Space Wave Propagation Questions and Answers

Answer: A

Common practice demands full first zone clearance at a k of four thirds together with about sixty per cent at a low k value, so the path stays clear under normal and adverse refraction. Optical clearance alone would leave diffraction loss, ten per cent is far too little, and three zones would raise mast heights for no benefit. A criterion with two conditions at two refraction states is how a varying atmosphere is accommodated in a fixed design.

Answer: B

A reflection from water or flat ground arrives with an amplitude comparable to the direct ray, and as refraction changes the path difference the two alternately reinforce and cancel, giving fades of twenty decibels or more. Siting the antennas so the reflection point falls on rough or shielded ground, or using space diversity, is the remedy. Reinforcement is only half the story, and polarisation and refraction sensitivity are separate matters. Two comparable paths always risk cancellation.

Answer: C

The geometry fixes where the reflection occurs, so adjusting the antenna heights can move that point onto broken ground, vegetation or a screening ridge where no coherent reflection forms. Space diversity is the alternative. Power cannot fill a cancellation null, lower frequencies reflect at least as strongly, and tilting the antennas costs gain on the direct path without removing the reflected one. Moving a problem to where it does not matter is often cheaper than fighting it.

Answer: D

Vertically separated antennas experience different path differences to the reflection point, so when one is in a fade the other generally is not, and combining or switching between them restores availability. Route diversity, sector antennas and polarisation diversity are different arrangements serving different purposes. Diversity works only where the two signals fade independently, and vertical separation on a reflective path is one of the situations where that independence is reliable.

Answer: A

Because a multipath fade is deep only over a narrow band, a second carrier a per cent or so away is usually unaffected, so switching to it maintains the link. It also protects against equipment failure as a side benefit, since two complete radio chains are involved. Rain affects a whole band together and is not helped, and interference is a separate problem. Diversity requires independence, and frequency selective fading provides it while rain attenuation does not.

Answer: B

Attenuation by rain varies slowly with frequency, so two carriers a few per cent apart fade together and no independence exists to exploit. Route diversity, with a second path outside the rain cell, is the effective answer where availability demands it. Rain does depolarise but affects both polarisations, fades last minutes rather than milliseconds, and the antenna match is unaffected. Choosing a diversity dimension in which the impairment is actually independent is the whole art of applying the technique.

Answer: C

Raindrops are a few millimetres across, so they scatter and absorb appreciably once the wavelength approaches that size, which happens above roughly ten gigahertz. Below that the drops are electrically tiny and the effect is negligible. At thirty gigahertz heavy rain can cost tens of decibels per kilometre. Comparing the size of a scatterer with the wavelength predicts at once whether it matters, and it is the first question to ask of any propagation impairment.

Answer: D

Specific attenuation follows a power law in rain rate with an exponent typically between one and one and a half, so heavy rain is disproportionately damaging. Since intense rain falls in small cells, long paths average over regions of different intensity and a path reduction factor is applied. Neither proportionality nor independence describes the relationship. A power law with an exponent above one means that the extremes of a distribution dominate the outage statistics.

Answer: A

Heavy rain falls in cells a few kilometres across, so a twenty kilometre link is rarely immersed in the peak rate along its whole length and the effective path is shorter than the geometric one. The factor accounts for that statistically. The formula itself is correct for a uniform rate, rain does not systematically favour the middle of a path, and drop size is a meteorological matter. Correcting a point measurement for the spatial structure of the phenomenon is a general requirement in path averaged quantities.

Answer: B

Trunk links are commonly designed for 99.99 or 99.999 per cent availability, corresponding to about fifty minutes or five minutes of outage a year, and the fade margin follows from the rain statistics at that percentage. Lower figures are accepted for less critical services. One hundred per cent is unattainable, since any margin can be exceeded by a sufficiently rare event. Availability is a design parameter with a cost, and choosing it is a commercial decision as much as a technical one.