Correct answer: the distance between the far end of the ground wave and where the refracted wave first returns to earth

The skip zone is the region where no signal is received because:

• the ground wave has become too weak to be detected
• the skywave has not yet returned to Earth

It lies between the point where the ground wave coverage ends and the point where the refracted skywave first returns.

• The distance from the antenna to the first return point is the skip distance.
• The distance between refracted waves is unrelated.
• It is not simply caused by “lost” sky waves.

Therefore, the skip zone is the distance between the far end of the ground wave and where the refracted wave first returns to earth.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — ionosphere

The ionosphere is a region of the upper atmosphere (roughly 60–1000 km altitude) where solar radiation ionises gas molecules, creating layers of free electrons and ions. These ionised layers (designated D, E, F1, and F2) are capable of refracting and reflecting high-frequency (HF) radio waves back toward the Earth's surface, enabling long-distance communication well beyond the line of sight.

• A. Biosphere — the zone of Earth supporting life; it has no special effect on radio wave propagation.
• B. Stratosphere — the atmospheric layer from about 12–50 km altitude; it contains the ozone layer but does not significantly reflect radio waves.
• D. Troposphere — the lowest atmospheric layer (0–12 km); it affects VHF/UHF propagation through ducting but does not reflect HF radio waves back to Earth in the same way.

Therefore, the ionosphere is the medium responsible for reflecting HF radio waves back to the Earth's surface, making long-distance amateur radio communication possible.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: B — MUF

The Maximum Usable Frequency (MUF) is the highest frequency that the ionosphere will reflect back to Earth at a given time, for a given path and direction. Frequencies above the MUF pass straight through the ionosphere into space and are not returned to Earth. The MUF varies with the time of day, season, solar activity, and the angle at which the radio wave strikes the ionosphere.

• A. UHF — Ultra High Frequency is a frequency band (300 MHz–3 GHz), not a propagation limit. UHF signals almost always pass through the ionosphere rather than being reflected.
• C. OWF — The Optimum Working Frequency is typically around 85% of the MUF and is the recommended operating frequency for reliable communication. It is related to, but not the same as, the MUF.
• D. LUF — The Lowest Usable Frequency is the minimum frequency that provides a usable signal on a given path; below the LUF, absorption losses become too great. It is the opposite end of the usable frequency range.

Therefore, the MUF defines the upper frequency boundary for ionospheric (skywave) propagation at any given time.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — 11 years

The solar cycle is the roughly 11-year periodic rise and fall in the Sun's activity, measured primarily by the number of sunspots visible on the solar surface. At solar maximum, sunspot numbers are high, solar radiation and particle flux are elevated, and ionospheric layers (particularly the F2 layer) become more densely ionised — greatly improving HF propagation, especially on the higher amateur bands. At solar minimum, the opposite applies and higher HF bands may become unreliable or dead for long periods.

• A — 1 year is incorrect; this is simply the Earth's orbital period around the Sun, unrelated to the sunspot cycle.
• B — 3 years is incorrect; no recognised solar activity cycle has this period.
• C — 6 years is incorrect; this is approximately half a solar cycle but is not the cycle length itself.

Therefore, amateur radio operators planning long-term HF operation should be aware that propagation conditions follow an approximately 11-year solar cycle, and band openings on frequencies such as 10 m and 15 m are far more common near solar maximum.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — ionospheric wave

The skywave is the term used for radio signals that travel upward from the transmitting antenna, are refracted (bent) by the ionosphere, and return to Earth at some distant point. Because this propagation mechanism depends entirely on the ionosphere, "skywave" and "ionospheric wave" are interchangeable terms. Skywave propagation is responsible for long-distance (DX) HF communication, sometimes spanning thousands of kilometres.

• B. tropospheric wave — Tropospheric propagation occurs in the lowest layer of the atmosphere (the troposphere) and is a distinct mechanism used mainly at VHF and above; it is not called a skywave.
• C. ground wave — The ground wave travels along the Earth's surface, hugging the terrain; it is the opposite propagation mode to the skywave and is used mainly at MF and lower HF frequencies over short to medium distances.
• D. inverted wave — "Inverted wave" is not a recognised propagation term in radio science or NZART terminology.

Therefore, "skywave" is simply another name for the ionospheric wave — a signal refracted back to Earth by the ionosphere.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: ground wave

Ground wave propagation refers to that portion of a radio signal that travels along and is directly affected by the surface of the Earth.

