Ground Wave Propagation Questions and Answers

Answer: C

Sommerfeld's treatment expresses attenuation as a function of a single dimensionless numerical distance that folds together the actual range, the frequency and the ground constants, so one universal curve serves every case. That is why published ground wave curves can cover so many conditions on a few graphs. The other options describe ordinary distances. Collapsing several variables into one dimensionless group is a standard way of making a family of problems into a single one.

Answer: D

The curves plot field strength for a stated radiated power against distance, with separate families for each frequency and each set of ground constants, and coverage planning consists largely of reading them. They are empirical summaries of the theory rather than regulations, and the ground wave is notably stable with time of day, which is one of its attractions. Reducing a calculation to a chart lookup is what allows planning to be done consistently by many people.

Answer: A

The surface wave travels along the earth and is unaffected by the ionosphere, so its strength depends only on ground constants and terrain, which change slowly if at all. That steadiness is why medium wave stations rely on it for their protected daytime service area. It is the sky wave arriving at night that causes fading and distant interference. Identifying which mechanism carries a signal predicts immediately how it will behave with time and season.

Answer: B

The absorbing D layer disappears at night, so sky wave returns from the E layer and arrives at distances where the ground wave is weak, producing fading where the two are comparable and interference from distant stations. The ground wave itself is unchanged. That is why medium wave stations have separate day and night coverage areas and often reduce power or use directional patterns after dark. Two mechanisms arriving together is what produces fading rather than either alone.

Answer: C

Where the two mechanisms arrive with similar amplitude but with a path difference that varies as the ionosphere moves, they alternately add and cancel and the signal fades deeply. The effect is worst at night and at the edge of the ground wave service area. Antennas with a reduced high angle radiation are used to push the fading zone further out. Interference between two paths of comparable strength is the general cause of fading in every propagation mechanism.

Answer: D

A mast about half a wavelength high or a little more concentrates radiation at low angles and puts a null in the direction that would return from the ionosphere nearby, so the fading zone is pushed beyond the intended service area. It is a pattern shaping solution to a propagation problem. Ground wave strength is set by the low angle radiation, uniform radiation would make matters worse, and no practical antenna switches polarisation in response to fading.

Answer: A

When a path crosses land and sea in succession, the simple curves do not apply, and Millington's method combines the curves for each section in a way that gives the correct result in both directions. It predicts the characteristic recovery of field strength when a wave passes from poor ground onto sea. Conductivity measurement, sky wave calculation and mast design are separate tasks. A method that handles a discontinuity by combining the solutions either side of it is a common engineering device.

Answer: B

Passing from poor ground onto highly conducting sea, the wave's attenuation rate drops abruptly and the field strength rises above what the land alone would have given, an effect sometimes called coastal recovery. No energy is regained; the rate of loss simply falls. Polarisation is unchanged throughout. The asymmetry of the effect, being different for a land to sea path and a sea to land one, is what Millington's method was devised to handle correctly.

Answer: C

The space wave is the vector sum of the direct path and the specularly reflected path, and it dominates whenever both antennas are elevated and the frequency is high. The surface wave is a separate guided component, ionospheric return is the sky wave, and diffraction is a further mechanism beyond the horizon. Line of sight microwave and mobile links are space wave propagation. Naming the mechanism correctly determines which model and which variables apply.

Answer: D

At grazing incidence the reflection coefficient approaches minus one, so the two rays almost cancel and what survives is proportional to the small path difference, which itself falls with range, giving an inverse fourth power law rather than an inverse square. That is why coverage falls off so sharply beyond a certain distance. Absorption, double attenuation and aperture are not the mechanism. A near cancellation leaving a small residue is a common source of unexpectedly steep power laws.