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Subel. ZLH
50 questions
Subelement ZLH

From Transmitter to Receiver

Section ZLH28

Propagation

A 'skip zone' is

  • the distance between the antenna and where the refracted wave first returns to earth
  • Correct Answer
    the distance between the far end of the ground wave and where the refracted wave first returns to earth
  • the distance between any two refracted waves
  • a zone caused by lost sky waves
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The medium which reflects high frequency radio waves back to the earth's surface is called the

  • biosphere
  • stratosphere
  • Correct Answer
    ionosphere
  • troposphere
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The highest frequency that will be reflected back to the earth at any given time is known as the

  • UHF
  • Correct Answer
    MUF
  • OWF
  • LUF
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All communications frequencies throughout the spectrum are affected in varying degrees by the

  • atmospheric conditions
  • ionosphere
  • aurora borealis
  • Correct Answer
    sun

Correct answer: sun

The sun is the primary energy source that drives radio propagation effects across the entire spectrum. Solar radiation creates and controls the ionosphere, affecting absorption, reflection, noise levels, and usable frequencies. Changes in solar activity such as the solar cycle, flares, and geomagnetic storms influence radio communications on many bands.

Solar heating also drives large-scale atmospheric behaviour, indirectly affecting refraction and attenuation at higher frequencies.

  • atmospheric conditions affect signal behaviour locally, but they are not the underlying cause of global propagation changes.
  • ionosphere mainly affects HF and some VHF, not all frequencies.
  • aurora borealis is a localized and intermittent effect.

Therefore, the fundamental influence affecting communications frequencies throughout the spectrum is the sun.

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Solar cycles have an average length of

  • 1 year
  • 3 years
  • 6 years
  • Correct Answer
    11 years
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The 'skywave' is another name for the

  • Correct Answer
    ionospheric wave
  • tropospheric wave
  • ground wave
  • inverted wave
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The polarisation of an electromagnetic wave is defined by the direction of

  • the H field
  • propagation
  • Correct Answer
    the E field
  • the receiving antenna

Correct answer: the E field

The polarisation of an electromagnetic wave is defined by the orientation of its electric field (E field) as the wave propagates through space.

For example:

  • if the E field oscillates vertically, the wave is vertically polarised
  • if the E field oscillates horizontally, the wave is horizontally polarised

Antennas are normally aligned to match the E field direction for maximum signal transfer.

  • the H field (magnetic field) is perpendicular to the E field and is not used to define polarisation.
  • propagation describes the direction the wave travels, not its polarisation.
  • the receiving antenna does not define the wave’s polarisation, it must align to the existing E field.

Therefore, the polarisation of an electromagnetic wave is defined by the direction of the E field.

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That portion of HF radiation which is directly affected by the surface of the earth is called

  • ionospheric wave
  • local field wave
  • Correct Answer
    ground wave
  • inverted wave
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Radio wave energy on frequencies below 4 MHz during daylight hours is almost completely absorbed by this ionospheric layer

  • C
  • Correct Answer
    D
  • E
  • F

Correct answer: D layer

During daylight, the D layer becomes strongly ionised by solar radiation and causes heavy absorption of low frequency radio waves, especially below about 4 MHz. Signals at these frequencies lose most of their energy as heat in the D layer and usually do not reach higher ionospheric layers to be refracted back to Earth.

  • C layer does not exist in the ionosphere model.
  • E layer mainly refracts signals that successfully pass through the D layer and is not responsible for heavy absorption.
  • F layer is responsible for long-distance skywave propagation and is reached only if signals are not absorbed first.

Therefore, radio wave energy below 4 MHz in daylight is almost completely absorbed by the D layer.

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Because of high absorption levels at frequencies below 4 MHz during daylight hours, only high angle signals are normally reflected back by this layer

  • C
  • D
  • Correct Answer
    E
  • F

Correct answer: E layer

During daylight, the D layer becomes strongly ionised and causes heavy absorption of radio waves below about \(4\ \mathrm{MHz}\), especially for low-angle signals. Most shallow-angle rays lose too much energy to reach higher layers.

Only high-angle (near-vertical) signals can pass through the D layer with enough remaining strength to reach the E layer, where they are refracted back to Earth, producing short-range skywave propagation.

  • C layer does not exist in the ionosphere model.
  • D layer mainly absorbs radio energy and does not normally reflect usable signals.
  • F layer reflections at these low frequencies are uncommon in daylight because most energy is absorbed before reaching it.

Therefore, the layer that normally reflects only high-angle signals under these conditions is the E layer.

