Despite a slow decline over the past two decades, the use of High Frequency (shortwave) bands for national and international radio broadcasting remains a uniquely effective and efficient medium for reaching millions of listeners with a single transmitter.
The physics of why shortwave transmissions behave in the way they do is complex, but with a well-planned service, using the right frequency at the right time of day, broadcasters can serve the population of an entire continent, beamed from a transmitter thousands of miles away. Shortwave knows no boundaries or international borders and continues to provide a lifeline service for audiences in remote corners of the planet, or in territories where access to state controlled news, information and education is tightly controlled.
Unlike other broadcast bands (notably FM and medium wave), shortwave signals can be steered towards the sky – rather than radiating horizontally along the ground – and reflected back to earth, illuminating large geographical areas several thousands of miles away. The same principle applies regardless of whether traditional analogue or digital (DRM) mode is used, which makes DRM digital radio on the SW bands a compelling proposition to deliver high quality audio to mass audiences over very wide areas.
The mechanism for this magic is the ionosphere, which extends from about 80 to 1,000 km above the earth's surface; it contains a high concentration of ions and free electrons which reflect radio waves. The ionosphere consists of several layers of these charged particles which are sensitive to sunlight – the effect of which is to vary the density and composition of these layers depending on a number of factors: local daylight hours and the time of year are two of the main influences, as the earth's inclined orbit around the sun moves our planet between the seasons.
However, the very useful property of shortwave signals to travel long distances also has a drawback. The same frequency cannot be used all day long, or indeed all year round. This is because the diurnal and seasonal changes in the number of daylight hours at any location has a direct effect on the optimum frequency band at any given time. The sun itself also undergoes a much longer period of change, known as the sunspot cycle. Based on observations over many centuries, this variation in solar activity peaks around every 11 years. Long term predictions can therefore be made with a reasonable degree of accuracy, and this is an important factor in the art of frequency planning.
As a general rule, higher frequencies work best during daylight hours and summer time (in the northern hemisphere) while lower frequencies propagate better in darkness – before dawn and during the long winter evenings, especially when the sunspot cycle is at its lowest ebb. The problem, of course, is that the same rules apply to everyone, so in periods of low solar activity such as we've experienced over the last couple of years, the lower frequency bands are crammed by every broadcaster trying to use the best possible frequency for their service.
Management of the shortwave broadcast bands is an international affair. While every country has its own national organisation or governing body for legislating internal broadcasting (such as Ofcom in the UK) at an international level the ITU (International Telecommunications Union) – an agency of the United Nations – performs the role of a worldwide administration of radio regulations. Every member state of the UN subscribes to the ITU, even if not all abide by its rules.
Within the auspices of the ITU, the High Frequency Co-ordination Committee (HFCC) manages and co-ordinates global databases of international shortwave broadcasting. Encompass' Frequency Managers represent the BBC and other customer's interests at this forum, which meets several times a year to co-ordinate the frequencies used by all of the world's major broadcasters.
The output from the HFCC is two seasonal frequency schedules – summer and winter – known as the 'A' and 'B' seasons. The changeover between seasons is internationally agreed to occur on the last Sunday in March (start of 'A' season) and the last Sunday in October (start of 'B' season), which coincides with start and end of 'Daylight Saving' in many countries that change their local clocks.
Thus the start of the new 'A20' season will be Sunday 29th March, and the frequencies agreed for all shortwave transmissions will continue until the beginning of the next season 'B20', on Sunday 25th October.