar sensors are sensors which allow the user to "see" through walls. By sending microwave signals from a directional transmitter (also known as horn transmitter), through a wall, reflecting it against the subject and back, the radar would be able to determine whether if there is a person behind the wall and how far away he is. Commercial applications for such through-wall radars would help police and military force greatly, allowing them to "see" their enemies before they are seen in the field. A detailed study on the properties of the electromagnetic wave propagation through a building wall is conducted to determine the important parameters that affect the performance capability of through-wall radar. Measurements are carried out to verify the results obtained from the field."

1. THE PROPAGATION PROPERTIES OF ELECTROMAGNETIC WAVES IN THE
APPLICATION OF THROUGH-WALL RADAR SENSORS
Teo Jun Wei Andre1
and K. H. Lee 2
Abstract
Through-wall radar sensors are sensors which allow the user to “see” through walls. By sending
microwave signals from a directional transmitter (also known as a horn transmitter), through a wall,
reflecting it against the subject and back, the radar would be able to determine whether if there is a person
behind the wall and how far away is he.
Commercial applications for such through-wall radars would help police and military force
greatly, allowing them to “see” their enemies before they are seen in the field.
A detailed study on the properties of the electromagnetic wave propagation through a building
wall is conducted to determine the important parameters that affect the performance capability of through-
wall radar. Measurements are carried out to verify the results obtained from the study.
Introduction
Microwaves are defined as alternating current signals with frequencies between 300 MHz and 300
GHz, complimented with a resulting electrical field perpendicular to it with a wavelength between 1 mm
and 1 m.
Assuming that the wall is a losseless medium, then σ (conductivity) = 0 and hence, µ
(permeability) and ε (dielectric constant), are real quantities. The propagation constant is then imaginary
and can be written as:
rr0jkjwj εµ=µε=β=γ (1)
where β is the phase coefficient
Equation (1) shows that the propagation constant is affected by the operating frequency and the
material or medium the wave is propagating.
If the wall is also assumed to be a good conductor, the propagation constant is now rewritten as:
( )
s
1
j1
2
)j1(j
δ
+=
ωµσ
+=β+α=γ (2)
where α is attenuation coefficient and δ is skin depth.
_____________________________________________________________
1
Raffles Junior College, SRP student
2
Sensor Systems Division, Defence Science & Technology Agency

2. Equation (2) proves that attenuation, or the weakening of the signal strength, is also reliant on the
operating frequency and material.
The reflection and transmission coefficient can be found using Snell’s Law:
t2r1i1 sinksinksink θ=θ=θ (3)
By simplifying it we will get a vastly simplified equation as shown:
i
2
1
t sinsin θ
ε
ε
=θ (4)
As iθ increases from 0 to 90 degrees, the refraction angle, tθ will increase at a faster rate than that of iθ .
The incidence angle for which tθ = 90°, is also known as the critical angle, cθ . Beyond the critical angel,
the incident wave will be totally reflected as it is not propagated into the medium. The significance of this
is that at the critical angle, none of the wave will get reflected away by the surface of the medium, in this
case the wall. Hence, we can rewrite Snell’s law in this way:
1
2
csin
ε
ε
=θ (5)
Using the computing software MATLAB® to solve equation (5) and assuming that the dielectric
constant of the wall is 10, the following relation is obtained when a wave propagates from air to the wall:
iθ tθ
0 0
0.1π 0.0979
0.2π 0.1870
0.3π 0.2587
0.4π 0.3055
0.5π 0.3218
0.6π 0.3055
0.7π 0.2587
0.8π 0.1870
0.9π 0.0979
π 0
Table 1 shows the relationship between incidence angle and refraction angle
From table 1, it is not difficult to observe that propagating wave at normal (90 degrees) penetrates better
into the 2nd
medium.

3. Figure 1 shows the plot of θt against θi using the data shown in table 1
Experiment
A signal generator tuned to a carrier frequency 2.4 GHz and output power 0 dBm is used to
simulate a transmitter, it is connected to a transmitting antenna which is a horn. A Spectrum Analyzer is
used to measure the signal after the wall; it is connected to an omni-directional antenna. The experimental
setup is shown in figure 2.
Figure 2 shows the experimental setup

4. The designed procedure to measure the signal penetrating into a wall is as follows:
i. Set up the apparatus as shown in figure 2.
ii. The signal generator is connected to the spectrum analyzer for verify the working condition of
the equipment, the measured strength include losses due to cables.
iii. Switch on the transmitter and take the reading of the signal strength, s, at 0 m (for computation of
antenna loss) and 0.50 m away from the transmitting antenna (this is to measure the signal
strength without the wall).
Figure 3 shows the measurement of signal at 0.5 m away from the transmitting antenna
iv. The transmitting antenna is now placed at directly behind a wall of thickness, 0.15m.
Figure 4 shows the transmitter setup, the transmitting antenna is pointing directly at the wall

5. v. A string 0.35 m long is attached at the opposite side of the wall, at the spot where the
transmitting antenna is pointing.
vi. Using the string and a compass, mark out positions of 0°, 20°, 45°, 70° and 90° (all at 0.35 m
away).
vii. With 0° as normal to the wall and 90° as along the wall. Mark the angles both in the clockwise
direction and anti-clockwise direction.
viii. Move from one position to another in the clockwise direction, starting from the normal. Take
readings from the signal analyzer and record them.
Figure 5 shows the measurement at the other side of the wall (at 90 degrees)
ix. Repeat step viii, starting from the normal and moving in a anti-clockwise direction.
x. Results are recorded in a table.
In the measurement s = -33 dBm when the distance, d = 0.50 m of free space (without the wall).
When the wall is present, the following measurements are obtained:
Signal Strength (dBm)
Incidence angle
clockwise anti-clockwise
0 -41 -40
20 -48 -50
45 -50 -51
70 -54 -54
90 -57 -57
Table 2 shows the strength of the signals obtained when a propagating wave penetrates a wall
of thickness of 15 cm

6. Discussion
From the results study and measurement, it is verified that best detection performance is obtained
if the radar sensor pointing is normal to the wall. The attenuation due to wall must also not be neglected,
from the study; it is shown that the wall (concrete) introduces 6 dB loss when the transmitting antenna is
normal to the wall. Higher losses are observed when the transmitting antenna is inclined at an angle. The
loss will be double in the case of detection using radar as the wave hits the target and return to the radar
sensor. Different types of walls give different attenuation.
Acknowledgement
The authors would like to express their gratitude to Temasek Engineering School, Temasek
Polytechnic for allowing them to use the Microwave, RF and Antenna Competency Unit to carry out the
experiment, without which, the study would not be fruitful.
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