Space Wave Propagation Questions and Answers

Q: Why does a repeater transmit on a different frequency from the one it receives?

With the transmitter and receiver on the same tower, even sixty or eighty decibels of antenna isolation would not prevent oscillation if both used the same frequency, so a frequency pair is used and the roles alternate along the route. Regulation follows the technical need, amplification efficiency is unaffected, and antennas handle multiple signals routinely. Positive feedback around a loop with gain is the risk being avoided, and the same argument governs any repeater in any medium.

Q: What is a regenerative repeater?

A regenerative repeater recovers the data, removes accumulated noise and distortion and transmits a fresh signal, so impairments do not build up along a long chain. A non-regenerative repeater amplifies noise along with the signal, and the degradation accumulates hop by hop. Power supply arrangements and passive reflection are different matters. Restoring a signal to its ideal form at each stage is what allows a digital chain to be arbitrarily long, and it is the central advantage of digital transmission.

Q: Why does noise accumulate along a chain of amplifying repeaters?

An analogue repeater cannot distinguish signal from noise, so it raises both and contributes its own noise figure, and the ratio degrades at every hop. Regeneration breaks the chain by making a decision and starting again. The signal is restored to level at each hop rather than weakening cumulatively, noise adds in power rather than coherently, and the atmospheric contribution is small at these frequencies. Whether impairments accumulate or are reset is the deciding difference between the two repeater types.

Q: What determines the number of hops on a long microwave route?

Hop length is set by line of sight geometry over the terrain and by the fade margin achievable at the chosen frequency, and the route is divided accordingly. Existing sites, capacity and spectrum influence the design but do not determine the hop count. In flat country hops of fifty kilometres are possible at lower microwave frequencies, while mountainous terrain may force much shorter ones. Geometry first, then margin, then everything else, is the order in which a route is planned.

Q: What is the advantage of siting a repeater on a hilltop?

Elevation extends the horizon and clears intervening terrain, so far fewer sites are needed than a route along a valley floor would require. The costs are access, power supply and exposure to weather and lightning. Rain attenuation is if anything similar, and a high site generally sees more interference rather than less, because it can see so far. Buying hop length with site difficulty is the classic trade in microwave route planning.

Q: Why does a high site see more interference than a low one?

The same elevation that gives a long clear path to the next repeater also gives clear paths to every other transmitter within the extended horizon, so co-channel and adjacent channel interference must be assessed carefully at hilltop sites. Antenna directivity, atmospheric noise and pressure are not the mechanism. A property that helps the wanted signal helping the unwanted one equally is a recurring difficulty, and it is why frequency coordination is done site by site.

Q: What antenna property most helps frequency coordination?

Coordination depends on how little a link radiates and receives in directions other than its own, so the side lobe envelope, specified as a mask, decides how closely other links can operate. Forward gain matters for the link's own budget rather than for its neighbours. Bandwidth and weight are practical properties with no bearing on interference. A specification written in terms of what must not be radiated is what protects a shared band, exactly as it does for satellite earth stations.

Q: Why are high performance antennas specified in congested areas?

A shrouded high performance dish radiates far less off boresight, so more links can share the band in the same area, which is precisely what a congested region needs. Its gain is similar to a standard dish of the same size, alignment becomes more rather than less critical, and robustness is incidental. Paying for a property that benefits everyone else in the band, rather than the link itself, is exactly the kind of externality that regulation exists to enforce.

Q: What is the general lesson of space wave propagation for system design?

A line of sight link succeeds or fails on the clearance of its Fresnel zone, the behaviour of the atmosphere over its route and the impairments the climate imposes, with power the least interesting variable. Free space loss is only the starting point of a budget. A generous margin helps but cannot substitute for correct geometry. Assessing the path before selecting the equipment is what distinguishes a designed link from an optimistic one.

91.

Why does a repeater transmit on a different frequency from the one it receives?

A.Because the antenna cannot handle two signals at one frequency
B.To prevent the transmitter from feeding back into its own receiver
C.Because the regulator allocates a separate frequency to each hop
D.To allow the signal to be amplified more efficiently

Answer: B

With the transmitter and receiver on the same tower, even sixty or eighty decibels of antenna isolation would not prevent oscillation if both used the same frequency, so a frequency pair is used and the roles alternate along the route. Regulation follows the technical need, amplification efficiency is unaffected, and antennas handle multiple signals routinely. Positive feedback around a loop with gain is the risk being avoided, and the same argument governs any repeater in any medium.

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92.

What is a regenerative repeater?

A.One powered by solar panels at a remote unattended site
B.One that reflects the signal passively towards the next station
C.One that demodulates and retransmits a clean copy of the signal
D.One that amplifies the signal without demodulating it at all

Answer: C

A regenerative repeater recovers the data, removes accumulated noise and distortion and transmits a fresh signal, so impairments do not build up along a long chain. A non-regenerative repeater amplifies noise along with the signal, and the degradation accumulates hop by hop. Power supply arrangements and passive reflection are different matters. Restoring a signal to its ideal form at each stage is what allows a digital chain to be arbitrarily long, and it is the central advantage of digital transmission.

Discuss in Forum

93.

