Uniform Plane Waves Questions and Answers

101.

An antenna designed for vertical polarisation receives almost nothing from a horizontally polarised transmitter. The remedy is

A.to increase the transmitted power
B.to raise the frequency of the transmission
C.to insert a quarter-wave matching layer at the receiver
D.to align the receiving antenna with the transmitted polarisation

Answer: D

Polarisation mismatch cannot be overcome by power, since the field has no component along the antenna, so the antenna must be turned or a circularly polarised one used. Circular polarisation costs three decibels against a matched linear pair but is immune to orientation.

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102.

A radar echo is much weaker than expected from a metal target. A likely cause is

A.the target being shaped to reflect the energy away from the receiver
B.the metal being a poor conductor at radar frequencies
C.the wave penetrating the metal rather than reflecting
D.the polarisation of the wave rotating in free space

Answer: A

Metal reflects almost perfectly, but the reflection is specular, so a surface angled away sends the energy elsewhere and little returns to the receiver. Shaping to achieve exactly that is the principle of a low-observable aircraft.

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103.

Comparing propagation in free space with propagation in sea water at 1 MHz, the wave in sea water

A.does not propagate at all
B.is attenuated severely within a fraction of a metre
C.travels faster and further than in free space
D.is unattenuated but travels more slowly

Answer: B

Sea water is a good conductor at a megahertz and the skin depth is of the order of a quarter of a metre, so the field falls by a factor of e in that distance. Communication with a submerged submarine therefore uses very low frequencies, where the skin depth is tens of metres.

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104.

The plane of polarisation of a linearly polarised wave contains

A.both field vectors
B.the direction of travel alone
C.the electric field and the direction of travel
D.the magnetic field and the direction of travel

Answer: C

The convention takes the electric field as defining the polarisation, so the plane of polarisation holds that field and the propagation direction. The magnetic field is perpendicular to that plane, and an antenna must be aligned with the electric field to receive.

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105.

The frequency of a wave is determined by

A.the permittivity and permeability of the medium it travels through
B.the wavelength together with the intrinsic impedance of the medium
C.the attenuation constant of the medium at that particular frequency
D.the source

Answer: D

Nothing along the path can change how often the field reverses, so the frequency is carried unchanged from medium to medium. The wavelength and the velocity both adjust to the medium, which is the whole content of refraction.

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106.

A wave travelling in the positive z direction with its electric field along x has its magnetic field along

A.y
B.z
C.x
D.the negative x direction

Answer: A

The three vectors form a right-handed set in the order electric field, magnetic field, direction of propagation, so the magnetic field must lie along the remaining axis. Reversing the direction of travel would reverse the magnetic field while leaving the electric field alone.

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107.

A radio wave is refracted on entering the ionosphere because the ionosphere has

A.a temperature far above that of the lower atmosphere
B.a different effective permittivity from free space
C.a higher conductivity than the atmosphere below it
D.a permeability greater than that of free space

Answer: B

Free electrons make its effective relative permittivity less than one and frequency dependent, so the phase velocity exceeds c and the wave bends away from the vertical. That bending is what returns short waves to earth and makes long-distance broadcasting possible.

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108.

Below the critical frequency a wave sent vertically into the ionosphere is

A.transmitted through into space
B.converted into a circularly polarised wave
C.reflected back to earth
D.absorbed entirely within the ionised layer

Answer: C

The effective permittivity falls to zero at that frequency, the wave can go no further and it returns. Above it the wave passes through, which is why satellite links use frequencies well above the critical value and broadcast services below it.

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109.

The depth of penetration of a wave into a conductor falls as the frequency rises. A practical consequence is

A.thicker metal is needed at higher frequencies for screening
B.screening becomes impossible above a certain frequency
C.the screen must be of magnetic rather than conducting material
D.thin metal foil screens high frequencies effectively

Answer: D

A few micrometres of aluminium is many skin depths at a gigahertz, so a metallised plastic film screens as well as a sheet of metal. At mains frequency, by contrast, several millimetres of copper are needed and high permeability material is preferred.

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110.

Sunlight reaching the earth is

A.unpolarised, being a random superposition of all polarisations
B.linearly polarised in the plane of the ecliptic
C.circularly polarised, with the sense set by the sun's rotation
D.elliptically polarised with an axial ratio near unity

Answer: A

Thermal emission from the sun's surface has no preferred direction, so all polarisations are present in random proportion. Scattering in the atmosphere partially polarises the light of the sky, which is why a polarising filter darkens it.

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