It follows the curvature of the Earth and is influenced by:

• ground conductivity

• terrain

• frequency (lower frequencies travel further)

• Ionospheric wave refers to skywave propagation.

• The other terms are not standard.

Therefore, the portion of HF radiation affected by the Earth’s surface is the ground wave.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: sporadic-E

Sporadic-E (Es) refers to irregular, patchy regions of high ionisation that form in the E layer of the ionosphere.

These patches:

• occur unpredictably, often seasonally

• can reflect higher-frequency signals (e.g. VHF) that normally pass through the E layer

• enable unusual long-distance contacts

• “patchy” and “random reflectors” are not standard terms.

• Trans-equatorial ionisation is a different propagation phenomenon.

Therefore, these ionised patches are called sporadic-E.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — less than 30 degrees

For long-distance (DX) HF propagation via the ionosphere, the antenna must launch energy at a low angle relative to the horizon — called the radiation angle or take-off angle. A low angle means the signal travels a long, shallow path up to the ionosphere and back down, covering a much greater ground distance per hop. As the radiation angle increases, each hop covers less distance, making it increasingly inefficient for reaching distant stations.

A radiation angle below 30 degrees is generally considered necessary for reliable DX work. Antennas designed for DX (such as a dipole at a good height, a Yagi, or a vertical) are optimised to maximise their radiation at these low angles.

• B (more than 30° but less than 45°) — still relatively low but not optimised for the longest distances; more suited to medium-distance paths.
• C (more than 45° but less than 90°) — high-angle radiation; useful for short-skip (NVIS) paths of a few hundred kilometres, not DX.
• D (90 degrees) — straight up (Near Vertical Incidence Skywave, NVIS); returns signal almost directly beneath the antenna, useful for regional communication within a few hundred kilometres but useless for long-distance DX.

Therefore, for long-distance ionospheric propagation, a radiation angle of less than 30 degrees from the horizon is required to achieve the long shallow hops needed to cover DX paths efficiently.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: straight line

At VHF and higher frequencies, radio waves typically propagate by line-of-sight (space wave) paths.

This means they travel:

• directly from the transmitting antenna to the receiving antenna

• in a straight-line path (subject to slight bending due to the atmosphere)

• Great circle paths apply to long-distance HF propagation.

• Ionospheric reflection is primarily an HF phenomenon.

• Circular paths are not relevant.

Therefore, the path is a straight line.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — fading

When a radio wave travels from transmitter to receiver by two or more different paths simultaneously (for example, a ground wave and a sky wave, or multiple ionospheric hops), the signals arrive with slightly different time delays and therefore different phases. These phase differences change slowly as ionospheric conditions vary, causing the received signals to alternately add constructively (stronger signal) or destructively (weaker signal). This slow, rhythmic variation in received signal strength is called fading, or more precisely multipath fading.

• A. absorption — Absorption is the loss of radio wave energy as it passes through a medium (such as the D-layer or dense atmosphere); it does not describe the multipath phase-difference effect.
• B. baffling — "Baffling" is not a recognised radio propagation term; it is a distractor.
• D. skip — Skip (or skip distance) refers to the minimum distance from a transmitter at which a sky wave returns to Earth; it describes coverage geometry, not the phase-interference effect.

Therefore, the slowly-changing phase differences produced by multipath propagation result in the phenomenon known as fading.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — ionospheric reflection

High Frequency (HF) signals in the 3–30 MHz range are refracted (bent) back toward Earth by the ionosphere — the electrically charged layers of the upper atmosphere (D, E, F1, and F2 layers) at altitudes of roughly 60–400 km. This mechanism allows HF signals to travel thousands of kilometres beyond the horizon, making ionospheric reflection (refraction) the cornerstone of long-distance HF communication.

• B. Tropospheric reflection — The troposphere (the lowest atmospheric layer, up to ~12 km) can scatter or reflect VHF/UHF signals over modest distances, but it has negligible effect on HF long-distance propagation.
• C. Ground reflection — Ground (surface) reflection plays a role in antenna radiation patterns close to the antenna, but it does not enable long-distance HF skip propagation.
• D. Inverted reflection — This is not a recognised propagation mechanism; it is a distractor with no technical meaning.