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Scattered patches of high ionisation developed seasonally at the height of one of the layers is called

  • Correct Answer
    sporadic-E
  • patchy
  • random reflectors
  • trans-equatorial ionisation
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For long distance propagation, the radiation angle of energy from the antenna should be

  • Correct Answer
    less than 30 degrees
  • more than 30 degrees but less than forty-five
  • more than 45 degrees but less than ninety
  • 90 degrees
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The path radio waves normally follow from a transmitting antenna to a receiving antenna at VHF and higher frequencies is a

  • circular path going north or south from the transmitter
  • great circle path
  • Correct Answer
    straight line
  • bent path via the ionosphere
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A radio wave may follow two or more different paths during propagation and produce slowly-changing phase differences between signals at the receiver resulting in a phenomenon called

  • absorption
  • baffling
  • Correct Answer
    fading
  • skip
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The distance from the far end of the ground wave to the nearest point where the sky wave returns to the earth is called the

  • skip distance
  • radiation distance
  • skip angle
  • Correct Answer
    skip zone

Correct answer: skip zone

When a signal propagates by both ground wave and sky wave, the ground wave eventually weakens and dies out, while the sky wave returns to Earth some distance away from the transmitter.

The region between the end of usable ground wave coverage and the point where the sky wave first returns to Earth is called the skip zone. In this area, little or no signal is received.

Some references define skip distance as the distance from the transmitter to the first sky-wave return point. However, this question specifically describes the gap between the two coverage regions, which corresponds to the skip zone in this exam pool.

  • skip distance refers to a distance measurement, not the reception gap itself in this context.
  • radiation distance is not a standard propagation term.
  • skip angle describes the launch angle of the transmitted wave, not a ground region.

Therefore, the distance between the far end of the ground wave and the nearest sky wave return region is called the skip zone.

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High Frequency long-distance propagation is most dependent on

  • Correct Answer
    ionospheric reflection
  • tropospheric reflection
  • ground reflection
  • inverted reflection
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The layer of the ionosphere mainly responsible for long distance communication is

  • C
  • D
  • E
  • Correct Answer
    F
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The ionisation level of the ionosphere reaches its minimum

  • just after sunset
  • Correct Answer
    just before sunrise
  • at noon
  • at midnight

Correct answer: just before sunrise

The ionisation of the ionosphere is caused primarily by solar radiation. During daylight, ultraviolet and X-ray energy from the sun continually creates ionisation in the ionospheric layers.

After sunset, the ionising radiation stops, but recombination (ions and electrons recombining into neutral atoms) takes time. Ionisation therefore gradually decays through the night.

The lowest ionisation level occurs just before sunrise, after the longest continuous period without solar input and before the sun begins re-ionising the atmosphere again.

  • just after sunset still has significant residual ionisation from the daytime.
  • at noon corresponds to near maximum ionisation due to strongest solar radiation.
  • at midnight ionisation has already declined, but it continues to fall further until just before sunrise.

Therefore, the ionisation level of the ionosphere reaches its minimum just before sunrise.

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One of the ionospheric layers splits into two parts during the day called

  • A & B
  • D1 & D2
  • E1 & E2
  • Correct Answer
    F1 & F2
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Signal fadeouts resulting from an 'ionospheric storm' or 'sudden ionospheric disturbance' are usually attributed to

  • heating of the ionised layers
  • over-use of the signal path
  • insufficient transmitted power
  • Correct Answer
    solar flare activity
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The 80 metre band is useful for working

  • in the summer at midday during high sunspot activity
  • long distance during daylight hours when absorption is not significant
  • all points on the earth's surface
  • Correct Answer
    up to several thousand kilometres in darkness but atmospheric and man-made noises tend to be high
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The skip distance of radio signals is determined by the

  • type of transmitting antenna used
  • power fed to the final amplifier of the transmitter
  • only the angle of radiation from the antenna
  • Correct Answer
    both the height of the ionosphere and the angle of radiation from the antenna
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Three recognised layers of the ionosphere that affect radio propagation are

  • A, E, F
  • B, D, E
  • C, E, F
  • Correct Answer
    D, E, F
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Propagation on 80 metres during the summer daylight hours is limited to relatively short distances because of

  • Correct Answer
    high absorption in the D layer
  • the disappearance of the E layer
  • poor refraction by the F layer
  • pollution in the T layer

The D layer is the bottom layer of the ionosphere during the daylight. It absorbs medium and high frequency waves, 10 MHz and below.

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The distance from the transmitter to the nearest point where the sky wave returns to the earth is called the

  • angle of radiation
  • maximum usable frequency
  • Correct Answer
    skip distance
  • skip zone
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A variation in received signal strength caused by slowly changing differences in path lengths is called

  • absorption
  • Correct Answer
    fading
  • fluctuation
  • path loss
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VHF and UHF bands are frequently used for satellite communication because

  • Correct Answer
    waves at these frequencies travel to and from the satellite relatively unaffected by the ionosphere
  • the Doppler frequency change caused by satellite motion is much less than at HF
  • satellites move too fast for HF waves to follow
  • the Doppler effect would cause HF waves to be shifted into the VHF and UHF bands.
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The 'critical frequency' is defined as the

  • highest frequency to which your transmitter can be tuned
  • lowest frequency which is reflected back to earth at vertical incidence
  • minimum usable frequency
  • Correct Answer
    highest frequency which will be reflected back to earth at vertical incidence
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The speed of a radio wave

