HF Propagation: Ionospheric Radio Propagation

Ionospheric propagation is one of the key modes of propagation used in the MF and HF bands enabling distances of thousands of kilometres to be reached.


Ionospheric propagation tutorial includes . . . .
Ionospheric propagation     Ionosphere     Ionospheric layers     Skywaves & skip     Critical frequency, MUF, LUF & OWF     Ionospheric absorption     Solar indices     Propagation software     NVIS     Transequatorial propagation     Sporadic E    


Ionospheric propagation is the main mode of radio propagation used in the MF and HF portions of the radio spectrum.

The basic concepts behind HF propagation using the ionosphere are easy to understand, and a study of it is not only fascinating, but also very useful for anyone involved in HF radio communications in any way.

HF ionospheric propagation applications

Using HF propagation via the ionosphere, radio signals can be heard around the globe – it was this form of communication that first opened up many global links to inaccessible regions, and also enabled international broadcasting.

HF propagation using the ionosphere was also used for ships, although they now utilise satellite communications.

Typical HF directional Yagi antenna used for communications using ionospheric propagation
Typical antenna used for HF communications via the ionosphere

Radio amateurs or radio hams also make widespread use of HF propagation via the ionosphere, often communicating with distant points on the globe with low powers and modest antenna systems.

Knowing how HF propagation varies and what influences the ionosphere enables the user to gain far more from this mode of propagation. A basic understanding can help considerably for many people.

HF propagation & skywaves

When using HF propagation via the ionosphere, the radio signals leave the transmitter on Earth's surface and travel towards the ionosphere where some of these are returned to Earth.

Basic concept of ionospheric radio signal propagation
Basic concept of ionospheric radio propagation
Notice how the signal is refracted as it enters the ionospheric layer

The radio signals travelling away from the Earth’s surface are termed sky waves for obvious reasons. If they are returned to Earth, then the ionosphere may (very simply) be viewed as a vast reflecting surface encompassing the Earth that enables signals to travel over much greater distances than would otherwise be possible.

Naturally this is a great over simplification because the frequency, time of day and many other parameters govern the reflection, or more correctly the refraction of signals back to Earth.

HF propagation & ionospheric regions

Within the ionosphere there levels of ionisation that affect the radio waves varies. There are some areas where the levels of ionisation are higher than others. As a result it is commonly stated that there are several layers within the ionosphere. More correctly there are a number of regions, as the level of ionisation does not reduce to zero, but instead there are several ionisation peaks.

The main regions are detailed below:

  • D region:   When a sky wave leaves the Earth's surface and travels upwards, the first region of interest that it reaches in the ionosphere is called the D region. This region attenuates the signals as they pass through. The level of attenuation depends on the frequency. Low frequencies are attenuated more than higher ones.
  • E region:   Once the signals have passed through the D region, they reach the E region. Although there is still a little attenuation of the signals, this region reflects, or more correctly refracts signals, sometimes sufficiently to return them back to earth. The level of refraction reduces with frequency and therefore higher frequency signals may pass through this region and on to the next region. The E region is of great importance for HF propagation at the lower end of the HF spectrum and even the MF spectrum.
  • F region:   The F region or layer is the one that enables HF propagation to provide worldwide communications. Signals that manage to pass through the D and E regions will reach the F region. Again this acts to refract signals and they can be returned to Earth. During the day this region often splits into two, known as the F1 and F2 regions.

. . . . . . Read more about ionospheric regions / layers.

HF propagation skip distance and skip zone

When using HF propagation, it is often convenient to define some of the distances involved.

  • Skip distance:   The skip distance for a signal using HF propagation via the ionosphere is the distance on the Earth's surface between the point where radio signals from a transmitter, transmitted to the ionosphere and refracted downwards by the ionosphere, to the point where they return to earth and are received.
  • Skip zone:   When signals are transmitted in the HF portion of the spectrum they will only extend for a small radius around the transmitter via the ground wave. Beyond this they are not audible until the sky-wave is returned to earth. The skip zone or silent zone is a region where a radio transmission can not be received. The zone is located between regions covered by the ground wave and where the sky-wave first returns to earth.

HF propagation & frequency selection

One of the key aspects of HF propagation is to use the right frequency. It may be possible for propagation to enable communications to exist with one area but not another.

