About HF (High Frequency) Radio

An Introduction to High Frequency Radio Propagation
Activity on the sun can have a wide-ranging effect on the earth. For example, radiation from the sun removes electrons from atoms in the upper regions of the earth's atmosphere, forming the ionosphere. The existence of the ionosphere allows the use of High Frequency (HF) radio as a means of communication over long distances.

How Does HF Radio Work Over Long Distances?
An HF signal transmitted from the earth may travel some way through the ionosphere before being "bent" back down towards the ground. This occurs due to the interaction between the HF signal and electrically charged particles in the ionosphere. The signal can then "bounce" off the ground back into the ionosphere, return to the earth again, and so on. The distance a given HF signal will travel depends on the frequency, transmitter power, take-off angle relative to the ground and the state of the ionosphere through which it is travelling.

For any given distance and time, there will be a certain range of HF frequencies that are most likely to provide successful communications; frequencies outside that range will work poorly or not at all. Simply increasing the power of an HF signal will not help if the frequency is too high for the distance required. Increasing the power may help if the frequency is too low, but using a higher, more suitable frequency is the best option. The highest frequency which may be used for reliable HF communications is known as the Maximum Usable Frequency (MUF).

How Do Conditions Affecting the Use of HF Radio Vary Over Time?
Extreme Ultraviolet (EUV) radiation from the sun creates the ionosphere. The EUV radiation arises from the bright and hot regions which overlie sunspots (areas of strong magnetic fields on the sun's surface). As the sun progresses through its eleven year cycle of activity, the number and size of sunspots will vary, as will the level of EUV radiation. Changes to the ionosphere that result from this mean that conditions affecting the use of HF radio will also change over the solar cycle.

At the low point of the solar cycle, only the lower frequency HF signals can be transmitted over a given distance. At the peak of the cycle, the higher frequencies in the HF band can be transmitted over the same distance. Other factors important in determining the range of usable HF frequencies include the seasons, the time of day and the relative locations of the transmit and receive points.

What Kind of Disturbances Can Degrade HF Communications?
Short-Wave Fadeouts - short lived (up to two hours) disturbances, in which solar flare activity results in the absorption of lower frequency HF signals. These will only affect signals passing through the daylight ionosphere.

Ionospheric Storms - large scale changes in the chemical composition of the ionosphere resulting in changes to the MUF. Decreased MUFs restrict the frequencies available for use over a given distance. Ionospheric storms normally last for one to two days.

What is Selcall?
Selective call - “Selcall”Selective call - Selcall - provides a simple and efficient method of calling stations within a network. With the combined “Selcall - Telcall” option fitted - all current derivatives of CCIR 493 format can be programmed into the your Selcall equipped transceiver on a channel by channel basis.

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This material has been sourced from I.P.S. Radio & Space Services