Section 7: Radio Wave Propagation Canadian Amateur Radio Basic Qualification (B-007)

7.1 Wave Propagation Types

Every radio signal that leaves your antenna reaches its destination by one (or more) of a handful of propagation modes. The mode that dominates depends primarily on frequency and distance. Two nearby VHF transceivers communicate by line-of-sight; an HF station working DX across the Atlantic relies on sky waves bouncing off the ionosphere; and a 160-metre signal hugging the ground uses the ground wave.

Ground wave (surface), sky wave (ionospheric), and line-of-sight (VHF/UHF)

Line-of-Sight (Direct Wave)

At VHF and higher frequencies, propagation is primarily line-of-sight -- the signal travels in a more or less straight path from transmitter to receiver. This is the type of wave propagation that usually occurs between two nearby VHF transceivers, such as a pair of 2-metre handhelds used across a parking lot or a neighbourhood.

Although line-of-sight sounds limiting, the radio horizon actually extends somewhat beyond the visible horizon. This happens because of normal refraction in the troposphere: the atmosphere's decreasing density with altitude slightly bends radio waves downward, letting them follow the Earth's curvature a little farther than a perfectly straight line would. The effect is modest, but it means your VHF/UHF radio can reach a few percent beyond where your eyes can see.

Ground Wave

The ground wave is the portion of a radio signal directly affected by the surface of the Earth. Rather than flying off into space, a ground wave hugs the terrain and follows the curvature of the planet. Ground waves are most effective at lower frequencies (longer wavelengths) because the Earth's surface attenuates shorter wavelengths more rapidly.

Among the HF amateur bands (from 160 metres down to 6 metres), the 160-metre band offers the greatest ground-wave propagation distance. At lower HF frequencies, ground wave is what makes radiocommunication out to about 200 km possible during the daytime, when ionospheric conditions may not support sky-wave propagation on those bands.

Sky Wave (Ionospheric Wave)

A sky wave is a radio wave that travels upward from the transmitter, enters the ionosphere, and is refracted (bent) back to Earth by layers of ionized gas high above the planet. The wave type commonly known as a "sky wave" is the ionospheric wave. When a signal is returned to Earth by the ionosphere, this is called sky-wave propagation.

The sky wave follows a path from the transmitter to the ionosphere and back to Earth. This is the mechanism that makes HF radio so powerful for long-distance communication: reception of HF radio waves beyond 4,000 km is generally made possible by the ionospheric wave, typically through multiple "hops" off the ionosphere and the ground.

Tropospheric Wave

The tropospheric wave describes the portion of a transmitted wave kept close to the Earth's surface due to bending in the lower atmosphere (the troposphere, which extends from the surface to roughly 12 km altitude). This is not the same as tropospheric ducting (covered in section 7.7), but rather the normal, gentle bending that occurs because the troposphere's refractive index decreases with altitude.

Near Vertical Incidence Skywave (NVIS)

NVIS is a specialized sky-wave technique where signals are launched nearly straight up and come back down in a broad footprint around the transmitter. This enables medium-range HF communications, especially in difficult terrain such as mountains or dense forest where line-of-sight is blocked and ground wave is attenuated. NVIS uses frequencies just below the critical frequency so the ionosphere reliably bends them back down.

Propagation Modes Comparison

7.2 The Ionosphere

What Creates the Ionosphere?

The ionosphere forms when solar radiation ionizes the outer atmosphere. Specifically, ultraviolet (UV) radiation from the sun carries enough energy to strip electrons from gas molecules (primarily oxygen and nitrogen) at high altitudes. This creates layers of ionized gas -- plasma -- that have the remarkable ability to refract radio waves back toward Earth. Without solar radiation, the ionosphere weakens dramatically, which is why propagation conditions change so much between day and night.

Ionospheric Layers

The ionosphere is not a uniform shell; it is organized into distinct regions (or layers) at different heights, each with its own characteristics. The regions are named D, E, and F -- in alphabetical order from the ground up.