Because the higher frequency signals can pass through the lower regions, signals on different frequencies will travel different distances. When using HF propagation via the ionosphere. As the higher frequencies tend to be reflected by higher regions, these are able to reach much greater distances as a result of the geometry.

There are a few definitions that are used within HF propagation circles:

  • Lowest Usable Frequency, LUF:   The LUF is the lowest frequency at which the received field intensity is sufficient to provide the required signal-to-noise ratio at a specific time of day.
  • Maximum usable Frequency MUF:   The MUF is the highest signal frequency that can be used for transmission between two points via reflection from the ionosphere at a given time.
  • Critical Frequency:   The critical frequency for a given layer or region in the ionosphere is the highest frequency at which a signal travelling vertically upwards is reflected back to ground. This gives a good indication of the state of the ionosphere.
  • Optimum Working Frequency:   The optimum working frequency is the highest effective frequency that is predicted to be usable for a specified path and time of day for 90% of the days of the month.

. . . . . . Read more about HF propagation frequency definitions.

HF propagation ionospheric reflection by D, E & F layers
Ionospheric reflection by D, E & F layers

Multiple hops

Whilst it is possible to reach considerable distances using the F region as already described, on its own this does not explain the fact that radio signals are regularly heard from opposite sides of the globe using HF propagation with the ionosphere. This occurs because the signals are able to undergo several "reflections". Once the signals are returned to earth from the ionosphere, they are reflected back upwards by the earth's surface, and again they are able to undergo another "reflection" by the ionosphere. Naturally the signal is reduced in strength at each "reflection", and it is also found that different areas of the Earth reflect radio signals differently. As might be anticipated the surface of the sea is a very good reflector, whereas desert areas are very poor. This means that signals that are "reflected" back to the ionosphere by the Pacific or Atlantic oceans will be stronger than those that use the Sahara desert or the red centre of Australia.

HF propagation with multiple ionospheric reflections
HF propagation with multiple ionospheric reflections

It is not just the Earth's surface that introduces losses into the signal path. In fact the major cause of loss is the D region, even for frequencies high up into the HF portion of the spectrum. One of the reasons for this is that the signal has to pass through the D region twice for every reflection by the ionosphere. This means that to get the best signal strengths it is necessary signal paths enable the minimum number of hops to be used. This is generally achieved using frequencies close to the maximum frequencies that can support communications using ionospheric propagation, and thereby using the highest regions in the ionosphere. In addition to this the level of attenuation introduced by the D region is also reduced. This means that a radio signal on 20 MHz for example will be stronger than one on 10 MHz if propagation can be supported at both frequencies.

The Sun and HF propagation

The ionisation in the ionosphere is chiefly caused by radiation from the Sun. As a result the state of the Sun and the radiation received from it governs the state of the ionosphere and HF propagation.

There are several key topics concerning the Sun and the radiation received from it.

  • The Sun:   The Sun is a fascinating star - discovering all about it is fascinating it is own right.   . . . . . . Read more about The Sun.
  • Sunspots & sunspot cycle:   Sunspots are areas on the surface of the Sun that are a little cooler than the surrounding areas. Their presence leads to higher levels of radiation being emitted and therefore this affects HF propagation.   . . . . . . Read more about sunspots & sunspot cycle.
  • Solar disturbances:   From time to time, massive disturbances occur on the surface of the Sun. Solar flares, and coronal mass ejections, CMEs also give rise to increased levels of radiation which in turn affects HF propagation.   . . . . . . Read more about solar disturbances & flares.
  • Sudden Ionospheric Disturbance, SID:   The Sudden Ionospheric Disturbance is normally the result of a coronal mass ejection. A CME has a major effect on HF propagation conditions.   . . . . . . Read more about Sudden Ionospheric Disturbance, SID.

HF propagation using the ionosphere is still a widely used as a form of radio communications. While not as reliable as satellite communications, it is not nearly as expensive, and can provide a useful back-up in case the satellite communications fail.

HF propagation is also widely used for broadcasting, military and many other organisations requiring long distance communications. HF propagation is also widely used by radio amateurs who are able to communicate across the globe.

As a result HF propagation using the ionosphere is likely to remain in use indefinitely as a form of radio communications technology.



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