Therefore, ionospheric reflection is the dominant mechanism that makes long-distance HF communication possible, with signals skipping between the ionosphere and Earth's surface over intercontinental paths.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — F

The F layer is the highest region of the ionosphere, sitting at roughly 150–400 km altitude. Because it is so high, radio waves refracted back from the F layer travel the greatest possible distances before returning to Earth — a single "hop" can cover 3,000 km or more. The F layer also persists through the night (unlike lower layers), making it the primary enabler of long-distance HF communication.

• C layer is not a recognised standard ionospheric layer; it is not a useful propagation medium.
• D layer exists only during daylight hours, sits low (60–90 km), and primarily absorbs HF signals rather than refracting them back to Earth — it impedes, rather than assists, long-distance communication.
• E layer (90–130 km) does support some medium-distance propagation (up to ~2,000 km), and sporadic-E can occasionally extend this, but its lower altitude means shorter hops and it cannot match the F layer for consistent long-distance paths.

Therefore, the F layer is the ionospheric layer mainly responsible for long-distance HF radio communication.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: F1 & F2

During daylight hours, increased solar radiation causes the F layer of the ionosphere to split into two distinct regions known as F1 and F2.

• F1 layer forms at a lower altitude and is mainly present during daylight.
• F2 layer forms higher, remains ionised longer, and is responsible for most long-distance HF propagation.

At night, reduced ionisation causes the F1 and F2 layers to merge back into a single F layer.

• A & B are not ionospheric layer designations.
• D1 & D2 are not recognised ionospheric layers.
• E1 & E2 are not standard ionospheric terms.

Therefore, the layer that splits into two parts during the day is the F layer, forming F1 and F2.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — solar flare activity

Ionospheric storms and sudden ionospheric disturbances (SIDs) are both caused by intense bursts of radiation and energetic particles from the Sun — primarily solar flares. A solar flare releases a sudden surge of X-ray and ultraviolet radiation that reaches Earth in approximately 8 minutes, dramatically increasing ionisation in the D-layer. This causes heavy absorption of HF signals, often producing a complete HF blackout on the sunlit side of Earth. Accompanying geomagnetic storms (from the slower-arriving charged particle stream) can then disturb the higher ionospheric layers for hours or days, causing signal fadeouts and erratic propagation.

• A — heating of the ionised layers: While solar radiation does heat the ionosphere, "heating" alone is not the recognised cause of storm-level fadeouts and SIDs; the driver is the abnormal radiation burst from a flare.
• B — over-use of the signal path: Radio propagation paths cannot be "worn out" or overloaded by traffic; this is not a physical mechanism.
• C — insufficient transmitted power: Fadeouts during ionospheric storms affect all stations regardless of power level; increasing power does not overcome D-layer absorption during a severe SID.

Therefore, signal fadeouts from ionospheric storms and sudden ionospheric disturbances are attributed to solar flare activity, which causes abnormal ionospheric absorption and disruption of HF propagation.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: up to several thousand kilometres in darkness but atmospheric and man-made noises tend to be high

The 80 metre band (around 3.5–4 MHz) is well suited for night-time skywave propagation.

After sunset:

• D-layer absorption decreases
• signals can be reflected by higher ionospheric layers (E and F)

This allows communication over distances of several thousand kilometres.

However:

• atmospheric noise (e.g. thunderstorms)
• man-made electrical noise

are often significant at these lower frequencies.

• Midday summer conditions usually favour higher frequencies.
• Daylight absorption limits long-distance HF communication on 80 m.
• It is not suitable for global coverage at all times.

Sanity check: \(f \approx \frac{300}{\lambda}\) gives the approximate centre frequency for the band.

Therefore, 80 metres is useful for working up to several thousand kilometres in darkness but atmospheric and man-made noises tend to be high.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — both the height of the ionosphere and the angle of radiation from the antenna

Skip distance is the minimum ground distance between a transmitter and the point where a sky-wave signal first returns to Earth after being refracted by the ionosphere. Two factors together control this distance: how high the refracting layer is (a higher layer pushes the return point further away) and at what angle the signal leaves the antenna (a shallower, more horizontal angle also increases the skip distance). Neither factor alone fully determines where the signal lands — both must be considered.

