FACULTY OF ENGINEERING

LAB SHEET

EMF4096 RADAR SYSTEMS DESIGNAND ANALYSIS

TRIMESTER 2 (2018/2019)

RS1: Radar Cross Section of Simple Targets

VENUE: APPLIED ELECTROMAGNETICS LAB

Students are advised to read through this lab sheet before doing experiment.

*Note: On-the-spot evaluation may be carried out during or at the end of the experiment. Students areadvised to read through this lab sheet before doing experiment. Your performance, teamwork effort, andlearning attitude will count towards the marks.

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RADAR CROSS SECTION OF SIMPLE TARGETS

Objective:To examine the radar cross section of flat plate and trihedral reflectors.

Apparatus:Klystron Power Supply Waveguide diode detectorKlystron Oscillator L-Section rotary jointIsolator Waveguide sectionsCavity Wave meter Tripod standSlotted-line probe detector Horn antennaSWR meter BNC coax cableVariable attenuator Flat-plate reflectorProtractor Trihedral reflector

Introduction / Theory:Radar is a system that allows the location, speed, and/or direction of a vehicle to be tracked. The word "radar" is actually an acronym for RAdio Detection And Ranging, since the device uses radio waves to detect targets. Radar works by sending out pulses of these electromagnetic waves and then "listening" for echoes that are bounced back by the target of interest.

Figure 1 Basic Elements of a Radar System

Even though a radar may transmit megawatts of power, only a tiny fraction of that energy is typically bounced back to be received by the radar antenna. The amount of power returned from a target depends on four major factors:1. The power transmitted in the direction of the target2. The amount of power that impacts the target and reflected back in the direction of the radar3. The amount of reflected power that is intercepted by the radar antenna4. The length of time in which the radar is pointed at the target

Factors that determine the energy returned by a target.

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A term used to describe the relationship between these variables is power density, sometimes also called power flux. The power transmitted by a radar is dissipated the further it travels because it is spread over an increasingly larger area. The area over which the power is spread is proportional to the square of the distance, or range (R), from the transmitting radar. The amount of power spread over a given area is called the power density, and this quantity decreases by the square of the range. The power density of the transmitted radar wave at the range of the target is called the incident power density (Pincident).

Effect of distance from the radar to the target on the power density

Once the radar power reaches the target, some portion of that power will be reflected back to its source. However, this reflected power also dissipates and spreads out as it echoes back to the radar receiver. Since the power density has already been reduced by a factor of 1/R2 by the time it reaches the target and is again reduced by 1/R2 on the return trip, the final power density of the energy received by the radar is proportional to 1/R4. The ability of radar to detect the target depends on whether the amount of powerreturned is large enough to be differentiated from internal noise, ground clutter, background radiation, and other sources of interference.

The amount of power that is reflected back to the radar depends largely on a quantity called the radar cross section (RCS.) Although RCS is technically an area and typically expressed in square meters (m2), it is helpful to break the term apart to better understand what it means. Radar cross section is usually represented by the Greek letter σ, and the quantity depends on three factors.

1. Geometric cross section:The geometric cross section refers to the area the target presents to the radar, or its projected area. This area will vary depending on the angle, or aspect, the target presents to the radar. The geometric cross section (A) determines how much power transmitted by the radar (P incident) is intercepted by the target (Pintercepted).

2. Reflectivity:Reflectivity refers to the fraction of the intercepted power that is reflected by the target, regardless of direction. In addition, some radar power is usually absorbed by the target. Regardless, the power that is reradiated, or scattered, after reflecting off the target is equal to the intercepted power less whatever portion of that power is absorbed by the target. Reflectivity is defined as the ratio of power scattered by the target (Pscatter) to the power intercepted by the target (Pintercepted).

