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 (conductivity) = 0 and hence, (permeability) and (dielectric constant), are real quantities. The propagation constant is then imaginary and can be written as: rr0jkjwj === (1) where 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 += +=+= (2) where is attenuation coefficient and 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 Snells Law: t2r1i1 sinksinksink == (3) By simplifying it we will get a vastly simplified equation as shown: i 2 1 t sinsin = (4) As i increases from 0 to 90 degrees, the refraction angle, t will increase at a faster rate than that of i . The incidence angle for which t = 90, is also known as the critical angle, c . 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 Snells law in this way: 1 2 csin = (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: i t 0 0 0.1 0.0979 0.2 0.1870 0.3 0.2587 0.4 0.3055 0.5 0.3218 0.6 0.3055 0.7 0.2587 0.8 0.1870 0.9 0.0979 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 t against i 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. References [1] David M. Prozar, Microwave Engineering 2nd Edition, 1997 [2] Merrill I. 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