• A — type of transmitting antenna used: The antenna type influences radiation pattern and polarisation, but it does not directly set skip distance. What matters is the angle of radiation, not merely which antenna produces it; and antenna type alone says nothing about ionospheric height.
• B — power fed to the final amplifier: Transmitter power affects signal strength and the maximum usable frequency, but it does not change the geometry of refraction. Higher power does not shift the skip zone closer or further.
• C — only the angle of radiation from the antenna: The radiation angle is indeed important, but ignoring ionospheric height gives an incomplete picture. The same angle from a higher refracting layer will produce a longer skip distance than from a lower layer.

Therefore, skip distance is governed jointly by the height of the ionospheric layer and the angle at which the radio wave is radiated from the antenna.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — D, E, F

The ionosphere is divided into distinct layers formed by solar radiation ionising the upper atmosphere at different altitudes. The three principal layers recognised as affecting HF radio propagation are the D layer, E layer, and F layer (which further splits into F1 and F2 during daylight hours).

• D layer (~60–90 km): Present only in daylight; primarily absorbs MF and lower HF signals rather than reflecting them.

• E layer (~90–140 km): Reflects lower HF signals and supports sporadic-E propagation.

• F layer (~150–400 km): The most important for long-distance HF communication; reflects higher HF frequencies back to Earth over great distances.

• Option A (A, E, F): There is no recognised "A layer" in ionospheric propagation theory.

• Option B (B, D, E): There is no recognised "B layer"; this combination is not used in standard ionospheric models.

• Option C (C, E, F): There is no recognised "C layer" in standard ionospheric propagation descriptions.

Therefore, the three recognised ionospheric layers that affect radio propagation are the D, E, and F layers.

Last edited by jim.carroll. Register to edit

Tags: none

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

Last edited by dogshed. Register to edit

Tags: none

Correct answer: skip distance

Skip distance is the distance from the transmitter to the nearest point on the Earth where the skywave returns after being refracted by the ionosphere.

This is the minimum distance at which a skywave signal can be received.

• Angle of radiation describes the launch angle of the transmitted signal.
• Maximum usable frequency (MUF) is the highest frequency that will be reflected by the ionosphere.
• The skip zone is the area between the end of the ground wave and the point where the skywave first returns.

Therefore, the distance from the transmitter to the nearest skywave return point is the skip distance.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: fading

Slow changes in received signal strength can occur when radio waves arrive at the receiver via different paths (multipath propagation).

Small changes in path length can cause the signals to:

• add together (constructive interference), or
• cancel each other (destructive interference)

This produces variations in signal strength over time, known as:

\[ \text{fading} \]

• Absorption reduces signal strength but does not cause variation.
• Fluctuation is not the correct technical term.
• Path loss refers to general signal attenuation.

Therefore, the variation is called fading.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: waves at these frequencies travel to and from the satellite relatively unaffected by the ionosphere

VHF and UHF signals pass through the ionosphere with relatively little refraction or reflection.

This allows signals to:

• travel directly between ground stations and satellites
• avoid the variability and absorption effects seen at HF

HF signals are often refracted or absorbed by the ionosphere, making them unsuitable for reliable satellite communication.

• Doppler shift exists at all frequencies.
• Satellite speed does not prevent HF propagation in that way.
• Doppler does not shift signals between bands.

Therefore, VHF and UHF are used because they are relatively unaffected by the ionosphere.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — highest frequency which will be reflected back to earth at vertical incidence

The critical frequency (also called the foF2 for the F2 layer) is the highest frequency at which a vertically transmitted radio signal is still reflected back to Earth by the ionosphere. At vertical incidence (straight up), the ionosphere can only bend signals back if the electron density is high enough. Above the critical frequency, the signal penetrates the ionosphere and escapes into space rather than returning to Earth.

The critical frequency depends on the maximum electron density of the ionospheric layer:

\[ f_c = 9\sqrt{N_{max}} \]

where \(N_{max}\) is the maximum electron density in electrons per cubic metre. A higher electron density produces a higher critical frequency.

• A — highest frequency to which your transmitter can be tuned: This describes transmitter tuning range, which is unrelated to ionospheric propagation.
• B — lowest frequency which is reflected back to earth at vertical incidence: This is the opposite of the definition; low frequencies are easily reflected, while it is the upper limit that defines the critical frequency.
• C — minimum usable frequency (MUF): The MUF applies to oblique (angled) paths between two stations, not vertical incidence, and is a different propagation concept.

