Space Wave Propagation Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.