Automatic Link Establishment, also known as ALE, is a de facto standard used for digitally connecting to HF radio channels.
ALE is a feature that is found in nearly any HF radio station, HF being high frequency.
This protocol is used in order to rapidly connect and call regardless of the changing HF ionospheric propagation and the various forms of interference. This is useful for everything from digital police scanners, to car radios.
A Primer on Digital Radio
In order to understand the importance of ALE, it’s useful to remember how digital radio operates.
Essentially, any radio signal works by causing modulations at certain wavelengths affecting the electromagnetic field that exists all over earth. In other word, changes to electron charges in the atmosphere occur in certain patterns, and these can then be identified and used to carry messages.
More specifically, a two-way radio receiver will look for fluctuations at certain wavelengths. The wavelength here is the frequency (the speed of the wave), while the data can be communicated via changes in the phase or amplitude. This is how traditional analog radio works.
Digital radio however works slightly differently: by stopping and starting the signal, or switching between two types of signal. This can then be translated to 0s and 1s, which effectively work like binary. This allows information to be carried at a more exact frequency, and with less shifting and changing.
The Role of ALE
If you recall the old days of analog radio, you had to consistently track and search for the frequency – which would occasionally move one a long journey!
This is not true of digital radio however, because the signal is more precise and therefore modern shortwave radio tends to be much clearer and require less manual configuration.
Due to the impact of atmospheric conditions on the ionosphere, HF radio as used by governmental organizations primarily required highly skilled radio operators. Today though, HF radios contain microprocessors and MFSK modems with unique ALE addresses – like a phone number – which defines its channel.
When a HF SSB transceiver is not broadcasting, it will be actively scanning through HF frequencies (channels) to listen for the ALE signals. Once that signcal is decoded, it can then lock on to the station and stay locked in.
This also makes it much easier to call a specific station. This is similar to dialing a phone number – the ALE controller finds the best idle channel for the destination channel and then uses a selective calling signal to communicate to the recipient that it will be transmitting on that channel.
The recipient will receive the ALE signal, then stop scanning and stay locked in on that channel.
This has important implications for everything from emergency radio to satellite phones. It is just another example of a technology many of us now take for granted, that has been the result of countless years of innovation and design. Without it, our world would be an extremely different place today!
Image Source: https://www.youtube.com/watch?v=y4USt8JFUHE