Therefore, the critical frequency is specifically the highest frequency reflected straight back to Earth by the ionosphere at vertical incidence — signals above it pass through and are lost to space.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: is the same as the speed of light

Radio waves are a form of electromagnetic radiation and travel at the speed of light in free space:

\[ c \approx 3 \times 10^8\ \mathrm{m/s} \]

• The speed does not depend on frequency in free space.
• It is not infinite.
• It is not less than half the speed of light.

Therefore, the speed of a radio wave is the same as the speed of light.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: B — maximum usable frequency

The MUF (Maximum Usable Frequency) is the highest frequency that can be used for skywave propagation between two points at a given time. Above the MUF, radio waves pass through the ionosphere rather than being refracted back to Earth, so the path fails. The MUF depends on the ionospheric layer's electron density, the angle of radiation, and the distance between stations, and changes throughout the day and with solar activity.

• A is wrong — the MUF is not a mean or average of two values; it is a specific upper boundary frequency.
• C is wrong — the minimum usable frequency (sometimes called the LUF, Lowest Usable Frequency) is a separate concept; signals below the LUF are too attenuated to be useful.
• D is wrong — "mandatory usable frequency" is not a recognised term in propagation or telecommunications.

Therefore, MUF stands for Maximum Usable Frequency, the upper frequency limit for reliable skywave propagation on a given path.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: below the F layer

The ionosphere is divided into several layers:

• F layer (highest)
• E layer
• D layer (lowest)

The E layer is located above the D layer and below the F layer, typically at an altitude of about 90 to 150 km.

• It is not above the F layer.
• It is not below the D layer.
• While sporadic-E can occur, this does not describe its normal position.

Therefore, the E layer is below the F layer.

Last edited by jim.carroll. Register to edit

Tags: none

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.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — reflection and refraction from the ionosphere

Skip distance is the minimum distance from a transmitter at which a sky wave (ionospheric wave) returns to Earth. When a radio signal travels upward into the ionosphere, it is bent (refracted) by the ionised layers and, depending on frequency and angle, is returned to Earth at a point some distance away. This process is often loosely described as "reflection," but refraction is the more accurate physical mechanism. The region between the transmitter and that first return point — where no signal arrives — is called the skip zone.

• B — selective fading of local signals: Fading describes variations in signal strength over time; it does not create a skip zone.
• C — high gain antennas: Antenna gain affects signal strength and direction but does not produce skip distance, which is a property of ionospheric propagation.
• D — local cloud cover: Clouds are meteorological phenomena affecting optical and microwave paths; HF sky waves pass through clouds without significant interaction and are refracted by the ionosphere, not clouds.

Therefore, skip distance is caused by the refraction (and partial reflection) of radio waves by the ionosphere, which returns signals to Earth only beyond a certain minimum ground distance from the transmitter.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — ionised layers

The ionosphere is a region of the upper atmosphere (roughly 60–1000 km altitude) where solar radiation ionises gas molecules, creating free electrons and ions. These free electrons interact with radio waves and, at appropriate frequencies, refract (bend) the signals back toward Earth — enabling long-distance HF communication well beyond the line of sight.

• Oxidised layers — oxidation is a chemical process and has no useful effect on radio wave propagation; no such radio-relevant atmospheric layer exists.
• Heavy cloud layers — clouds affect weather and can attenuate microwave signals, but they are far too low in the atmosphere and are not ionised; they do not return HF signals to Earth.
• Sun spot layers — sunspots are disturbances on the Sun's surface, not atmospheric layers. They influence ionospheric conditions indirectly by varying solar radiation output, but are not themselves a layer that returns signals.

Therefore, ionised layers in the ionosphere are the atmospheric structures responsible for refracting radio signals back to Earth and enabling long-distance radio communication.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — is formed from layers of ionised gases around the earth

The ionosphere is a region of Earth's upper atmosphere (roughly 60 km to 1000 km altitude) where solar ultraviolet and X-ray radiation ionises gas molecules, stripping electrons free and creating layers of ionised gas (plasma). These layers — commonly labelled D, E, F1, and F2 — are capable of refracting or reflecting radio waves back to Earth, making long-distance HF communication possible.