  HEIGHT (km)                  DAYTIME                        NIGHTTIME
                     +-------------------------+    +-------------------------+
  500 ----------     |                         |    |                         |
                     |       F2 REGION         |    |                         |
  400 ----------     |  (highest, key for HF   |    |       F REGION          |
                     |   long-distance DX)     |    |  (F1 and F2 merge       |
  300 ----------     |- - - - - - - - - - - - -|    |   into single layer)    |
                     |       F1 REGION         |    |                         |
  200 ----------     |  (only exists daytime)  |    |                         |
                     |=========================|    |=========================|
  150 ----------     |                         |    |                         |
                     |       E REGION          |    |       E REGION          |
  100 ----------     |  (sporadic-E possible)  |    |  (weakens at night)     |
                     |=========================|    |=========================|
   80 ----------     |                         |    |                         |
                     |       D REGION          |    |                         |
   60 ----------     |  (absorbs HF, daytime   |    |   (disappears at        |
                     |   only!)                |    |    night)               |
                     +-------------------------+    +-------------------------+
                         SUN IS SHINING                  SUN HAS SET

Ionospheric Layer Comparison

The D region is the ionospheric region closest to the Earth. It is also the least useful for long-distance radio-wave propagation because it primarily absorbs HF signals rather than refracting them. The E region sits above the D region and below the F region. The F2 region is mainly responsible for the longest-distance radio-wave propagation because it is the highest ionospheric region -- signals refracted from higher up travel farther per hop before returning to Earth.

Day vs Night: The Key Differences

The ionosphere transforms dramatically between day and night because its very existence depends on ongoing solar radiation. During the day, all regions are present and the F layer splits into two sub-regions: F1 and F2. At night, several important changes occur:

• The D region disappears entirely (no solar radiation to sustain it)
• The F1 and F2 regions merge back into a single F layer
• The E region weakens but does not vanish completely

These changes explain why certain bands behave so differently after sunset. During the day, the D region absorbs lower-frequency HF signals (160m and 80m bands), limiting them to short-distance ground-wave communication. At night, the D region vanishes, allowing these signals to pass through to the higher F layer and propagate over long distances. This is why AM broadcast stations from hundreds of kilometres away suddenly become audible after dark.

When Is the Ionosphere Strongest?

Since the ionosphere is created by solar radiation, its ionization level follows the sun across the sky. The ionosphere is most ionized at midday, when solar radiation is strongest and hitting the atmosphere most directly. Ionization is at a minimum shortly before dawn, after a full night without solar radiation to replenish the charged particles that have been gradually recombining.

  Ionization
  Level
    ^
    |              /‾‾‾‾‾‾\             MAXIMUM
    |            /          \             at
    |          /              \          MIDDAY
    |        /                  \
    |      /                      \
    |    /                          \
    |  /                              \           MINIMUM
    |/                                  \         before
    ╰----+----+----+----+----+----+----+----▸     DAWN
       Dawn  8am  10am Noon  2pm  4pm Dusk Dawn
                    Time of Day

7.3 Skip Zone and Skip Distance

Understanding the Skip Zone

                          IONOSPHERE
                    /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\
                  /        Refracted        \
                /           wave             \
              /                                \
            /                                    \
          /                                        \
  ------/----------------------------------------------\--------
  TX )))                                                 RX
      |         |                              |
      | GROUND  |          SKIP ZONE           |
      |  WAVE   |  (no signal received here!)  |
      | RANGE   |                              |
      +---------+                              |
      |         |                              |
      |<-- Ground wave -->|                    |
      |   fades out here                       |
      |                                        |
      |<---------- SKIP DISTANCE ------------->|
      |         (first sky-wave return point)   |

The skip zone is an area too distant for reception of ground waves, but too close for reception of ionospheric waves. Specifically, it is the zone between the end of the ground wave and the point where the first ionosphere-refracted wave returns to Earth. If you are standing in the skip zone, you will hear nothing from the transmitter -- the ground wave has faded out, and the sky wave overshoots you.

The skip distance is the minimum distance reached by a signal after one reflection by the ionosphere. It is measured from the transmitter to the nearest point where the sky wave returns to Earth. "Skip" itself is a term associated with signals from the ionosphere, and what causes skip is refraction by the ionosphere.

Maximum Distance Per Hop

The higher the ionospheric layer doing the refracting, the farther the signal can travel in a single hop. This is simple geometry -- a higher "mirror" sends the signal farther before it returns to Earth.

Multi-Hop Propagation

For distances beyond the single-hop maximum, radio waves can perform multi-hop propagation: the signal bounces off the ionosphere, returns to Earth, reflects off the Earth's surface, and travels back up to the ionosphere again. On a double-hop path using the Earth's surface as a middle bounce point, reflection is the phenomenon that returns the wave from the ground back to the ionosphere for the second hop.

Each F2 hop covers ~4,000 km. Two hops can reach ~8,000 km.

Factors Affecting Skip Distance

Two main geometric factors determine how far your sky wave travels before returning to Earth: the radiation angle (how steeply the signal leaves the antenna) and the height of the refracting layer.

