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.

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Ionospheric propagation tutorial includes . . . .
Ionospheric propagation     Ionosphere     Ionospheric layers     Skywaves & skip     Critical frequency, MUF, LUF & OWF     How to use ionospheric propagation     Multiple reflections & hops     Ionospheric absorption     Signal fading     Solar indices     Propagation software     NVIS     Transequatorial propagation     Grey line propagation     Sporadic E     Spread F    

Solar aspects:   Solar effects on radio propagation     Sunspots     Solar disturbances     SID sudden ionospheric disturbance     Auroras & propagation    

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.

Using HF ionospheric radio propagation, it is possible to hear and talk to other stations around the globe, but a knowledge of the modes of propagation and the ways they vary means that the right times can be chosen for the best results.

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 maritime two way radio communications, although they now utilise satellite communications.

HF radio is also used for broadcasting, a backup for aircraft as well as for a variety of other forms of point to point radio communications, including for the military.

Although HF radio communications is not as widely used as it once was, it is nevertheless still important.

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

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

HF propagation & skywaves

When using HF propagation via the ionosphere, the radio signals leave the transmitting radio antenna 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 . . . . the ionospheric regions / layers.

HF propagation skip distance and skip zone

When signals travel towards the ionosphere and away from the Earth' surface, they are know as skywaves for obvious reasons.

As they travel away from the Earth's surface towards the ionosphere, they virtually only attenuated as a result of the distances they travel until they reach the ionosphere.

However the signals propagating close to the ground do suffer some levels of attenuation dependent upon the frequency of the transmission. The are soon attenuated to the point where they cannot be heard.

At a greater distance the signals can then be heard again once they have been reflected, or more correctly refracted back to Earth.

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.
Read more about . . . . Skywaves, Skip Distance & Skip Zone.

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.

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

Multiple reflections

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 can be 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

In reality, the state of the ionosphere is not as clean and clinical as we might like, and there are many ways in which signals can be reflected multiple times achieve very long distances, sometimes being reflected on to another reflection by the ionosphere. Sometimes they may be ducted between the layers or regions.

Radio propagation & signal losses

It is not just the Earth's surface and the reflections that introduce 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. This can be a key factor when trying to establish two way radio communications.

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. Despite this, our Sun is the main source of radiation that creates the ionosphere.

  • 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.

    Sunspots have been recognised in the surface of the Sun for very many years, and their affect of radio propagation was noted once the way in which signals travelled over long distances started to be understood. It was found that there was a correlation between sunspots and the conditions for HF radio propagation and radio communications.

    Read more about . . . . Sunspots & Radio Propagation.

  • 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.

    Smaller increases in radiation level can improve the HF radio conditions, but as they increase, it can even lead to a radio blackout on HF.

    Visible signs of solar disturbances can be visible auroras at the poles. For large solar disturbances, ionisation levels at the poles increase significantly and can allow some specialist propagation modes at VHF allowing radio communications to be established at these frequencies. Here stations point their antennas northwards and reflections can often be heard over reasonably long distances.

    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.

Under some circumstances it is possible to use low power levels and simple antennas to establish radio communications over long distances.

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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