• A is wrong — a "magnetised belt" describes the Van Allen radiation belts, which are a separate phenomenon involving charged particles trapped by Earth's magnetic field, not the ionosphere.
• B is wrong — the ionosphere is not composed of magnetised particles; it is ionised gas (plasma) produced by solar radiation acting on the existing atmosphere.
• D is wrong — the ionosphere is not solar radiation itself; it is the atmospheric region created by solar radiation acting on gas molecules.

Therefore, the ionosphere is correctly defined as layers of ionised gases in the upper atmosphere, formed by solar radiation ionising the thin air at high altitudes.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: angle of radiation is smallest

Skip distance is the distance from the transmitter to the point where the skywave first returns to Earth after reflection from the ionosphere.

When the angle of radiation is small (low take-off angle), the radio wave travels farther before being refracted back to Earth by the ionosphere.

This increases the distance between the transmitter and the first return point, giving a greater skip distance.

• Greater ionisation generally improves reflection but does not maximise skip distance.
• Signal strength does not determine skip distance.
• Polarisation does not affect the geometry of skywave return.

Therefore, skip distance is greatest when the angle of radiation is smallest.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — becomes greater

Skip distance is the minimum distance from a transmitter at which a sky-wave signal (reflected from the ionosphere) returns to Earth. It depends on the angle at which the radio wave leaves the antenna and the height of the reflecting layer.

When the ionospheric reflecting layer rises higher, the wave must travel further before it is refracted back toward Earth. As a result, the point where the wave returns to the ground moves further from the transmitter — the skip distance increases. Think of it geometrically: if the "ceiling" is higher, a ball bounced off it at the same angle will land further away.

• A — stays the same: Incorrect. A change in layer height directly changes the geometry of the sky-wave path, so skip distance cannot remain constant.
• B — decreases: Incorrect. A higher layer pushes the return point further away, not closer; a lower layer would decrease skip distance.
• C — varies regularly: Incorrect. While ionospheric height does change over time (diurnally and seasonally), this option does not correctly answer the direct relationship asked — when height increases, skip distance specifically becomes greater.

Therefore, a higher ionospheric reflecting layer causes the skip distance to increase, because the sky-wave path subtends a longer ground distance before returning to Earth.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: maximum usable frequency

The maximum usable frequency (MUF) is the highest frequency that can be refracted back to Earth by the ionosphere for a given path.

If the transmitted frequency is increased above the MUF:

• the signal passes through the ionosphere

• no skywave reflection occurs

• the signal is lost to space

• The speed of light is unrelated.

• “Sun spot frequency” is not a defined term.

• Skip distance refers to distance, not frequency limits.

Therefore, the signal is above the maximum usable frequency.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: ground wave

Line-of-sight communication occurs when radio waves travel directly from one antenna to another without reflection from the ionosphere.

At VHF and UHF frequencies, this direct path propagation is mainly by ground wave (space wave).

• The ionosphere is involved in skywave (HF) propagation.
• The troposphere may influence signals but is not the primary mechanism.
• Sky wave involves ionospheric reflection, not line-of-sight.

Therefore, line-of-sight transmission mainly uses the ground wave.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: is less at higher frequencies

Ground waves travel along the Earth’s surface and are attenuated as they propagate.

Attenuation increases with frequency, so higher-frequency signals are absorbed more rapidly by the ground.

This results in:

• greater range at lower frequencies

• shorter range at higher frequencies

• It is not the same for all frequencies.

• Maximum usable frequency (MUF) applies to skywave propagation.

Therefore, the distance travelled by ground waves in air is less at higher frequencies.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — sky wave

A sky wave is the radio wave that travels upward from the transmitter, is refracted (bent) back toward the Earth by the ionosphere, and arrives at a distant receiving location. This is the primary propagation mode used on the HF bands for long-distance (DX) communication, and is the standard NZART term for this path.

• B. skip wave — "Skip wave" is not the correct technical term. The skip zone is the silent area between the end of the ground wave and the first sky wave return, but the wave itself is not called a skip wave.
• C. Surface wave — A surface wave (also called a ground wave) travels along the Earth's surface rather than up to the ionosphere; it is a different propagation mode used mainly at lower frequencies.
• D. F layer — The F layer is a region of the ionosphere that refracts sky waves, not the name for the wave or propagation path itself.

