Sky Wave and Ionospheric Propagation Questions and Answers

Answer: A

The two limits move towards each other: the maximum usable frequency falls as the F layer is depressed while absorption raises the lowest usable frequency, and when they cross there is no frequency that both reaches and survives the path. The circuit is then simply unavailable. The ionosphere does not vanish, equipment is not the limitation, and interference is a separate problem. A window defined by two independently moving bounds can close, which is why both must be predicted rather than only one.

Answer: B

When no frequency works, a robust low rate digital mode may still get a message through, or a relay via a different geographical path may avoid the disturbed region, or a satellite or cable may be used instead. Power helps against absorption but cannot restore refraction that has gone, polarisation is not the limitation, and the solar cycle operates far too slowly to be an operational response. Having a planned alternative for a medium known to fail occasionally is the only sound approach.

Answer: C

Confining the signal to a few hertz and applying strong error correction lowers the signal to noise ratio required by tens of decibels, so a path that cannot carry speech may still carry a message. Frequency selection is the same for every mode, power is equal or lower for a given result, and multipath must still be handled, often by interleaving. Trading data rate for sensitivity is the fundamental bargain of communication theory and it is nowhere more visible than on a marginal high frequency circuit.

Answer: D

Signals arriving by several paths spread symbols in time, so the rate must be low enough that the spread is a small fraction of a symbol, and the few kilohertz of channel bandwidth caps it further. Adaptive equalisation and multicarrier modulation push the limit but do not remove it. Power, receiver noise and antenna gain affect the achievable signal to noise ratio rather than the channel's time dispersion. A limit imposed by the channel's structure cannot be overcome by improving the equipment at either end.

Answer: A

Several ionospheric paths differ in length by hundreds of kilometres, so their arrival times differ by a few milliseconds, and any symbol shorter than that suffers interference from its predecessor. Ionospheric response time, protocol delay and Doppler spread are separate quantities, the last being the frequency domain counterpart of this time domain one. A channel is characterised by its spread in both time and frequency, and each limits a different aspect of what it can carry.

Answer: B

The reflecting layer rises, falls and moves horizontally, so each path imposes a slightly different Doppler shift and a transmitted carrier arrives spread over a fraction of a hertz to several hertz. It limits how long a receiver can coherently integrate and hence how narrow a mode can usefully be. Station motion, magneto-ionic splitting and modulation bandwidth are different quantities. Time spread and frequency spread together define a channel completely, and each sets its own limit on the modulation that can be used.

Answer: C

The maximum contribution comes from the ultra-violet spectrum.

The ionosphere is formed by solar radiation ionising the upper atmosphere. Most of that ionisation is caused by extreme ultraviolet radiation from the Sun, which is strongly absorbed between about 60 and 1000 km and frees electrons from oxygen and nitrogen, with solar X-rays adding to the lowest layers.

Cosmic rays contribute a small amount of ionisation, mainly in the lowest D region at night. Gamma rays from the Sun do not reach these heights in significant amounts, and beta radiation is not a source of ionospheric ionisation.