Radiation Angle

The angle at which the signal leaves the antenna relative to the ground is critical. A lower radiation angle (more nearly horizontal) means the signal hits the ionosphere at a more oblique, grazing angle and travels farther before returning to Earth. Conversely, a higher radiation angle (more nearly vertical) decreases skip distance because the wave goes more steeply up and comes back down closer to the transmitter.

                      IONOSPHERE
            /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\
          /    \  High angle      Low angle /    \
        /        \  (steep)    (shallow)  /        \
      /            \                    /             \
    /                \                /                 \
  /                    \            /                     \
 ------------------------\--------/-------------------------\------
TX )))                  SHORT    |                          LONG
                       SKIP      |                         SKIP
                      DISTANCE   |                        DISTANCE

Height of the Refracting Region

An increase in the height of the refracting region increases skip distance. Think of it like raising the ceiling -- when the "mirror" is higher, the ball bounces farther from the thrower. This is why F2-layer hops cover more distance than E-layer hops.

7.4 Fading and Multipath

D-Region Absorption

Before discussing fading, it is important to understand the D region's role as an absorber. The D region is the lowest ionospheric layer and primarily absorbs (rather than refracts) HF signals, especially at lower frequencies. This absorption is strongest during the day and at lower HF frequencies.

This is why you cannot hear distant 160-metre and AM broadcast stations during the day -- the ionization of the D region absorbs the signals before they can reach the higher refracting layers. At night, the D region disappears, and those signals can pass through to the F layer for long-distance propagation.

Multipath Propagation and Fading

                    IONOSPHERE
         /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\
       /     Path 1 (one hop)                    \
     /        --------------------->               \
   /                                                 \
  /              Path 2 (two hops)                     \
 /         ---------------------------------->           \
 --------------------------------------------------------------
TX )))                     Earth                          RX
                          bounce
                                              Signals arrive
                                              at DIFFERENT TIMES
                                              with DIFFERENT PHASES
                                              = FADING

When a radio transmission follows two or more different paths during propagation (for example, one-hop and two-hop paths), the signals arrive at the receiver at different times. The resulting phase differences cause the signals to alternately reinforce and cancel each other. The effect at the receiver is fading -- the signal strength rises and falls, sometimes rapidly. If the multipath also causes different frequency components to be affected differently, phase distortion can occur as well.

When a signal reaches a receiver by both one-hop and two-hop paths, even small changes in the ionosphere can shift the relative phase of these paths, causing variations in signal strength.

Multipath in Urban VHF/UHF

Multipath is not just an HF phenomenon. When using a 2-metre hand-held transceiver in an urban setting, you may notice that moving less than one metre can severely attenuate your received signal. This happens because signals arriving on different paths cancel one another (destructive interference). At 2-metre wavelength (~146 MHz), a half-wavelength shift (about 1 metre) can change from constructive to destructive interference. Buildings, vehicles, and other structures create multiple reflected paths that interfere at your antenna.

Sudden Ionospheric Disturbance (SID)

A Sudden Ionospheric Disturbance is caused by a solar flare dramatically increasing D-region ionization. This can cause a complete blackout of HF communications on the sunlit side of the Earth. During a SID, HF communications can be maintained by trying a higher frequency band. Higher frequencies are less affected by D-region absorption, so moving up in frequency can restore communication.

Selective Fading

Selective fading occurs when different frequency components of a signal experience different amounts of fading. It is caused by phase differences between radio wave components of the same transmission as experienced at the receiving station. Because different frequency components travel slightly different paths, they fade independently.

The bandwidth of the transmitted signal matters: selective fading is more pronounced at wide bandwidths. A wider-bandwidth signal has more frequency components that can experience different fading, making the distortion worse. This is one reason why narrow-bandwidth modes like CW are more robust under poor conditions.

Faraday Rotation and Polarization

As HF signals pass through the ionosphere, the Earth's magnetic field causes the wave's polarization to rotate -- this is called Faraday rotation. Together with refraction and reflection, Faraday rotation changes the polarization of a radio wave as it passes through the ionosphere.

This has a practical consequence: on HF bands, the polarization of the receiving antenna relative to the transmitting antenna is relatively unimportant. The ionosphere scrambles the polarization anyway, so on HF it does not matter much whether you use a horizontal or vertical antenna. On VHF and UHF bands, however, where signals travel by line-of-sight and do not pass through the ionosphere, polarization matching between transmitting and receiving antennas is very important. The reason polarization is unimportant on HF is specifically because the refraction in the ionosphere changes the wave's polarization.

7.5 Solar Activity

Sunspots and the Solar Cycle

Sunspots are dark regions on the sun's surface associated with intense magnetic activity and increased radiation output. The number of sunspots rises and falls in a roughly 11-year cycle called the solar cycle (or sunspot cycle). This cycle is the major cause of cyclical changes in HF propagation.

