Space Wave Propagation Questions and Answers

Answer: A

Greater diffraction and clutter loss shrink the area a single site can serve, so a network that used a handful of high power stations may need hundreds of smaller ones. The additional antenna gain available at higher frequency does not compensate for the terrain and clutter losses. It is why spectrum below one gigahertz is so valuable for wide area services, and why its reallocation is contested. A propagation property translating directly into infrastructure cost is what makes spectrum policy an economic question.

Answer: B

A line of sight path is one in which the direct ray, with adequate Fresnel clearance, reaches the receiver without obstruction, and it is the basis of microwave links, satellite paths and most mobile coverage near a base station. Surface guidance, ionospheric return and scatter are separate mechanisms with their own bands and behaviours. Establishing which mechanism applies before predicting anything is the first step in every propagation problem.

Answer: C

The horizon distance follows from the antenna heights and the effective earth radius, and beyond it the path is obstructed by the earth itself whatever the power. Raising the antennas is the only geometric remedy. Power affects the margin rather than the geometry, bandwidth affects the noise, and ground conductivity governs the surface wave rather than this mechanism. A limit set by geometry cannot be overcome by improving the equipment.

Answer: D

Raising the antennas pushes the horizon further and lifts the beam clear of buildings and trees so that the Fresnel zone is unobstructed, which is usually the dominant consideration in siting. Interference and safety are secondary benefits, and antenna gain is a property of the structure rather than of its height. The height of a tower is bought to satisfy a geometric requirement, and calculating that requirement precisely is what a path profile is for.

Answer: A

Any intrusion into the first Fresnel zone removes part of the contributing wavefront, so loss appears even while the direct ray remains geometrically clear, and the margin that was protecting the link against rain is consumed by a permanent term. Planning consultation over microwave paths exists for this reason. Reflection can occur too but does not compensate, and polarisation is unaffected. A permanent loss eating a margin intended for a varying one is a particularly damaging kind of degradation.

Answer: B

A margin is calculated so that time varying fades exceed it only for the permitted outage period, and a constant loss simply reduces the whole budget, leaving less to absorb those fades. The correct treatment is to include the obstruction as a fixed loss and recompute the margin. Frequency, seasonal applicability and alignment are unrelated. Distinguishing a constant term from a statistical one determines where in a budget it belongs and how it must be covered.

Answer: C

Additional height pushes the horizon further and lifts the ray away from obstacles, improving clearance and often removing a ground reflection. Free space loss depends on distance rather than height, so it does not fall, and rain and frequency are unaffected. The two ray height gain that applies to low mobile antennas is a different regime. Knowing which regime a path is in decides whether height buys signal or merely clearance.

Answer: D

The height gain arises from the near cancellation of a strong reflected ray, and a link designed to avoid that reflection is left with the direct ray alone and free space loss. Antenna directivity helps by discriminating against the reflection, which is part of good design rather than an automatic property. The ground does reflect at microwave frequencies, and the reflected ray reaches the receiver readily on a flat path. A model applies only where its assumptions hold, and here they have been deliberately broken.

Answer: A

An active repeater receives on one frequency, amplifies or regenerates the signal and retransmits it on another towards the next site, breaking a long route into hops each within line of sight. Passive reflectors achieve a change of direction without electronics, and filters and dual beam antennas are components rather than stations. Chaining short hops to cross a long distance is what makes terrestrial microwave routes possible over any terrain.

Answer: B

With the transmitter and receiver on the same tower, even sixty or eighty decibels of antenna isolation would not prevent oscillation if both used the same frequency, so a frequency pair is used and the roles alternate along the route. Regulation follows the technical need, amplification efficiency is unaffected, and antennas handle multiple signals routinely. Positive feedback around a loop with gain is the risk being avoided, and the same argument governs any repeater in any medium.