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Understanding the way in which HF radio signals actually propagate can help the effects of ionospheric propagation to be used to its best.
Skywaves, skip zone and skip distances are three key concepts show why radio communications signals are heard in some places and not others .
In order to cover large distances using ionospheric radio propagation the concepts of skywaves, skip distance and skip zone are important. They have an impact on aspects of radio communications links including the times and frequencies chosen, the antennas used, the transmitters and receivers employed and a variety of other aspects.
The skywave refers to the signal that travels away from the Earth’s surface towards the ionosphere. Unlike a ground wave it does not follow the contour of the ground, but instead it is directed towards the ionosphere.
The angle between the line of the skywave signal and the Earth’s surface at that point may be shallow or steep.
The skip distance is the distance over the Earth's surface between the point where a radio signal is transmitted, and the point where it is received having travelled to the ionosphere, and been refracted back by the ionosphere.
The signals leave the antenna and travel away from it, eventually reaching the ionosphere. Normally they will leave the earth at an angle called the angle of radiation. Whether it is low, i.e. almost parallel to the Earth, or high, i.e. at a high angle upwards, they will reach the ionosphere at some point.
The skip distance is dependent upon a variety of factors:
- Frequency: The frequency of operation has a major influence on the skip distance that can be achieved. Typically as the frequency increases a lower angle of radiation is needed to return the signals to Earth in a shorter distance. Also higher frequencies tend to be reflected or refracted by higher layers or regions in the ionosphere. This will mean that higher frequencies tend to lead to longer skip distances.
- Ionospheric conditions: The ionospheric conditions play a major role in governing the skip distance. Under some circumstances when ionisation levels are high it may be possible for signals to achieve very short skip distances.
- Angle of radiation: The angle of radiation from the transmitting antenna will also have an impact on the skip distance. A lower angle of radiation will lead to longer skip distances as a result of the geometry. This means that radio communications links can cover greater distances if low angles of radiation are used. However for some applications where a specific target area is required, the angle of radiation can be tailored so that the required target region is covered in conjunction with the specific ionospheric region.
The skip distance is dependent upon a variety of different factors. Although some users such as radio amateurs often require long distance radio communications and can accommodate linking with others anywhere on the globe.
For other users such as broadcasters and some diplomatic radio communications, specific target areas may be needed. Here adjusting the angle of radiation from the antenna in conjunction with using a specific band of frequencies and times of days, etc enable the best performance in terms of the skip distance to be obtained.
The skip zone, which may also be called a silent zone or dead zone, is a region where a radio transmission can not be received. The skip zone is the region between the point where the ground wave signals can no longer be heard and the point where the skywave first returns to Earth.
The presence of the skip zone can sometimes be beneficial, and at other times it can give problems.
If continuous localised coverage is needed, then the skip zone phenomenon can be an issued. However if local coverage is not needed, for example for long distance radio communications, then it not an issue. The skip zone can also help to reduce the levels of interference because it means that the number of stations that are received is less, and hence the levels of interference can be reduced.
As one of the key limiting aspects of the skip zone close in is the attention of the ground wave, it helps to understand a little more about it.
Note on Ground Wave propagation:
Ground wave propagation occurs when signals follow the contour of the Earth, bending so that the signals are able to be detected beyond the horizon. It is this form of propagation that is used by LF and MF, Long Wave and Medium Wave Band broadcast stations..
Read more about Ground Wave propagation
The skip zone or dead zone depends upon a variety of factors:
- Ground wave coverage: The local coverage around the transmitter is governed by the ground wave. The extent of the coverage and the start of the skip zone will be dependent upon the frequency used. The ground wave coverage will be greater for lower frequencies. At MF it may extend as far as 100 miles or so, but for higher frequencies, e.g. 10MHz and above it may only extend for a mile or two. It can be surprising how short the ground wave coverage for signals at HF can be, even when high powers are used in conjunction with good antennas.
Skywave minimum skip distance: The minimum skip distance is influenced by a number of factors including the frequency of operation; the state of the ionosphere and the angle of radiation.
Under some circumstances near vertical incidence skywaves can be used and returned from the ionosphere. This normally only occurs for frequencies lower in the radio spectrum, but using this, the skywave coverage can be very localised and provide local coverage before the ground wave disappears.
The skip zone is normally experienced for most frequencies in the HF portion of the spectrum. It means that stations further away can be heard much better than local ones.
If the skip zone needs to be reduced so that better coverage is obtained closer to the transmitter and its antenna, then the frequency of transmission can be reduced. This has two effects, not only does it increase the ground wave coverage because attenuation of the ground wave reduces as the frequency is reduced. Also it enables higher angle radiation to be returned to Earth more easily, but it is also dependent upon the prevailing radio propagation conditions.
Skywaves & frequencies
To gain a better idea of the characteristics of HF propagation using the ionosphere, it is worth viewing what happens to a radio communications signal if the frequency is increased across the frequency spectrum. First it starts with a signal in the medium wave broadcast band. During the day signals on these frequencies only propagate using the ground wave. Any signals that reach the D region are absorbed. However at night as the D region disappears signals reach the other regions and may be heard over much greater distances.
If the frequency of the signal is increased, a point is reached where the signal starts to penetrate the D region and signals reach the E region. Here it is reflected and will pass back through the D region and return to earth a considerable distance away from the transmitter.
As the frequency is increased further the signal is refracted less and less by the E region and eventually it passes right through. It then reaches the F1 region and here it may be reflected passing back through the D and E regions to reach the earth again. As the F1 region is higher than the E region the distance reached will be greater than that for an E region reflection.
Finally as the frequency of the radio communications signal rises still further the it will eventually pass through the F1 region and onto the F2 region. This is the highest of the regions in the ionosphere and the distances reached using this are the greatest. As a rough guide the maximum skip distance for the E region is around 2500 km and 5000 km for the F2 region.
The concepts of skywaves, skip distance and skip zone are at the core of HF radio propagation. Understanding these basic concepts provides a foundation for other concepts associated with HF broadcasting or forms of HF two way radio communications, etc. With distances of many thousands of miles being able to be covered using ionospheric radio propagation, skywaves, skip distance and skip zone are concepts that are encountered all the time.
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