Therefore, the correct term for the radio wave that travels from the transmitter up to the ionosphere and back down to Earth is the sky wave.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: D — sky wave

At high frequencies (HF, roughly 3–30 MHz), the ionosphere acts as a refracting layer that bends radio waves back toward Earth. A signal transmitted at an appropriate angle travels upward, is refracted by the ionosphere, and returns to the ground at a great distance — often thousands of kilometres away. This mode of propagation is called the sky wave (also known as ionospheric propagation).

Beyond about 4000 km, sky wave is essentially the only practical propagation path, making it the basis of long-distance HF communication (international broadcasting, amateur DX contacts, etc.).

• A. Ground wave — ground wave propagation follows the Earth's surface but is heavily attenuated at HF and is only effective over relatively short distances (up to a few hundred kilometres at most).
• B. Skip wave — "skip wave" is not standard propagation terminology; the skip zone is the silent zone between where the ground wave fades and where the sky wave returns, but "skip wave" is not the name of a propagation mode.
• C. Surface wave — surface wave is another name for ground wave and is subject to the same distance limitations; it does not reach beyond 4000 km at HF.

Therefore, reception of HF signals beyond 4000 km is achieved via the sky wave, which relies on ionospheric refraction to return signals over intercontinental distances.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: B — during daylight hours

28 MHz falls in the 10-metre HF band, which relies heavily on ionospheric propagation via the F-layer. At this frequency, the critical factor is solar ultraviolet and X-ray radiation ionising the upper atmosphere. During daylight hours, solar radiation keeps the F-layer (particularly the F2 layer) densely ionised and at sufficient height to refract 28 MHz signals back to Earth over thousands of kilometres. This makes daytime conditions — especially around the solar maximum — ideal for long-distance (DX) propagation on 10 metres.

• A — reducing transmitter power makes propagation less likely over great distances, not more. Lower power reduces the signal-to-noise ratio at the distant receiver.
• C — only during the night is incorrect; at night the F-layer thins and its electron density often drops too low to reliably refract 28 MHz signals back to Earth. Lower HF bands (e.g. 40 m, 80 m) benefit more from night-time conditions.
• D — at full moon has no meaningful effect on ionospheric propagation. The Moon reflects negligible ionising radiation compared to the Sun, and lunar phase does not influence the F-layer.

Therefore, 28 MHz long-distance propagation is most reliably achieved during daylight hours when solar radiation maintains the ionisation needed to refract the signal back to Earth.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: A — the highest frequency at which ionospheric reflection can occur

Long-distance HF communication relies on the ionosphere reflecting radio waves back to Earth. The ionosphere's ability to reflect a signal depends on its level of ionisation, which varies with solar activity, time of day, and season. At any given moment there is a maximum frequency the ionosphere can reflect — known as the Maximum Usable Frequency (MUF). Only HF bands with frequencies below the MUF can support skywave propagation at that time, so the number of usable bands rises and falls directly with the MUF.

• B — the number of frequencies the receiver can tune: The receiver's tuning range is a hardware characteristic; it does not affect whether the ionosphere will actually reflect a signal on any given band.
• C — the power being radiated by the transmitting station: Higher power can improve signal strength on an already-open band, but it cannot force the ionosphere to reflect frequencies above the MUF. Power does not determine which bands are open.
• D — the height of the transmitting antenna: Antenna height influences low-angle radiation and ground-wave range, but the ionosphere's reflective ceiling is governed by ionisation levels, not by how high the transmitting antenna is mounted.

Therefore, the number of HF bands available for long-distance communication at any moment is determined by the highest frequency the ionosphere can reflect — the MUF — which itself depends on current ionospheric conditions.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — years

The ionosphere's long-term behaviour is strongly influenced by the 11-year solar cycle. Solar activity (measured by sunspot numbers) rises to a maximum and falls to a minimum roughly every 11 years. During solar maximum, increased ultraviolet and X-ray radiation from the Sun produces a more densely ionised ionosphere, improving HF propagation on higher frequencies. During solar minimum, the ionosphere is less ionised and the usable frequency range narrows.

• Days — Diurnal (daily) changes do occur as the Sun rises and sets, but these are not the "regular changes" described by an 11-year cycle.
• Months — Seasonal variations occur over months, but the major regular long-term cycle is measured in years, not months.
• Centuries — Longer-term solar variations exist, but the well-defined, regular cycle relevant to amateur radio propagation is approximately 11 years, not centuries.