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3. Directivity:Directivity is related to reflectivity but refers to the power scattered back in the direction of the transmitting radar. The power that is reflected toward the radar is called the backscattered power (Pbackscatter). We've already noted that radar energy is not reflected evenly, but directivity is defined as the ratio of the power that is backscattered in the direction of the radar to the power that would have been scattered in that direction if thescattering were in fact uniform in all directions. If the power were to scatter equally, itwould form a sphere expanding uniformly in all directions from the target. This type of behavior is called isotropic expansion. Isotropic power (Pisotropic) is defined as the powerthat is scattered in a perfect sphere over a unit solid angle of that sphere.

It is mentioned that the power reflected by the target can be much stronger in somedirections than in others. As a result, that reflected power will be much greater or muchsmaller than the isotropic power depending on how the target is oriented to thetransmitting radar. The directivity, therefore, will be much greater than 1 when thetarget returns a strong backscatter in the direction of the radar and much less than 1when the backscatter is small.

These three factors can be combined to determine the complete radar cross section fora target.

The importance of radar cross section can best be understood by looking at an equationrelating the RCS of the target to the energy received by the radar.

Pr=PtG

2 λ2σ

(4 π )3R4

wherePr = power received by the radarPt = power transmitted by the radarG = gain of the radar antennaσ= radar cross section of the targetR = range to the target

Procedure:

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CW radar set-up

1. The equipment should have been set-up as shown above. Verify the set-up.2. The Klystron sources together with the waveguide components are used as the continuous wave transmitter. Two sets of horn antennas are used as the transmitting and receiving antennas. The receiver antenna is connected to a diodedetector and SWR/Power meter.3. The targets are placed on the turn table in the direction of the antennas. The turntable is used for the accurate angle measurement.4. Place the target on the turn table and align it to face the horn antennas. You needto ensure that the target and horn antennas are aligned properly. Take your time to align as this is a critical step in this experiment.5. Switch on the Klystron source and wait for a few minutes to allow the source toreach operating temperature.6. Tune the repeller voltage, level adjust knob and freq knob of the Klystron power supply repeatedly to attain maximum power generation.7. Note that if alignment was done properly, the return power would be maximum.8. The power meter connected to the slotted line probe measures the transmitted power and the power meter connected to the detector measures the receivedpower.

RCS dependence on aspect angle

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1. Two simple targets are provided for this experiment. A flat plate and a trihedral reflector.2. Place the flat plate on the turn table (with the plate facing the horn antennas), on the 0°-180° line (90° is pointing to the horn antennas).3. Adjust the horn antennas and the position of the experiment set, to achievemaximum return power.4. Turn the flat plate at small incremental angles. Record down the power received.Continue the measurement for angles up to 60°.5. Plot the received power vs. angle in a graph.6. Replace the flat plate with trihedral reflector and repeat steps 2 to 5.

Evaluation of RCS for simple targets

1. Place the flat plate on the turn table (with the plate facing the horn antennas), on the 0°-180° line (90° is pointing to the horn antennas).2. Record down the maximum received power for this case and the distancebetween target and radar antennas.3. Measure the transmitted power.4. If the gains of the horn antennas are given as 17dB, and the frequency of thesource is 8.5GHz, calculate the RCS of the flat plate.5. Repeat steps 1 to 4 for the trihedral reflector.6. Calculate the theoretical RCS of the flat plate and trihedral reflector and comparethem with their respective measured RCS. Equations for the theoretical RCS aregiven below.

PRECAUTION:

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Never look directly into horn antenna when it is transmitting.Do not touch the Klystron source while operating.The horn antennas must not be touching each other while operating.

Questions / Discussions:

1. Compare the calculated (theoretical) RCS and measured RCS. Briefly discuss the causes for the difference between calculated and measured results.2. Reflection can cause errors in RCS measurements. Describe all the items in the lab that can cause this reflection in your experiment.3. Are the targets in the far field of the antenna? If yes, demonstrate about it.4. Illustrate the advantages of a trihedral reflector compared to a flat plate, with regards to RCS?

Report guidelines:

Your report should include:1. A brief introduction of the experiment.2. Summary of the steps taken.3. Results and analysis.4. Discussion on the results.5. Answers to the above questions.6. Conclusions.

Report is to be submitted to the Applied EM Lab within 10 working days from your lab date.

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