  • varies indirectly to the frequency
  • Correct Answer
    is the same as the speed of light
  • is infinite in space
  • is always less than half the speed of light
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The MUF for a given radio path is the

  • mean of the maximum and minimum usable frequencies
  • Correct Answer
    maximum usable frequency
  • minimum usable frequency
  • mandatory usable frequency
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The position of the E layer in the ionosphere is

  • above the F layer
  • Correct Answer
    below the F layer
  • below the D layer
  • sporadic
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A distant amplitude-modulated station is heard quite loudly but the modulation is at times severely distorted. A similar local station is not affected. The probable cause of this is

  • transmitter malfunction
  • Correct Answer
    selective fading
  • a sudden ionospheric disturbance
  • front end overload

Correct answer: selective fading

In amplitude modulation (AM), the signal consists of a carrier and two sidebands. During long-distance propagation, especially via the ionosphere, these components can take slightly different paths and experience different fading conditions.

When one sideband or the carrier fades more than the others, the recovered audio becomes distorted, even though the signal strength may still be strong. This effect is known as selective fading and is most noticeable on distant AM stations.

The local station is not affected because its signal arrives primarily by ground wave or short paths, so all components fade together, preserving the modulation.

  • transmitter malfunction would affect all listeners, not just distant reception.
  • a sudden ionospheric disturbance typically causes widespread signal loss or severe attenuation, not selective distortion of modulation.
  • front end overload would distort strong local signals as well, which is not observed here.

Therefore, the probable cause of the distorted modulation on the distant AM station is selective fading.

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Skip distance is a term associated with signals through the ionosphere. Skip effects are due to

  • Correct Answer
    reflection and refraction from the ionosphere
  • selective fading of local signals
  • high gain antennas being used
  • local cloud cover
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The type of atmospheric layers which will best return signals to earth are

  • oxidised layers
  • heavy cloud layers
  • Correct Answer
    ionised layers
  • sun spot layers
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The ionosphere

  • is a magnetised belt around the earth
  • consists of magnetised particles around the earth
  • Correct Answer
    is formed from layers of ionised gases around the earth
  • is a spherical belt of solar radiation around the earth
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The skip distance of a sky wave will be greatest when the

  • ionosphere is most densely ionised
  • signal given out is strongest
  • Correct Answer
    angle of radiation is smallest
  • polarisation is vertical
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If the height of the reflecting layer of the ionosphere increases, the skip distance of a high frequency transmission

  • stays the same
  • decreases
  • varies regularly
  • Correct Answer
    becomes greater
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If the frequency of a transmitted signal is so high that we no longer receive a reflection from the ionosphere, the signal frequency is above the

  • speed of light
  • sun spot frequency
  • skip distance
  • Correct Answer
    maximum usable frequency
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A 'line of sight' transmission between two stations uses mainly the

  • ionosphere
  • troposphere
  • sky wave
  • Correct Answer
    ground wave
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The distance travelled by ground waves in air

  • is the same for all frequencies
  • Correct Answer
    is less at higher frequencies
  • is more at higher frequencies
  • depends on the maximum usable frequency
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The radio wave from the transmitter to the ionosphere and back to earth is correctly known as the

  • Correct Answer
    sky wave
  • skip wave
  • surface wave
  • F layer
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Reception of high frequency radio waves beyond 4000 km normally occurs by the

  • ground wave
  • skip wave
  • surface wave
  • Correct Answer
    sky wave
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A 28 MHz radio signal is more likely to be heard over great distances

  • if the transmitter power is reduced
  • Correct Answer
    during daylight hours
  • only during the night
  • at full moon
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The number of high frequency bands open to long distance communication at any time depends on

  • Correct Answer
    the highest frequency at which ionospheric reflection can occur
  • the number of frequencies the receiver can tune
  • the power being radiated by the transmitting station
  • the height of the transmitting antenna
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Regular changes in the ionosphere occur approximately every 11

  • days
  • months
  • Correct Answer
    years
  • centuries
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When a HF transmitted radio signal reaches a receiver, small changes in the ionosphere can cause

  • consistently stronger signals
  • a change in the ground wave signal
  • Correct Answer
    variations in signal strength
  • consistently weaker signals
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The usual effect of ionospheric storms is to

  • increase the maximum usable frequency
  • Correct Answer
    cause a fade-out of sky-wave signals
  • produce extreme weather changes
  • prevent communications by ground wave
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Changes in received signal strength when sky wave propagation is used are called

  • ground wave losses
  • modulation losses
  • Correct Answer
    fading
  • sunspots
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Although high frequency signals may be received from a distant station by a sky wave at a certain time, it may not be possible to hear them an hour later. This may be due to

  • Correct Answer
    changes in the ionosphere
  • shading of the earth by clouds
  • changes in atmospheric temperature
  • absorption of the ground wave signal
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VHF or UHF signals transmitted towards a tall building are often received at a more distant point in another direction because

  • these waves are easily bent by the ionosphere
  • Correct Answer
    these waves are easily reflected by objects in their path
  • you can never tell in which direction a wave is travelling
  • tall buildings have elevators
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