Therefore, the regular long-term ionospheric cycle tied to solar activity repeats approximately every 11 years, making years the correct answer.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: variations in signal strength

HF signals often propagate via the ionosphere (skywave).

Small changes in ionospheric conditions can cause:

• constructive interference (signals add)
• destructive interference (signals cancel)

This leads to:

\[ \text{fading} \]

i.e., variations in received signal strength over time.

• It does not consistently increase or decrease strength.
• Ground wave is not the main HF long-distance mode.

Therefore, the result is variations in signal strength.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: B — cause a fade-out of sky-wave signals

Ionospheric storms are disturbances in the ionosphere caused by solar activity (such as coronal mass ejections and solar flares). During a storm, the ionosphere becomes highly disturbed and irregular, causing increased absorption and scattering of radio waves. High-frequency (HF) sky-wave signals that rely on reflection from the ionosphere are severely attenuated or completely absorbed, resulting in a fade-out or blackout of sky-wave communications. These events are sometimes called "radio blackouts" or "shortwave fadeouts."

• A — increase the maximum usable frequency: Incorrect. Ionospheric storms generally lower the MUF as the ionosphere becomes weakened and less able to refract higher frequencies back to Earth.
• C — produce extreme weather changes: Incorrect. The ionosphere exists far above the troposphere where weather occurs; ionospheric storms have no direct effect on surface weather.
• D — prevent communications by ground wave: Incorrect. Ground wave propagation follows the Earth's surface and does not depend on the ionosphere, so it is largely unaffected by ionospheric storms.

Therefore, the usual effect of an ionospheric storm is to cause a fade-out of sky-wave signals due to increased absorption of HF radio waves in the disturbed ionosphere.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: C — fading

When a signal travels via sky wave (ionospheric) propagation, it is refracted back to Earth by the ionosphere. The ionosphere is not static — its electron density varies due to solar activity, time of day, and atmospheric conditions. These variations cause the signal to arrive at the receiver along slightly different paths or with varying strength, producing fluctuations in received signal level known as fading. In CW and SSB operation, fading is sometimes so severe it completely drops the signal — a phenomenon called a "deep fade."

• A. Ground wave losses — Ground wave is a different propagation mode (following the Earth's surface); its attenuation is a steady path loss, not a fluctuation in received strength.
• B. Modulation losses — No such standard term exists in this context; modulation does not cause the signal strength variations described.
• D. Sunspots — Sunspot activity influences ionospheric conditions and thus affects propagation, but sunspots themselves are not the name given to signal strength changes; they are a cause, not the phenomenon.

The Q-code for this phenomenon is QSB, which means "your signals are fading" — a useful mnemonic for the examination.

Therefore, variations in received signal strength caused by sky wave propagation are correctly called fading.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: changes in the ionosphere

Skywave propagation depends on the state of the ionosphere, which can change over short periods due to:

• time of day
• solar radiation
• ionisation levels in different layers (D, E, F)

These changes affect:

• how well signals are refracted back to Earth
• which frequencies are supported

As a result, a signal that is receivable at one time may disappear later as ionospheric conditions change.

• Clouds do not significantly affect HF propagation.
• Temperature changes have little direct effect.
• Ground wave is not involved in long-distance HF communication.

Therefore, the change is due to changes in the ionosphere.

Last edited by jim.carroll. Register to edit

Tags: none

Correct answer: B — these waves are easily reflected by objects in their path

VHF and UHF signals travel primarily as line-of-sight waves and do not bend significantly through the ionosphere. However, because their wavelengths are short (centimetres to metres), they reflect effectively off large solid structures such as tall buildings, hills, and water towers. This reflection can redirect the signal in a completely different direction, allowing it to be received well beyond the original line-of-sight path — a phenomenon commonly experienced in urban environments.

• A is wrong because VHF and UHF frequencies are too high to be refracted by the ionosphere under normal conditions; they pass straight through it, which is why they are used for satellite communication.
• C is wrong because the direction of wave travel is entirely deterministic — it follows known laws of reflection and propagation.
• D is wrong because the presence of elevators is irrelevant to radio wave propagation.

Therefore, VHF/UHF signals can be received in unexpected directions because their short wavelengths reflect readily off large structures such as tall buildings.

Last edited by jim.carroll. Register to edit

Tags: none