Space Wave Propagation Questions and Answers

Answer: C

Below about ten gigahertz rain is almost irrelevant and gaseous absorption negligible, so a modest fade margin covers a hop of tens of kilometres. Antenna gain for a given size falls rather than rises at lower frequency, the Fresnel zone is wider, and regulation follows physics. The two disadvantages are accepted because the atmospheric advantage dominates on a long path, which is a good example of one term in a budget outweighing several others.

Answer: D

At thirty eight gigahertz a thirty centimetre dish gives high gain and narrow beams that allow dense reuse, and wide channels are available, which suits hops of a few kilometres between rooftops. Rain is a limitation rather than an advantage and is accepted because the hops are short. Building reflection is a nuisance, and lower bands can carry high rates given the spectrum. Matching the frequency to the hop length and the spectrum availability is the design decision.

Answer: A

A dish with a beam of a degree or two radiates and receives almost nothing outside it, so links can cross the same area on the same frequency provided their beams do not intersect. Side lobe performance and cross polar discrimination determine how tightly. Gain concentrates rather than increases the total power, distance is only one factor, and easy alignment is a convenience bought at a high cost in reuse. Spatial selectivity is a spectrum resource in its own right.

Answer: B

With a beamwidth of a degree, a small structural movement costs several decibels, and towers do move with temperature, wind loading and foundation settlement. Periodic realignment and rigid mounts are standard practice. Frequency drift is an equipment matter, antenna gain does not age appreciably, and the far field distance is fixed by geometry. A mechanical tolerance expressed as a fraction of an electrical beamwidth is what ties structural engineering to radio performance.

Answer: C

Sending two independent channels on horizontal and vertical polarisation doubles the capacity of an allocation, and how well that works depends on how little energy leaks between them. Rain degrades that discrimination just when the margin is smallest, which is why cross polar cancellers are fitted on high capacity links. Rain attenuation, rearward rejection and operational flexibility are separate matters. A capability that depends on a specification makes that specification a system parameter rather than a component detail.

Answer: D

Because the interference is a known signal, being the other channel, an adaptive circuit can estimate the leakage and subtract it, restoring twenty decibels or more of discrimination during rain. It is the same principle as an echo canceller. Mechanical rotation is far too slow, falling back to one polarisation halves the capacity, and raising power raises the leakage too. Cancelling a known interferer rather than merely tolerating it is possible whenever the interferer is available as a reference.

Answer: A

A microwave hop needs only two sites and can be working in days where a fibre route would require wayleaves and civil works taking months or years, which is why they dominate mobile backhaul in many regions. Fibre carries far more capacity and is unaffected by rain, and no regulation mandates a radio backup, though diverse routing is common practice. Speed of deployment and independence from civil works, rather than performance, are what keep the technology in use.

Answer: B

Beyond the radio horizon a wave still reaches into the shadow by diffraction around the bulge, with a loss that increases rapidly with distance and with frequency. It sets the coverage of broadcast and mobile services beyond line of sight. Reflection, beam spreading and refraction are separate mechanisms, though refraction determines where the horizon lies. Coverage that continues beyond the geometric limit, with a steeply rising loss, is the characteristic signature of diffraction.

Answer: C

Diffraction becomes weaker as the wavelength shrinks relative to the obstacle, so a hundred megahertz signal reaches much further beyond the horizon than a two gigahertz one, which is one reason low band broadcasting covers such large areas. Absorption and antenna gain are separate terms, and the horizon distance depends on height and refraction rather than frequency. Comparing a wavelength with the scale of an obstacle predicts the strength of diffraction everywhere it occurs.

Answer: D

Longer waves diffract more readily past terrain and are attenuated less by buildings and vegetation, so coverage from a single high site extends far further than at higher frequencies. That is why moving a service upward in frequency requires many more transmitters. Power limits and receiving antennas are secondary, and the ionosphere returns these frequencies only sporadically. The cost of a frequency change is measured in transmitter sites, which makes this relationship commercially as well as technically important.