Space Wave Propagation Questions and Answers

Q: What is a radio hole caused by ducting?

Energy trapped in a duct is not available above it, so an aircraft flying over a ducting layer may find a surface radar unable to see it at all. The same trapping that extends surface coverage creates the hole. Terrain shadowing, interference nulls and pattern nulls are different causes of coverage gaps. Recognising that redistribution of energy creates deficits as well as surpluses is the key to predicting anomalous propagation correctly.

Q: What clearance criterion is usually applied to a microwave link?

Common practice demands full first zone clearance at a k of four thirds together with about sixty per cent at a low k value, so the path stays clear under normal and adverse refraction. Optical clearance alone would leave diffraction loss, ten per cent is far too little, and three zones would raise mast heights for no benefit. A criterion with two conditions at two refraction states is how a varying atmosphere is accommodated in a fixed design.

Q: What is the effect of a strong ground reflection on a line of sight link?

A reflection from water or flat ground arrives with an amplitude comparable to the direct ray, and as refraction changes the path difference the two alternately reinforce and cancel, giving fades of twenty decibels or more. Siting the antennas so the reflection point falls on rough or shielded ground, or using space diversity, is the remedy. Reinforcement is only half the story, and polarisation and refraction sensitivity are separate matters. Two comparable paths always risk cancellation.

Q: How can a specular ground reflection be avoided on a link?

The geometry fixes where the reflection occurs, so adjusting the antenna heights can move that point onto broken ground, vegetation or a screening ridge where no coherent reflection forms. Space diversity is the alternative. Power cannot fill a cancellation null, lower frequencies reflect at least as strongly, and tilting the antennas costs gain on the direct path without removing the reflected one. Moving a problem to where it does not matter is often cheaper than fighting it.

Q: What is space diversity on a microwave link?

Vertically separated antennas experience different path differences to the reflection point, so when one is in a fade the other generally is not, and combining or switching between them restores availability. Route diversity, sector antennas and polarisation diversity are different arrangements serving different purposes. Diversity works only where the two signals fade independently, and vertical separation on a reflective path is one of the situations where that independence is reliable.

Q: What is frequency diversity used against?

Because a multipath fade is deep only over a narrow band, a second carrier a per cent or so away is usually unaffected, so switching to it maintains the link. It also protects against equipment failure as a side benefit, since two complete radio chains are involved. Rain affects a whole band together and is not helped, and interference is a separate problem. Diversity requires independence, and frequency selective fading provides it while rain attenuation does not.

Q: Why does frequency diversity not help against rain attenuation?

Attenuation by rain varies slowly with frequency, so two carriers a few per cent apart fade together and no independence exists to exploit. Route diversity, with a second path outside the rain cell, is the effective answer where availability demands it. Rain does depolarise but affects both polarisations, fades last minutes rather than milliseconds, and the antenna match is unaffected. Choosing a diversity dimension in which the impairment is actually independent is the whole art of applying the technique.

Q: At what frequency does rain attenuation become significant?

Raindrops are a few millimetres across, so they scatter and absorb appreciably once the wavelength approaches that size, which happens above roughly ten gigahertz. Below that the drops are electrically tiny and the effect is negligible. At thirty gigahertz heavy rain can cost tens of decibels per kilometre. Comparing the size of a scatterer with the wavelength predicts at once whether it matters, and it is the first question to ask of any propagation impairment.

Q: How does rain attenuation depend on rain rate?

Specific attenuation follows a power law in rain rate with an exponent typically between one and one and a half, so heavy rain is disproportionately damaging. Since intense rain falls in small cells, long paths average over regions of different intensity and a path reduction factor is applied. Neither proportionality nor independence describes the relationship. A power law with an exponent above one means that the extremes of a distribution dominate the outage statistics.

11.

What is a radio hole caused by ducting?

A.A region where two signals cancel and produce a null
B.A part of the antenna pattern in which no power is radiated
C.A region above the duct that receives very little signal
D.A gap in coverage caused by an obstacle on the path

Answer: C

Energy trapped in a duct is not available above it, so an aircraft flying over a ducting layer may find a surface radar unable to see it at all. The same trapping that extends surface coverage creates the hole. Terrain shadowing, interference nulls and pattern nulls are different causes of coverage gaps. Recognising that redistribution of energy creates deficits as well as surpluses is the key to predicting anomalous propagation correctly.

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12.

Why is the first Fresnel zone relevant to a line of sight link?

A.It defines the width of the antenna beam at the mid point
B.It marks the boundary beyond which the wave is no longer coherent
C.It is the region within which the ground reflection takes place
D.Obstruction of it causes loss even when the direct ray is clear

Answer: D

Waves reaching the receiver by paths within half a wavelength of the direct route arrive nearly in phase and contribute constructively, so blocking part of that zone removes energy that would otherwise have arrived. Optical clearance alone is therefore insufficient. Beamwidth, coherence and the reflection point are separate concepts. Diffraction rather than geometric optics governs what an obstacle actually does, and the Fresnel zone is the practical expression of it.

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13.

What clearance criterion is usually applied to a microwave link?

