Ground Wave Propagation Questions and Answers

Q: Why can a small loop antenna give good medium wave reception?

A ferrite loop is enormously inefficient in absolute terms, but since atmospheric noise arrives through it as well, the signal to noise ratio is nearly as good as a large antenna would give. Its directional null is a further practical benefit. Its gain is far below a mast's, efficiency comparisons between small antennas are marginal, and the two field components carry the same information. An antenna that would be useless for transmitting can be entirely adequate for receiving, and the reason is the noise environment.

Q: Why has man made noise become a growing problem for low frequency reception?

Every switched mode supply, lighting driver and data cable radiates broadband noise, and their proliferation has raised urban noise floors by many decibels over recent decades, which is one reason long wave and medium wave services have declined. Atmospheric noise is unchanged, receiver sensitivity is irrelevant in an externally limited system, and transmitter power reductions follow the decline rather than causing it. An environmental change that undermines a technology is worth recognising as such.

Q: What does a directional receiving antenna achieve against local noise?

A loop or a small array has a sharp null that can be turned onto a dominant local interferer, improving the ratio by twenty decibels or more against that one source. It cannot reject noise arriving from every direction at once, and distributed noise is only modestly reduced. Gain raises signal and distributed noise together, and no antenna changes the character of noise. Exploiting the geometry of a specific problem, rather than seeking a general improvement, is often the only effective remedy.

Q: What is the practical conclusion of comparing surface wave and space wave mechanisms?

The surface wave dominates at low frequency with antennas near the ground, and the space wave dominates at higher frequency with elevated antennas, so a service is designed around whichever suits its band and geometry. Neither is universally stronger, the space wave works over any surface, and the two are physically distinct components of the total ground wave. Knowing which mechanism will carry a signal before designing for it is the single most useful habit in propagation work.

101.

Why can a small loop antenna give good medium wave reception?

A.External noise dominates, so its low efficiency costs little
B.Its gain at those frequencies is comparable with a large mast
C.It is more efficient than a whip antenna of the same volume
D.It receives the magnetic field, which is stronger than the electric

Answer: A

A ferrite loop is enormously inefficient in absolute terms, but since atmospheric noise arrives through it as well, the signal to noise ratio is nearly as good as a large antenna would give. Its directional null is a further practical benefit. Its gain is far below a mast's, efficiency comparisons between small antennas are marginal, and the two field components carry the same information. An antenna that would be useless for transmitting can be entirely adequate for receiving, and the reason is the noise environment.

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102.

What is man made noise at medium and high frequencies?

A.Noise produced by lightning discharges in distant thunderstorms
B.Interference from switching devices, motors and power lines
C.The thermal noise generated by the receiver's own circuits
D.Noise arriving from cosmic sources through the ionosphere

Answer: B

Switching power supplies, electric motors, lighting and power line discharges radiate broadband interference that in urban areas often exceeds atmospheric noise, and it has risen steadily with the spread of electronic equipment. Receiver noise, cosmic noise and lightning are the other contributors, each dominating in a different band or environment. A noise budget must name the dominant source before any improvement is attempted, and in a city at medium frequency the answer is usually man made.

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103.

Why has man made noise become a growing problem for low frequency reception?

A.Receivers have become more sensitive and so pick up more noise
B.Transmitter powers have been reduced to save energy
C.The number of switching electronic devices has risen sharply
D.Atmospheric noise has fallen as the climate has changed

Answer: C

Every switched mode supply, lighting driver and data cable radiates broadband noise, and their proliferation has raised urban noise floors by many decibels over recent decades, which is one reason long wave and medium wave services have declined. Atmospheric noise is unchanged, receiver sensitivity is irrelevant in an externally limited system, and transmitter power reductions follow the decline rather than causing it. An environmental change that undermines a technology is worth recognising as such.

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104.

What does a directional receiving antenna achieve against local noise?

A.It reduces the total noise power reaching the receiver by its gain
B.It rejects all noise arriving from directions other than the wanted one
C.It converts the noise into a form the receiver can filter out
D.It can null a specific interfering source

Answer: D

A loop or a small array has a sharp null that can be turned onto a dominant local interferer, improving the ratio by twenty decibels or more against that one source. It cannot reject noise arriving from every direction at once, and distributed noise is only modestly reduced. Gain raises signal and distributed noise together, and no antenna changes the character of noise. Exploiting the geometry of a specific problem, rather than seeking a general improvement, is often the only effective remedy.

Discuss in Forum

105.

Why is a null more useful than gain against a single interferer?

A.The null is deep and narrow while gain raises everything equally
B.A null can be steered electronically whereas gain cannot
C.Nulls are unaffected by the polarisation of the interfering signal
D.Gain would overload the receiver front end at close range

Answer: A

Raising gain amplifies the wanted signal and the interferer alike if both are in the beam, whereas a null attenuates one direction by tens of decibels while barely affecting the other. Improving a ratio requires treating the two differently, which only a directional feature can do. Steerability and polarisation are secondary, and overload is a separate risk. The principle that amplification never improves a signal to interference ratio is worth internalising, since it recurs constantly.

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106.

What is the practical conclusion of comparing surface wave and space wave mechanisms?

A.The two are simply different names for the same mechanism
B.Each dominates in a different band and geometry
C.The surface wave is always the stronger of the two components
D.The space wave is only significant over sea water paths

Answer: B

The surface wave dominates at low frequency with antennas near the ground, and the space wave dominates at higher frequency with elevated antennas, so a service is designed around whichever suits its band and geometry. Neither is universally stronger, the space wave works over any surface, and the two are physically distinct components of the total ground wave. Knowing which mechanism will carry a signal before designing for it is the single most useful habit in propagation work.

Discuss in Forum

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