The relationship between sunspots and propagation is straightforward: the more sunspots there are, the greater the ionization of the ionosphere. More ionization means higher frequencies can be refracted back to Earth, opening up bands that would otherwise be useless for long-distance communication. When sunspot numbers are high (solar maximum), frequencies up to 40 MHz or even higher become usable for long-distance communication. At solar minimum, the higher HF bands (10m, 12m, 15m) may not support long-distance propagation at all.

  Sunspot
  Number
    ^
    |         /\
    |        /  \
    |       /    \          /\
    |      /      \        /  \
    |     /        \      /    \
    |    /          \    /      \
    |   /            \  /        \
    |  /              \/          \
    | /                            \
    ╰--+----+----+----+----+----+---▸ Time (years)
       0    5    11   16   22   27

       |<- Cycle 1 ->|<- Cycle 2 ->|
         (~11 years)     (~11 years)

  Solar Max: Higher MUF, more HF bands open
  Solar Min: Lower MUF, fewer HF bands usable

Solar Flux

Solar flux is the radio frequency energy emitted by the sun. It is measured at a specific frequency (2800 MHz / 10.7 cm wavelength) daily at the Dominion Radio Astrophysical Observatory in Penticton, BC. The solar-flux index is a measure of solar activity that is taken at a specific frequency. A higher solar-flux index generally correlates with better HF propagation conditions because it indicates more solar radiation reaching the ionosphere.

Solar Influence on Propagation

Solar radiation influences all radiocommunication beyond ground wave or line-of-sight ranges. In fact, all communication frequencies throughout the spectrum are affected in varying degrees by the sun, though the effects are most dramatic on HF.

On a daily basis, the sun's activity influences ionospheric propagation through electromagnetic and particle radiation. The ability of the ionosphere to refract high-frequency radio signals depends fundamentally on the amount of solar radiation -- more radiation means more ionization, which means higher frequencies can be bent back to Earth.

7.6 Maximum Usable Frequency (MUF)

Key Frequency Definitions

The mathematical relationship is: \( f_{\text{critical}} < f_{\text{optimum}} < f_{\text{MUF}} \)

What Causes MUF to Vary?

The MUF varies primarily based on the amount of radiation received from the sun. More solar radiation means more ionization, which means the ionosphere can refract higher frequencies. When solar UV radiation increases daily (morning into midday), the maximum usable frequency increases. The MUF also changes with the solar cycle, the season, and the specific path geometry between transmitter and receiver.

Why Frequencies Above the Critical Frequency Still Work

At first glance, it seems contradictory that communication can occur at frequencies above the critical frequency. The critical frequency is the highest frequency that reflects when sent straight up. But real communication signals are not sent straight up -- they travel at an angle. When a signal enters the ionosphere at an oblique (inclined) angle, it passes through more ionized material than a vertical signal would, giving the ionosphere more opportunity to bend it back to Earth. This is why the MUF for any given path is always higher than the local critical frequency.

                        IONOSPHERE
              /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\
            /                                   \
          /     Vertical: only works below        \
        /        critical frequency     ▲           \
      /              /                  | (passes     \
    /              /  Oblique: works     |  through     \
  /             /      up to MUF         |  if > MUF)    \
 ------------/--------------------------|----------------------
           TX )))                       |
                                   Signal at vertical
                                   incidence > critical
                                   frequency escapes to space

What Happens Below and Above the MUF?

Radio waves at frequencies below the MUF are bent back to Earth by the ionosphere -- they are successfully refracted and reach the intended destination. Waves above the MUF pass through the ionosphere into space and are lost. This is why finding the right frequency matters so much.

Checking Propagation Conditions

One practical way to determine if the MUF is high enough to support 28 MHz propagation to a given area (such as western Europe) is to listen for 10-metre beacon stations. Beacons transmit continuously on known frequencies; if you can hear a beacon in the direction you want to communicate, the band is open on that path.

Band Selection by Time and Season

Different bands perform best under different conditions. The 20-metre band (14 MHz) is the workhorse of amateur HF -- it usually supports worldwide propagation during daylight hours at any point in the solar cycle. Unlike higher bands (10m, 6m) that require solar maximum, 20 metres is reliably open for DX throughout the entire 11-year cycle, making it the most popular DX band.

Propagation on the 80-metre band is generally least effective during daytime in summer -- this is the worst combination because D-region absorption is at its peak and atmospheric noise from thunderstorms is highest. During summer daytime, the bands most difficult for communications beyond ground wave are 160 metres and 80 metres.

