Within the ionosphere the 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.



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.



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.



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Grey line propagation

Grey line propagation is a form of radio signal propagation that provides surprisingly long distance radio communications at dawn and dusk sometimes when other forms of ionospheric propagation may not be expected to provide signal paths of these distances. Grey line propagation is only present around dawn and dusk and therefore it cannot be used to support global radio communications at any time. Accordingly it tends to be used chiefly by radio amateurs and a few other users who can accommodate the timing and other limitations of its availability.

For grey line propagation signals travel along the grey or twilight zone between night and day. This is area where night and day meet and it is also known as the terminator. In this region signals on some frequencies are attenuated much less than might normally be experienced and as a result signals can be received at surprisingly high levels over very long distances - even from the other side of the globe. The improved propagation conditions around the grey line are most noticeable primarily on the lower frequency bands in the HF portion of the spectrum where the level of ionisation in the D layer has a much greater effect on signals that on those frequencies higher up.

Current grey line (at time of loading page)

WSPR - Weak Signal Propagation Reporter

To assist radio amateurs understand propagation they can utilise the Weak Signal Propagation Network. The network consists of a group of like-minded radio amateurs who operate using K1JT's MEPT-JT digital mode to report real time radio frequency conditions. Transmissions are made using low power of very low power (QRP or QPRp).

Data can be uploaded to a central database and displayed for use by the wider amateur community. Below are some examples of my WSPR transmissions.

Weather forecast for the QTH of G0FEA

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