A.Full first Fresnel clearance at standard refraction
B.Optical line of sight clearance under all refraction conditions
C.Clearance of ten per cent of the first Fresnel zone radius
D.Clearance of three complete Fresnel zones at every point

Answer: A

Common practice demands full first zone clearance at a k of four thirds together with about sixty per cent at a low k value, so the path stays clear under normal and adverse refraction. Optical clearance alone would leave diffraction loss, ten per cent is far too little, and three zones would raise mast heights for no benefit. A criterion with two conditions at two refraction states is how a varying atmosphere is accommodated in a fixed design.

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14.

What is the effect of a strong ground reflection on a line of sight link?

A.The link becomes immune to changes in atmospheric refraction
B.The two rays can cancel and cause deep fading
C.The received signal is always stronger than the direct ray alone
D.The polarisation of the received signal is rotated by ninety degrees

Answer: B

A reflection from water or flat ground arrives with an amplitude comparable to the direct ray, and as refraction changes the path difference the two alternately reinforce and cancel, giving fades of twenty decibels or more. Siting the antennas so the reflection point falls on rough or shielded ground, or using space diversity, is the remedy. Reinforcement is only half the story, and polarisation and refraction sensitivity are separate matters. Two comparable paths always risk cancellation.

Discuss in Forum

15.

How can a specular ground reflection be avoided on a link?

A.Use a lower frequency so that the reflection coefficient is smaller
B.Point the antennas slightly upward to miss the ground entirely
C.Choose antenna heights so the reflection point falls on rough terrain
D.Increase the transmitter power until the fade is no longer significant

Answer: C

The geometry fixes where the reflection occurs, so adjusting the antenna heights can move that point onto broken ground, vegetation or a screening ridge where no coherent reflection forms. Space diversity is the alternative. Power cannot fill a cancellation null, lower frequencies reflect at least as strongly, and tilting the antennas costs gain on the direct path without removing the reflected one. Moving a problem to where it does not matter is often cheaper than fighting it.

Discuss in Forum

16.

What is space diversity on a microwave link?

A.Two links following geographically separated routes to the same place
B.Two antennas pointing in different directions from the same site
C.Two polarisations received simultaneously on one antenna
D.Two receiving antennas at different heights on the same tower

Answer: D

Vertically separated antennas experience different path differences to the reflection point, so when one is in a fade the other generally is not, and combining or switching between them restores availability. Route diversity, sector antennas and polarisation diversity are different arrangements serving different purposes. Diversity works only where the two signals fade independently, and vertical separation on a reflective path is one of the situations where that independence is reliable.

Discuss in Forum

17.

What is frequency diversity used against?

A.Frequency selective multipath fading on a link
B.Interference from other users of the same frequency band
C.Rain attenuation, which affects all frequencies equally
D.Equipment failure in the transmitter or the receiver

Answer: A

Because a multipath fade is deep only over a narrow band, a second carrier a per cent or so away is usually unaffected, so switching to it maintains the link. It also protects against equipment failure as a side benefit, since two complete radio chains are involved. Rain affects a whole band together and is not helped, and interference is a separate problem. Diversity requires independence, and frequency selective fading provides it while rain attenuation does not.

Discuss in Forum

18.

Why does frequency diversity not help against rain attenuation?

A.Rain changes the antenna match rather than the path loss
B.Rain attenuates the whole band nearly equally
C.Rain affects only one polarisation at a time
D.Rain fades are too short for a switch to operate in time

Answer: B

Attenuation by rain varies slowly with frequency, so two carriers a few per cent apart fade together and no independence exists to exploit. Route diversity, with a second path outside the rain cell, is the effective answer where availability demands it. Rain does depolarise but affects both polarisations, fades last minutes rather than milliseconds, and the antenna match is unaffected. Choosing a diversity dimension in which the impairment is actually independent is the whole art of applying the technique.

Discuss in Forum

19.

At what frequency does rain attenuation become significant?

A.Above about one hundred megahertz in any climate
B.Only above one hundred gigahertz, where the drops resonate
C.Above about ten gigahertz
D.Above about one gigahertz in temperate climates

Answer: C

Raindrops are a few millimetres across, so they scatter and absorb appreciably once the wavelength approaches that size, which happens above roughly ten gigahertz. Below that the drops are electrically tiny and the effect is negligible. At thirty gigahertz heavy rain can cost tens of decibels per kilometre. Comparing the size of a scatterer with the wavelength predicts at once whether it matters, and it is the first question to ask of any propagation impairment.

Discuss in Forum

20.

How does rain attenuation depend on rain rate?

A.It rises exactly in proportion to the rain rate in millimetres per hour
B.It is independent of rain rate once rain is falling at all
C.It falls as the rain rate rises because the drops coalesce
D.It rises faster than in proportion to the rain rate

Answer: D

Specific attenuation follows a power law in rain rate with an exponent typically between one and one and a half, so heavy rain is disproportionately damaging. Since intense rain falls in small cells, long paths average over regions of different intensity and a path reduction factor is applied. Neither proportionality nor independence describes the relationship. A power law with an exponent above one means that the extremes of a distribution dominate the outage statistics.

Discuss in Forum

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