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Amateur Radio Info & Exams - Propagation 2 - Ionospheric layers, Critical angle, and Near Vertical Incidence Sky-wave

This covers further aspects of HF propagation.

Layers of the Ionosphere

The layers are the D, E, F1, and F2 layers, from lowest to highest. These extend to several hundred kilometres above the Earth's surface, depending on the time of day.

During the day the D later absorbs signals from 160 to 40 metres, and in the new 630 metre band. Thus these bands, and MF and low SW broadcast bands only provide local ground-wave coverage during the day. Higher frequency signals can penetrate this layer, both mid to upper HF frequencies which may be then refracted back to earth by higher layers; and VHF, UHF, and microwave frequencies, which can reach satellites and more distant space vehicles. Likewise, these signal can come from objects in space to Earth, including GNSS (GPS, etc) signals.

In the evening the D-layer dissipates, and MF and lower HF bands provide good regional coverage. Certainly, when I was younger Radio Moscow used to broadcast into western Europe in the AM broadcast band at night, as well as Shortwave, and BBC Radio Scotland on 810 kHz is audible in Norway. 2ZB from New Zealand on 1035 kHz can be heard in eastern Australia if you are away from the SBS transmitter in Wollongong.

A rule-of-thumb used by sailors, applicable to all, is "The higher the sun, the higher the frequency".

No A, B, or C layers? Initially the E-layer inherited its name, as it was the layer from which the "Electric waves" were returned. The below it was named the D-layer, based on an assumption that lower layers may be discovered, but these were not.

Critical Angle & HF Scatter

There is an angle above which signals hitting the ionosphere will not refract. This is the critical angle, and it means that there is a zone, called the "skip zone", where signal are not returned to earth. This is varies, by time and frequency.

However, as with VHF signals and the troposphere, HF signals can be randomly scattered back to earth by small dense areas in the ionosphere. These signals can suffer flutter and distortion.


Bush search and rescue use small Q-MAC HF radios with dipole antennas which can be thrown out across the brush, or even on dry ground, and by transmitting on frequencies such as 3.8 to 4 MHz, and around 5 MHz, they can report their position, and their intention (where the will search next), or receive instructions.

The same effect can be used on 80, 60, or 40 metres to operate emergency and radio club nets. Low dipoles work best.

Relevant Questions

These are actual questions from the General exam pool.

Which ionospheric layer is closest to the surface of the Earth?
A. The D layer
B. The E layer
C. The F1 layer
D. The F2 layer

Layers are named from the surface up, so it is the D layer, answer A.

Where on the Earth do ionospheric layers reach their maximum height?
A. Where the Sun is overhead
B. Where the Sun is on the opposite side of the Earth
C. Where the Sun is rising
D. Where the Sun has just set

This is where the sun is overhead, answer A.

Why is the F2 region mainly responsible for the longest distance radio wave propagation?
A. Because it is the densest ionospheric layer
B. Because it does not absorb radio waves as much as other ionospheric regions
C. Because it is the highest ionospheric region
D. Because of meteor trails at that level

It is because it is the highest layer, so signals travel further before they are returned to Earth, and they also land further away, answer C.

What does the term "critical angle" mean as used in radio wave propagation?
A. The long path azimuth of a distant station
B. The short path azimuth of a distant station
C. The lowest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions
D. The highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions

This is the highest take-off angle that will be returned under particular conditions, answer D.

This means that there is often a zone between where the ground-wave peters out, and where skywave coverage commences, where the signal is not returned, this being the "skip zone", as signals skip over it.

Why is long distance communication on the 40-meter, 60-meter, 80-meter and 160-meter bands more difficult during the day?
A. The F layer absorbs signals at these frequencies during daylight hours
B. The F layer is unstable during daylight hours
C. The D layer absorbs signals at these frequencies during daylight hours
D. The E layer is unstable during daylight hours

The D layer absorbs them signal, answer C.

You may have noticed that MW (AM) signals are local only during the day, but travel hundreds of km, or more at night. This is due the the behaviour of the D layer mentioned above.

What is a characteristic of HF scatter signals?
A. They have high intelligibility
B. Signals have a fluttering sound
C. There are very large, sudden swings in signal strength
D. Scatter propagation occurs only at night

These have a fluttering sound, answer B.

What makes HF scatter signals often sound distorted?
A. The ionospheric layer involved is unstable
B. Ground waves are absorbing much of the signal
C. The E-region is not present
D. Energy is scattered into the skip zone through several different radio wave paths

The scatter is from different regions in the ionosphere which are randomly more dense than the rest, and these varying paths cause distortion, answer D.

Why are HF scatter signals in the skip zone usually weak?
A. Only a small part of the signal energy is scattered into the skip zone
B. Signals are scattered from the magnetosphere which is not a good reflector
C. Propagation is through ground waves which absorb most of the signal energy
D. Propagation is through ducts in the F region which absorb most of the energy

Only a small amount of the signal is bounced back to the station in the skip zone, answer A.

What type of radio wave propagation allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky-wave propagation?
A. Faraday rotation
B. Scatter
C. Sporadic-E skip
D. Short-path skip

Signals can be scattered by the ionosphere, answer B. These will be weak and variable.

What is Near Vertical Incidence Skywave (NVIS) propagation?
A. Propagation near the MUF
B. Short distance MF or HF propagation using high elevation angles
C. Long path HF propagation at sunrise and sunset
D. Double hop propagation near the LUF

NVIS provides short distance MF or HF propagation, with signals refracting at high elevation angles, answer B.

Near Vertical and high elevation sounds like the same thing, so yes, it is shorter range propagation in which signals travel up, and are returned over a distance from zero to several hundred kilometres.

Which ionospheric layer is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz?
A. The F2 layer
B. The F1 layer
C. The E layer
D. The D layer

The D layer absorbs low HF and MF signals during the day, answer D.

10 MHz (30 metres) itself can perform well during the day.

On to: Amateur Practices 1 - Station Set-up and Test Equipment

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, March 2022.

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