eee334_2005_exam

Data Provided: None

The University of Sheffield

DEPARTMENT OF ELECTRONIC AND ELECTRICAL ENGINEERING Spring Semester 2004-2005 (2 hours) Antennas, Radar and Navigation 3

Answer THREE questions. No marks will be awarded for solutions to a fourth question. Solutions will be considered in the order that they are presented in the answer book. Trial answers will be ignored if they are clearly crossed out. The numbers given after each section of a question indicate the relative weighting of that section.

1. a. In the context of antenna design define the terms: gain, directivity, efficiency and

effective area.

b.

A 10GHz satellite comms link consists of a 3.0m diameter dish transmit antenna with an aperture efficiency of 0.7, and a receive dish antenna of 1.4m diameter with an aperture efficiency of 0.55. If the distance between the link is 35787km and the the transmit power is 100W, calculate the magnitude of the received power. c.

An electrically large aperture antenna has 3dB far-field principal plane beamwidths of βx and βyrespectively. Derive an approximate expression for the

gain of the antenna, stating any assumptions you make

d.

Estimate the gain of an antenna with azimuth and elevation 3dB beamwidths of 3 degrees

2. a. With the aid of simple sketches, explain how the radiation pattern of an array can

be expressed in terms of the element pattern and an array factor

b.

Derive an expression for the normalised array factor of a simple 2-element array with element spacing d and uniform, equal phase excitation

Plot the array factor for an element spacing of λ/2.

c.

List the some of the advantages of a two-dimensional electronically scanned phased-array antenna compared to mechanically scanned reflector antenna

3. a. Derive the radar range equation.

b.

A radar system operating at 9.4GHz uses a common transmit/receive antenna with an effective aperture size of 3.4m (horizontal) by 0.75m (vertical) and a first sidelobe level of –20dB relative to the main beam. Calculate the approximate EEE334 1

TURN OVER

(4)

(8)(6)(2)

(4)

(12)

(4)(6)

EEE334

azimuth and elevation beamwidth and estimate the antenna gain.

If the transmit power is 12kW, the noise level is –130dBW and the total losses c.

are 4dB, calculate the maximum range at which an aircraft with an RCS of 1m2 could be detected with a SNR of 13dB. Calculate the RCS of a target that would produce a similar output level at half d.

(5)

(5)EEE334 2 this range if detected by the antenna’s first sidelobe.

(4)

CONTINUED

EEE334

4. a. Explain the terms: spot jamming, sweep jamming and barrage jamming.

What are the three standard jamming tactics used in EW?

b.

A 10GHz radar with a boresight gain of 40dB and a peak transmit power of 100kW is used to track a target with an RCS of 1m2. The radar antenna also receives a jamming signal in a sidelobe which has a gain of 10dB. The jammer operates at a distance of 100km from the radar and has an antenna gain of 30dB and a transmit power level of 1kW. Calculate the burnthrough range. c.

With the aid of a block diagram, describe the basic operation of a continuous wave Doppler radar system.

In Figure 1, A denotes a moving police car with an onboard CW Doppler radar operating at 10GHz. The radar detects returns from each of the targets B to F simultaneously and without obstruction. Sketch a graph of the frequency spectrum of the radar receive signal indicating the features that correspond to each of the targets B to F. Assume that the magnitude of the received signals from each target is identical. The speed of each target is given in the table below and the direction of travel is indicated by the arrows in Figure 1.

Object Description A Police car with 10GHz radar travelling at 80km/H B Car travelling at 130km/H C Car travelling at 60km/H D Car travelling at 100km/H E Car travelling at 100km/H F Stationary road sign

45o A F BD E C Figure 1

AT / GGC

EEE334 3 END OF PAPER

(3)

(7)(4)

(6)