Why does noise accumulate along a chain of amplifying repeaters?

A.The signal weakens at each hop while the noise stays constant
B.The repeaters are not synchronised and their noise adds coherently
C.Atmospheric noise is picked up afresh at every site along the route
D.Each repeater amplifies the noise it receives and adds its own

Answer: D

An analogue repeater cannot distinguish signal from noise, so it raises both and contributes its own noise figure, and the ratio degrades at every hop. Regeneration breaks the chain by making a decision and starting again. The signal is restored to level at each hop rather than weakening cumulatively, noise adds in power rather than coherently, and the atmospheric contribution is small at these frequencies. Whether impairments accumulate or are reset is the deciding difference between the two repeater types.

Discuss in Forum

94.

What determines the number of hops on a long microwave route?

A.The terrain and the maximum reliable hop length at that frequency
B.The number of repeater sites the operator happens to own already
C.The total capacity that the route is required to carry
D.The regulatory limit on the number of frequencies available

Answer: A

Hop length is set by line of sight geometry over the terrain and by the fade margin achievable at the chosen frequency, and the route is divided accordingly. Existing sites, capacity and spectrum influence the design but do not determine the hop count. In flat country hops of fifty kilometres are possible at lower microwave frequencies, while mountainous terrain may force much shorter ones. Geometry first, then margin, then everything else, is the order in which a route is planned.

Discuss in Forum

95.

What is the advantage of siting a repeater on a hilltop?

A.Interference from other users is reduced at greater height
B.Longer hops are possible because the horizon is further away
C.The equipment is easier to access for maintenance visits
D.Rain attenuation is lower at higher altitude on the route

Answer: B

Elevation extends the horizon and clears intervening terrain, so far fewer sites are needed than a route along a valley floor would require. The costs are access, power supply and exposure to weather and lightning. Rain attenuation is if anything similar, and a high site generally sees more interference rather than less, because it can see so far. Buying hop length with site difficulty is the classic trade in microwave route planning.

Discuss in Forum

96.

Why does a high site see more interference than a low one?

A.Atmospheric noise increases with altitude above sea level
B.The equipment is more sensitive at lower air pressure
C.It has line of sight to many more distant transmitters
D.Its antennas are less directive because of the exposure

Answer: C

The same elevation that gives a long clear path to the next repeater also gives clear paths to every other transmitter within the extended horizon, so co-channel and adjacent channel interference must be assessed carefully at hilltop sites. Antenna directivity, atmospheric noise and pressure are not the mechanism. A property that helps the wanted signal helping the unwanted one equally is a recurring difficulty, and it is why frequency coordination is done site by site.

Discuss in Forum

97.

What is frequency coordination between microwave links?

A.Synchronising the transmitters of a route to a common clock
B.Aligning the polarisations of neighbouring links in a region
C.Sharing a single frequency between several links in rotation
D.Assigning frequencies so that links do not interfere with each other

Answer: D

Before a link is licensed its potential to interfere with existing links, and their potential to interfere with it, is calculated from the geometry and the antenna patterns, and a frequency and polarisation are chosen to keep both within limits. Clock synchronisation, polarisation alignment and time sharing are different techniques. A shared resource allocated by calculation rather than by first come convention is what allows dense reuse of a fixed band.

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98.

What antenna property most helps frequency coordination?

A.A low side lobe envelope that limits radiation off boresight
B.A high forward gain that concentrates the beam on the wanted path
C.A wide bandwidth allowing the link to change frequency easily
D.A low weight that reduces the loading on the supporting tower

Answer: A

Coordination depends on how little a link radiates and receives in directions other than its own, so the side lobe envelope, specified as a mask, decides how closely other links can operate. Forward gain matters for the link's own budget rather than for its neighbours. Bandwidth and weight are practical properties with no bearing on interference. A specification written in terms of what must not be radiated is what protects a shared band, exactly as it does for satellite earth stations.

Discuss in Forum

99.

Why are high performance antennas specified in congested areas?

A.Their construction is more robust in exposed urban locations
B.Their tighter side lobe masks allow denser frequency reuse
C.Their higher gain permits lower transmitter power to be used
D.Their narrower beams make alignment less critical in practice

Answer: B

A shrouded high performance dish radiates far less off boresight, so more links can share the band in the same area, which is precisely what a congested region needs. Its gain is similar to a standard dish of the same size, alignment becomes more rather than less critical, and robustness is incidental. Paying for a property that benefits everyone else in the band, rather than the link itself, is exactly the kind of externality that regulation exists to enforce.

Discuss in Forum

100.

What is the general lesson of space wave propagation for system design?

A.Only the free space loss needs to be considered on a clear path
B.Propagation effects can be ignored if the fade margin is large
C.Geometry, the atmosphere and the terrain must all be assessed
D.Transmitter power is the main variable available to the designer

Answer: C

A line of sight link succeeds or fails on the clearance of its Fresnel zone, the behaviour of the atmosphere over its route and the impairments the climate imposes, with power the least interesting variable. Free space loss is only the starting point of a budget. A generous margin helps but cannot substitute for correct geometry. Assessing the path before selecting the equipment is what distinguishes a designed link from an optimistic one.

Discuss in Forum

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