Space Wave Propagation Questions and Answers

Q: What is the space wave?

The space wave is the sum of the ray travelling directly between the antennas and the ray reflected from the ground between them, and it dominates whenever both antennas are elevated and the frequency is above about thirty megahertz. Satellite paths are free space propagation, surface guidance is the surface wave and ionospheric return is the sky wave. Naming the mechanism correctly determines which variables the prediction will depend on, and here they are antenna height, terrain and the atmosphere.

Q: What does the k factor represent?

Multiplying the earth's radius by k lets rays be drawn as straight lines on a path profile while reproducing the effect of atmospheric refraction, and the standard value of four thirds corresponds to the normal gradient. Values below one indicate sub-refraction and a bulging earth, and large or negative values indicate ducting. It is a geometric device rather than a power ratio, a zone count or a polarisation correction. Replacing a curved ray with a modified earth is what makes profile drawing possible.

Q: What does a k factor less than one mean?

Sub-refraction bends the ray less than usual or even upward, which on a profile appears as a more sharply curved earth and can put an obstacle into the path that was clear under normal conditions. Links are therefore checked for clearance at a low k value as well as at four thirds. Stronger downward bending is a large k, ducting is an extreme case of that, and a k of exactly one would be the no refraction case. Designing for the worst case of a varying parameter is what a clearance criterion does.

Q: What conditions favour the formation of a tropospheric duct?

A layer in which temperature rises with height or humidity falls sharply produces a steep refractivity gradient that traps the wave, and calm anticyclonic weather over water is the classic setting. Mixing destroys the gradient, uniformity gives no gradient at all, and wind disperses any layer that forms. Recognising the meteorological conditions that produce a propagation anomaly allows its occurrence to be forecast, which is what interference planning requires.

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Multiplexing is the sending of a separate signals together, over the same cable or bearer simultaneously without interference. Time division multiplex is a method of interleaving in the time domain pulses belonging to different transmissions. These pulses are generally narrow and separation between successive pulses is rather wide. That being the case it is possible, provided the two ends of a link are synchronized, to use the wide spaces for pulses belonging to other transmissions. Frequency division multiplexing concerns itself with combining continuous signals. It may be thought of as an outgrowth of independent side band transmission on a much enlarged scale. As will seen 12 or 16 channels are combined into a group, 5 groups into a subgroup. Each group and subgroup is then sent as a whole unit one microwave link cable or other broadband system. Continental broadband systems are then treated followed by coaxial cables, fibre optic cable, microwave links and troposcatter systems.

Microwave link repeaters are typically 50 km apart

A.because of atmospheric attenuation
B.because of output tubepower limitations
C.because of earths curvature
D.to ensure that the applied dc voltage is not excessive

Exam Year(s)

UGC NET - 2013,

Answer: C

Microwave links work on line of sight, and because of the curvature of the earth the antennas on towers can see each other only up to about 50 km, so repeaters are placed at about that spacing. Atmospheric attenuation and the output power of the tubes matter for the link budget but do not fix the spacing, and the dc voltage supplied has nothing to do with it. Radio relay towers are usually about 50 km apart for this reason, and an antenna height of about 50 m is needed to reach that distance, because the radio horizon grows with the square root of the height. The answer is because of the earth's curvature.

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What is the space wave?

A.The combination of the direct and ground reflected rays
B.The wave that travels through space between a satellite and the earth
C.The wave guided along the boundary between the air and the ground
D.The wave returned to earth after refraction in the ionosphere

Answer: A

The space wave is the sum of the ray travelling directly between the antennas and the ray reflected from the ground between them, and it dominates whenever both antennas are elevated and the frequency is above about thirty megahertz. Satellite paths are free space propagation, surface guidance is the surface wave and ionospheric return is the sky wave. Naming the mechanism correctly determines which variables the prediction will depend on, and here they are antenna height, terrain and the atmosphere.

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What is the refractivity of the atmosphere usually expressed in?

A.Percentage of the speed of light at the surface of the earth
B.N units, being the excess refractive index times a million
C.Decibels relative to the refractive index of free space
D.Radians per kilometre of the ray path through the atmosphere

Answer: B

Because the refractive index differs from unity only in the sixth decimal place, it is quoted as N units, the excess over one multiplied by a million, giving values near three hundred at the surface. The gradient of N with height, typically minus forty N units per kilometre, is what bends the ray. Decibels, angles and percentages are not the convention. Scaling a quantity so that its interesting variation occupies convenient numbers is a small but useful piece of engineering practice.

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What is the standard vertical gradient of refractivity?

