Ground Wave Propagation Questions and Answers

Q: What are the two components of the ground wave?

The ground wave comprises a surface wave guided along the boundary between air and earth and a space wave made up of the direct and ground reflected rays. The surface wave dominates at low frequencies and short ranges over good ground, while the space wave dominates at higher frequencies and with elevated antennas. Ionospheric reflection is the sky wave, a quite separate mechanism, and the ordinary and extraordinary waves belong to magneto-ionic theory. Sorting a received signal into its propagation mechanisms is the first step in predicting it.

Q: Why does a horizontally polarised wave attenuate so rapidly over ground?

A horizontal electric field must be nearly zero at the surface of a good conductor, so the image is in antiphase and the direct and reflected fields very nearly cancel near the ground. A vertical field has an in phase image instead and is reinforced. The same reasoning explains why a horizontal dipole close to ground radiates almost entirely upward. Boundary conditions at a conducting surface, rather than absorption in any material, decide which polarisation can propagate.

Q: What is the tilt of the wavefront in surface wave propagation?

Because the earth is not a perfect conductor, a small horizontal component of electric field appears at the surface and the resultant vector leans forward in the direction of travel. That component drives current in the ground and is exactly the mechanism by which the wave loses energy. It is also what allows a horizontal receiving wire to pick up a surface wave at all. A departure from ideal boundary conditions producing both a new field component and a loss mechanism is a common pairing.

Q: How does surface wave attenuation vary with frequency?

Ground losses rise steeply with frequency, which is why the surface wave is useful for hundreds of kilometres at low frequency, tens at medium wave and only a few at high frequency. That single relationship explains the choice of band for every long range surface wave service. There is no resonance of the ground, and the attenuation certainly does not fall with frequency. Knowing the direction and steepness of the frequency dependence lets a coverage estimate be sanity checked in a moment.

Q: Over which surface does the ground wave travel furthest?

Sea water has a conductivity thousands of times greater than dry ground, so the surface wave loses far less energy and ranges of many hundreds of kilometres are achieved. Dry sand and rock are the worst surfaces, and ice is a poor conductor rather than a good one. Terrain height helps the space wave rather than the surface wave. Conductivity of the surface, rather than anything above it, is what governs surface wave range.

Q: What ground constants describe a surface for propagation purposes?

Propagation over a surface depends on its conductivity in siemens per metre and its relative permittivity, and published maps give both for use in coverage prediction. Moisture strongly affects conductivity, which is why the same terrain behaves differently in wet and dry seasons, but it enters through the conductivity rather than directly. Density, temperature and roughness matter only through their effect on these two constants. Reducing a complicated medium to two numbers is what makes prediction tractable.

Q: Which quantity determines whether ground behaves as a conductor or a dielectric?

Comparing the conduction current, proportional to conductivity, with the displacement current, proportional to frequency and permittivity, decides which behaviour dominates. The ground looks like a conductor at low frequencies and increasingly like a lossy dielectric as the frequency rises, which is part of why surface wave attenuation grows so rapidly. Roughness and layer depth matter separately. A dimensionless ratio deciding which of two regimes applies is a recurring pattern in electromagnetics.

Q: What is skin depth in the ground?

Skin depth is the distance in which the field falls by a factor of e, and it decreases with frequency and with conductivity. At medium wave over average ground it is a few metres, which is why buried radials need only be shallow. It is not a reflection depth and certainly not a current free layer, since it is precisely where the current flows. Knowing the depth over which a medium participates tells you how much of it matters to the problem.

Q: Why are radials buried only shallowly beneath a broadcast antenna?

Ground current concentrates in the top few metres, so radials laid a few tens of centimetres deep intercept essentially all of it while being protected from ploughing and traffic. Depth beyond that adds nothing electrically. Water table, access and resonance are secondary or irrelevant. Understanding where the current actually flows determines how much copper must be laid and where, which is a substantial part of the cost of a broadcast installation.

What are the two components of the ground wave?

A.The surface wave and the space wave
B.The direct wave and the wave reflected from the ionosphere above
C.The ordinary and the extraordinary waves produced by the earth's field
D.The vertically and the horizontally polarised parts of the transmitted signal

Answer: A

The ground wave comprises a surface wave guided along the boundary between air and earth and a space wave made up of the direct and ground reflected rays. The surface wave dominates at low frequencies and short ranges over good ground, while the space wave dominates at higher frequencies and with elevated antennas. Ionospheric reflection is the sky wave, a quite separate mechanism, and the ordinary and extraordinary waves belong to magneto-ionic theory. Sorting a received signal into its propagation mechanisms is the first step in predicting it.

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Which polarisation is used for surface wave propagation?

A.Either, since the ground affects both polarisations equally at low frequency
B.Vertical
C.Horizontal
D.Circular, so that the wave is unaffected by the ground's orientation

Answer: B

The earth is a reasonably good conductor at low frequencies, so a horizontally polarised wave is short circuited by it and attenuates very rapidly, while a vertically polarised wave survives. That is why medium wave broadcast antennas are vertical masts. The electric field of the vertical wave is perpendicular to the surface and is not cancelled by the image. Choosing the polarisation that the boundary condition permits is the single most important decision in ground wave working.

