Space Wave Propagation Questions and Answers

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.

Answer: C

Fade margin is the number of decibels by which the received signal in clear conditions exceeds the level at which the link fails, and it is chosen so that fades exceed it only for the permitted outage time. It is not simply the difference between transmitter and receiver specifications, which ignores the path. Antenna gain and alignment enter the budget separately. A margin sized from the statistics of the impairment, rather than chosen arbitrarily, is what turns a prediction into a service level.

Answer: D

Large raindrops flatten as they fall and tend to align, presenting different attenuation and phase to horizontal and vertical polarisation, so a dual polarised system loses isolation. That matters most where frequency reuse depends on cross polar discrimination. Reflection, bulk birefringence and antenna wetting are not the mechanism, though a wet radome adds loss of its own. A scatterer whose shape and orientation are not random will always couple energy between polarisations.

Answer: A

Molecular oxygen has a complex of magnetic dipole transitions near sixty gigahertz that gives around fifteen decibels per kilometre at sea level, effectively blocking terrestrial paths but making the band attractive for short range systems that benefit from isolation. Water vapour has lines at twenty two and a hundred and eighty three gigahertz, carbon dioxide absorbs in the infrared, and nitrogen is transparent. A band that is useless for range and excellent for reuse is a good illustration of a property being an asset or a liability according to the application.

Answer: B

Absorption that kills a signal within a few hundred metres also prevents it from interfering with a system a kilometre away, so the same frequency can be reused very densely. Wide unlicensed allocations exist for that reason. Rain still attenuates the band severely, regulation exists but is light, and antennas are small rather than notably cheap. Turning a propagation liability into a system asset by choosing the right application is an instructive piece of spectrum engineering.

Answer: C

Between the oxygen and water vapour lines lie ranges near thirty five, ninety four and a hundred and forty gigahertz where clear air attenuation is comparatively low, and imaging and radar systems are placed there. The windows are properties of the molecular spectrum rather than of time, regulation or geography. Choosing an operating frequency from the structure of the medium's absorption spectrum is the same reasoning that placed infrared astronomy at particular wavelengths.

Answer: D

Turbulent mixing produces small variations in refractive index that focus and defocus the wave, giving fluctuations of a fraction of a decibel to a few decibels at rates of a few hertz, worst at low elevation angles and in humid weather. Seasonal change, rain fading and ground reflection are separate effects with different timescales. Distinguishing impairments by the timescale on which they act is the quickest way to identify which one a record is showing.

Answer: A

A path at five degrees elevation traverses many times more of the turbulent lower atmosphere than one directly overhead, so the accumulated phase perturbation and the resulting amplitude fluctuation are much greater. The same geometry increases gaseous absorption and sky noise temperature. Antenna gain is unchanged with pointing, rain is a separate impairment, and geostationary satellites do not move appreciably. Path length through an impairing medium is the governing variable for every low elevation problem.

Answer: B

The model gives path loss as a function of frequency, distance and antenna heights, with correction terms for urban, suburban and open environments, fitted to extensive measurements in Tokyo. It became the standard first estimate for cellular planning. Rain, diffraction and ducting are handled by quite different models. An empirical model named after the measurements behind it carries their conditions with it, which is why its stated range of validity matters as much as its formula.