MSc Map Antennas and Propagation module exam 2000 (DJJ questions)


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Antennas notes.

Question 1.

(a)
Define the terms effective aperture, gain, efficiency, E-plane radiation pattern, boresight direction, null, half-power beamwidth and polarisation for a large Cassegrain reflector antenna. Use diagrams to illustrate your answers, where appropriate.

(b)
Explain why all practical antennas necessarily have maximum directivity greater than unity.

(c)
Give three methods which might be used to generate circular polarisation for a low-earth-orbit satellite antenna communication system.

(d)
A deep space communication system uses a Cassegrain antenna of diameter 70m at a frequency of 8.45 GHz.

(i)
Determine the gain of this dish (in dBi) assuming an aperture efficiency of 80%

(ii)
Determine the power received by this dish from a transmission from a satellite having antenna gain 2.2 dBi and transmitter power of 10 W at a distance of 180 million kilometres. Assume a receiver noise temperature of 70 K and a receiver bandwidth of 10 Hz. Estimate the maximum receiver signal-to-noise ratio.

(iii)
Estimate the half-power beamwidth of this 70 m Cassegrain antenna at 8.45 GHz.


Question 2.

(a)
Define the terms element, element factor, array factor, pattern multiplication, and total radiation pattern for an array antenna. State what constraints have to be applied to the individual elements for pattern multiplication to be possible.

(b)
Distinguish between active arrays and passive arrays and discuss to what extent the method of moments calculation process for antenna structures may be applied to array antennas.

(c)
An active array antenna is to be constructed from four half-wave dipoles.

(i)
Sketch the azimuth and elevation pattern for a half-wave dipole. Explain which pattern is an E-plane section and which is an H-plane section.

(ii)
Sketch the array factor for two isotropes spaced (a) lambda/4, (b) lambda/2, and (c) lambda apart.

(iii)
Choose an element spacing and suitable drive amplitudes for the elements so that the four-isotrope "array factor" has only two main lobes, but no side lobes.

(iv)
Choose an orientation and spacing for the dipole elements so that the entire array antenna has maximum directivity of about 8 dBi.



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25th March 2003.