Space Wave Propagation Questions and Answers

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.

Q: Why does an evaporation duct matter to maritime radar?

Trapping in the duct carries the signal along the surface with little spreading loss, so targets are detected at ranges far beyond line of sight, though the coverage above the duct is correspondingly reduced. Both effects matter operationally. Absorption, backscatter and depolarisation are separate phenomena. A mechanism that gives more range in one region at the cost of a hole in another is a trade rather than a pure benefit, and it must be understood as such.

Q: What is a radio hole caused by ducting?

Energy trapped in a duct is not available above it, so an aircraft flying over a ducting layer may find a surface radar unable to see it at all. The same trapping that extends surface coverage creates the hole. Terrain shadowing, interference nulls and pattern nulls are different causes of coverage gaps. Recognising that redistribution of energy creates deficits as well as surpluses is the key to predicting anomalous propagation correctly.

11.

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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12.

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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13.

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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14.

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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15.

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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16.

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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17.

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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18.

Why does an evaporation duct matter to maritime radar?

A.It changes the polarisation of the returned echo from a target
B.It extends detection ranges well beyond the geometric horizon
C.It absorbs the radar signal and reduces the detection range
D.It reflects the radar signal back towards the transmitting ship

Answer: B

Trapping in the duct carries the signal along the surface with little spreading loss, so targets are detected at ranges far beyond line of sight, though the coverage above the duct is correspondingly reduced. Both effects matter operationally. Absorption, backscatter and depolarisation are separate phenomena. A mechanism that gives more range in one region at the cost of a hole in another is a trade rather than a pure benefit, and it must be understood as such.

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19.

What is a radio hole caused by ducting?

A.A region where two signals cancel and produce a null
B.A part of the antenna pattern in which no power is radiated
C.A region above the duct that receives very little signal
D.A gap in coverage caused by an obstacle on the path

Answer: C

Energy trapped in a duct is not available above it, so an aircraft flying over a ducting layer may find a surface radar unable to see it at all. The same trapping that extends surface coverage creates the hole. Terrain shadowing, interference nulls and pattern nulls are different causes of coverage gaps. Recognising that redistribution of energy creates deficits as well as surpluses is the key to predicting anomalous propagation correctly.

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20.

Why is the first Fresnel zone relevant to a line of sight link?

A.It defines the width of the antenna beam at the mid point
B.It marks the boundary beyond which the wave is no longer coherent
C.It is the region within which the ground reflection takes place
D.Obstruction of it causes loss even when the direct ray is clear

Answer: D

Waves reaching the receiver by paths within half a wavelength of the direct route arrive nearly in phase and contribute constructively, so blocking part of that zone removes energy that would otherwise have arrived. Optical clearance alone is therefore insufficient. Beamwidth, coherence and the reflection point are separate concepts. Diffraction rather than geometric optics governs what an obstacle actually does, and the Fresnel zone is the practical expression of it.

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