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Why does a horizontally polarised wave attenuate so rapidly over ground?

A.Its electric field is absorbed by vegetation and buildings
B.It is refracted downward into the ground and does not return
C.Its image in the conducting earth is in antiphase and cancels it
D.It couples strongly into the ionosphere and is lost upward

Answer: C

A horizontal electric field must be nearly zero at the surface of a good conductor, so the image is in antiphase and the direct and reflected fields very nearly cancel near the ground. A vertical field has an in phase image instead and is reinforced. The same reasoning explains why a horizontal dipole close to ground radiates almost entirely upward. Boundary conditions at a conducting surface, rather than absorption in any material, decide which polarisation can propagate.

Discuss in Forum

What is the tilt of the wavefront in surface wave propagation?

A.A sideways tilt caused by the earth's magnetic field
B.An upward tilt that carries energy away into the ionosphere
C.A backward lean produced by the antenna's own ground system
D.A forward lean caused by the finite conductivity of the ground

Answer: D

Because the earth is not a perfect conductor, a small horizontal component of electric field appears at the surface and the resultant vector leans forward in the direction of travel. That component drives current in the ground and is exactly the mechanism by which the wave loses energy. It is also what allows a horizontal receiving wire to pick up a surface wave at all. A departure from ideal boundary conditions producing both a new field component and a loss mechanism is a common pairing.

Discuss in Forum

How does surface wave attenuation vary with frequency?

A.It increases rapidly with frequency
B.It decreases with frequency because the wavelength becomes shorter
C.It is independent of frequency for a given ground conductivity
D.It is greatest at the frequency where the ground becomes resonant

Answer: A

Ground losses rise steeply with frequency, which is why the surface wave is useful for hundreds of kilometres at low frequency, tens at medium wave and only a few at high frequency. That single relationship explains the choice of band for every long range surface wave service. There is no resonance of the ground, and the attenuation certainly does not fall with frequency. Knowing the direction and steepness of the frequency dependence lets a coverage estimate be sanity checked in a moment.

Discuss in Forum

Over which surface does the ground wave travel furthest?

A.Mountainous rock, because the height carries the wave above obstacles
B.Sea water
C.Dry sandy desert, because there is no vegetation to absorb the wave
D.Frozen ground

Answer: B

Sea water has a conductivity thousands of times greater than dry ground, so the surface wave loses far less energy and ranges of many hundreds of kilometres are achieved. Dry sand and rock are the worst surfaces, and ice is a poor conductor rather than a good one. Terrain height helps the space wave rather than the surface wave. Conductivity of the surface, rather than anything above it, is what governs surface wave range.

Discuss in Forum

What ground constants describe a surface for propagation purposes?

A.Temperature and moisture content measured at the surface
B.Roughness and the average height of vegetation on it
C.Conductivity and relative permittivity
D.Density and mechanical hardness of the surface material

Answer: C

Propagation over a surface depends on its conductivity in siemens per metre and its relative permittivity, and published maps give both for use in coverage prediction. Moisture strongly affects conductivity, which is why the same terrain behaves differently in wet and dry seasons, but it enters through the conductivity rather than directly. Density, temperature and roughness matter only through their effect on these two constants. Reducing a complicated medium to two numbers is what makes prediction tractable.

Discuss in Forum

Which quantity determines whether ground behaves as a conductor or a dielectric?

A.The ratio of the wavelength to the depth of the soil layer
B.The ratio of the surface roughness to the wavelength in use
C.The ratio of the permittivity to the permeability of the ground
D.The ratio of conduction current to displacement current

Answer: D

Comparing the conduction current, proportional to conductivity, with the displacement current, proportional to frequency and permittivity, decides which behaviour dominates. The ground looks like a conductor at low frequencies and increasingly like a lossy dielectric as the frequency rises, which is part of why surface wave attenuation grows so rapidly. Roughness and layer depth matter separately. A dimensionless ratio deciding which of two regimes applies is a recurring pattern in electromagnetics.

Discuss in Forum

What is skin depth in the ground?

A.The depth at which the field falls to about a third of its surface value
B.The depth of soil that must be removed before a radial system is buried
C.The depth to which a surface wave penetrates before being reflected
D.The thickness of the layer of ground that carries no current at all

Answer: A

Skin depth is the distance in which the field falls by a factor of e, and it decreases with frequency and with conductivity. At medium wave over average ground it is a few metres, which is why buried radials need only be shallow. It is not a reflection depth and certainly not a current free layer, since it is precisely where the current flows. Knowing the depth over which a medium participates tells you how much of it matters to the problem.

Discuss in Forum

10.

Why are radials buried only shallowly beneath a broadcast antenna?

A.Deeper radials would resonate at a lower frequency than intended
B.The return current flows within a skin depth of the surface
C.Deeper burial would be prevented by the water table at most sites
D.The radials must remain within reach for inspection and repair

Answer: B

Ground current concentrates in the top few metres, so radials laid a few tens of centimetres deep intercept essentially all of it while being protected from ploughing and traffic. Depth beyond that adds nothing electrically. Water table, access and resonance are secondary or irrelevant. Understanding where the current actually flows determines how much copper must be laid and where, which is a substantial part of the cost of a broadcast installation.

Discuss in Forum

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