A.Zero, since the atmosphere is well mixed below the tropopause
B.About minus four hundred N units per kilometre near the surface
C.About minus forty N units per kilometre
D.About plus forty N units per kilometre in temperate climates

Answer: C

Pressure and humidity both fall with height, so refractivity falls at roughly forty N units per kilometre in the standard atmosphere, which bends a ray gently downward. A gradient near minus one hundred and fifty makes the ray follow the earth's curvature exactly, and steeper gradients trap it in a duct. A positive gradient bends the ray upward and shortens the horizon. One number, the gradient, determines the whole refraction behaviour of a path.

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What does the k factor represent?

A.The ratio of the received to the transmitted power on a link
B.The number of Fresnel zones cleared by an obstacle on the path
C.The correction applied for the polarisation mismatch at the receiver
D.The multiplier applied to the earth's radius to straighten the rays

Answer: D

Multiplying the earth's radius by k lets rays be drawn as straight lines on a path profile while reproducing the effect of atmospheric refraction, and the standard value of four thirds corresponds to the normal gradient. Values below one indicate sub-refraction and a bulging earth, and large or negative values indicate ducting. It is a geometric device rather than a power ratio, a zone count or a polarisation correction. Replacing a curved ray with a modified earth is what makes profile drawing possible.

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What does a k factor less than one mean?

A.The earth appears more curved and clearance is reduced
B.The ray is bent downward more strongly than usual
C.The path is ducted and the signal reaches far beyond the horizon
D.The atmosphere has no effect at all on the ray's trajectory

Answer: A

Sub-refraction bends the ray less than usual or even upward, which on a profile appears as a more sharply curved earth and can put an obstacle into the path that was clear under normal conditions. Links are therefore checked for clearance at a low k value as well as at four thirds. Stronger downward bending is a large k, ducting is an extreme case of that, and a k of exactly one would be the no refraction case. Designing for the worst case of a varying parameter is what a clearance criterion does.

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Why is a microwave path checked for clearance at both high and low k values?

A.The regulator requires two separate profiles to be submitted
B.Refraction varies, and each extreme threatens the path differently
C.The link must work at every frequency in its allocated band
D.The antennas may be moved during the life of the installation

Answer: B

At low k the earth bulges and an obstacle may intrude, while at high k the ray flattens and a strong ground reflection can arrive to interfere with the direct signal, so both conditions must be considered. Typical criteria demand full first Fresnel clearance at four thirds and sixty per cent at a low value such as two thirds. Frequency, antenna moves and paperwork are not the reason. A parameter that varies must be checked at both extremes rather than at its average.

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What is super-refraction?

A.The complete absence of any refraction in the lower atmosphere
B.Refraction caused by the ionosphere rather than the troposphere
C.Downward bending stronger than standard but not enough to trap the ray
D.Bending of the ray upward away from the earth's surface

Answer: C

A refractivity gradient steeper than standard curves the ray more sharply downward, extending the horizon and often strengthening a link, and if the gradient exceeds about minus a hundred and fifty N units per kilometre the ray is trapped and a duct forms. Upward bending is sub-refraction, and the ionosphere acts at much greater heights. A continuum of behaviour governed by one gradient, with named regimes along it, is easier to remember as a scale than as a list.

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What conditions favour the formation of a tropospheric duct?

A.Strong vertical mixing caused by daytime surface heating
B.Uniform temperature and humidity throughout the lower atmosphere
C.High wind speeds that stir the atmosphere over a large area
D.A temperature inversion or a sharp fall in humidity with height

Answer: D

A layer in which temperature rises with height or humidity falls sharply produces a steep refractivity gradient that traps the wave, and calm anticyclonic weather over water is the classic setting. Mixing destroys the gradient, uniformity gives no gradient at all, and wind disperses any layer that forms. Recognising the meteorological conditions that produce a propagation anomaly allows its occurrence to be forecast, which is what interference planning requires.

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10.

What is an evaporation duct?

A.A shallow duct over the sea caused by the humidity gradient above the water
B.A duct formed when rain evaporates as it falls through dry air
C.A duct produced by the evaporation of dew from a land surface at dawn
D.A duct that forms and disperses within a few minutes over any surface

Answer: A

Air immediately above the sea is saturated while air a few metres higher is drier, so a steep humidity gradient produces a duct a few metres to a few tens of metres deep that is present over the ocean most of the time. It supports maritime radar and communication far beyond the horizon. Rain evaporation, dew and short lived structures are not the mechanism. A duct that is usually present rather than occasional changes from an anomaly into a design consideration.

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Space Wave Propagation Questions and Answers

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