7.7 VHF/UHF Propagation

Sporadic-E

Sporadic-E consists of patches of dense ionization at E-region height (~100 km). These patches appear unpredictably -- they can form during any season, though they are most common in summer. When they occur, they can refract VHF signals over hundreds or even thousands of kilometres, well beyond the normal line-of-sight range.

The extended-distance propagation effect of sporadic-E is most often observed on the 6-metre band (50 MHz). At higher VHF frequencies (2 metres, 70 cm), sporadic-E becomes increasingly rare because the ionization patches are usually not dense enough to refract those higher frequencies. The E region is also the ionospheric region that most affects sky-wave propagation on the 6-metre band.

Tropospheric Effects

Tropospheric Bending and Ducting

Under normal conditions (no enhanced propagation), VHF tropospheric propagation reaches approximately 800 km. But when a temperature inversion creates a tropospheric duct, the effect on 2-metre radio waves is that it lets you contact stations farther away than normal -- sometimes dramatically farther. A VHF signal propagating over 800 km is most likely due to tropospheric ducting.

Temperature inversion traps VHF/UHF signals, extending range well beyond line-of-sight

Auroral Propagation

During geomagnetic storms, the aurora borealis (northern lights) creates a curtain of ionization at E-region height that can scatter VHF signals. Auroral activity occurs in the ionosphere at E-region altitude, concentrated in a ring around the magnetic poles.

To take maximum advantage of auroral propagation in the northern hemisphere, a directional antenna should be pointed North -- toward the aurora. However, auroral signals have a characteristic "buzz" or "hiss" that makes voice modes difficult to copy. The most reliable analog emission mode for auroral propagation is CW (Morse code) because it is readable even when distorted by the auroral scatter.

Tropospheric Wave

The tropospheric wave describes the portion of a transmitted wave kept close to the Earth's surface due to bending in the atmosphere. This is distinct from tropospheric ducting -- it is the normal, slight downward bending that occurs even without a temperature inversion.

VHF/UHF Propagation Modes Summary

7.8 Scatter Propagation

What Is Scatter?

Scatter propagation occurs when radio waves encounter irregularities in the propagation medium -- small variations in ionization, density, temperature, or moisture -- and are redirected in many directions. Only a small portion of the transmitted energy reaches the receiver, so signals are typically weak. The effect of scattering on a radio wave is that the wave gets redistributed in multiple directions, with only a fraction heading toward any particular receiver.

HF Scatter (Ionospheric Scatter)

Scatter-mode HF propagation allows weak signals from the skip zone to be heard. Normally the skip zone is a dead area with no signal, but ionospheric irregularities scatter some energy into it. This is the kind of unusual HF propagation that allows weak signals from the skip zone to be heard.

If you receive a weak, distorted signal close to the maximum usable frequency, scatter propagation is probably occurring. Similarly, a weak HF signal heard at a distance too far for ground-wave propagation but too near for normal sky-wave propagation is likely due to scatter.

HF scatter signals are often distorted because the energy is scattered into the skip zone through several radio-wave paths (multipath from scattering). They are usually weak because only a small part of the signal energy is scattered into the skip zone. On the HF bands, scatter propagation is most likely involved when you receive weak and distorted signals near the MUF.

                        IONOSPHERE
                  /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\
                /    Normal sky wave        \
              /      (refracts back)         \
            /              * * *               \         * = scatter
          /             *  Scattered  *          \         points
        /            *    energy        *          \
      /           *     (weak, many       *          \
    /          *         directions)         *         \
  ----------------------------------------------------------
  TX )))         |         ↓ ↓ ↓          |            RX
                 |    Weak signals in      |
                 |      SKIP ZONE          |
                 |<-- Skip Zone ---------->|

Tropospheric Scatter

Tropospheric scatter enables VHF/UHF communications well beyond the radio horizon. It works because of small variations in the properties of the lower atmosphere -- density, temperature, and water-vapour content create irregularities that scatter some signal energy forward. The type of VHF/UHF propagation that depends upon small variations in density and water-vapour content is tropospheric scatter.

Meteor Scatter

When meteors enter the atmosphere, they leave brief trails of ionized gas at E-region height. Radio signals can be reflected off these trails for a few seconds at a time, enabling brief but often strong communication bursts over distances of 500-2,200 km.

Meteor scatter is most effective on the 6-metre band. More precisely, the frequency range where meteor scatter is most effective for extended-range communication is 10 MHz to 30 MHz (upper HF through low VHF). Because the signal bursts are so brief, operators typically use high-speed CW or digital modes to exchange information in just a few seconds.

Scatter Propagation Comparison

Quick Reference Summary