In 1969, years before the start of Television in South Africa, we were entertained by radio programs. In South Africa itself, we could listen on Short-wave and in all the bigger centres we had Medium-Wave. Both of these are what we call AM (Amplitude-Modulated) systems. Later on we got FM and the quality was amazing.
You cannot broadcast sound, i.e. audio, directly. Sounds produce pressure waves that attack our ears at a rate of between 50 and 15 000 pulses/second. The electrical signals that correspond to these sound waves are accordingly of extremely low frequency when compared to radio waves and they could not be radiated from an antenna. To radiate from an antenna, we need to use signals that vibrate at a couple of hundred thousand cycles/second up to many millions of cycles/second. So what we do is, we generate these higher frequencies and push them into antennas that will radiate them into the space around and above the antenna. Medium-wave and short wave transmissions work by radiating the energy away into near-space where it is refracted (bent) by magnetic layers in the atmosphere and caused to bend back towards the earth. It then touches down again at a distance of many km from the transmitting antenna. The exact skip-distance (technical term) from origin to reception point depends on the frequency of the transmitted wave, the angle of the antenna to the surface of the earth, and also to some degree on magnetic conditions in the atmosphere. These conditions change all the time as radiation from the sun ionizes various layers of atmosphere. In this way it is possible to shoot a signal into the sky in say Johannesburg and receive it in Cape Town or London.
Higher frequencies than those used for medium and short-wave radio can also be transmitted. As the frequency rises however, the way that the wave is propagated through the atmosphere changes gradually. When we get above the short-wave band, say roughly to wavelengths shorter than 10 metres i.e frequencies higher than 30 MHz (30 million vibrations per second), waves sent off into space will no longer be refracted back to Earth. At those frequencies the energy in the wave will simply carry it straight on out into space where it will travel forever or until it bumps into something and gets absorbed. But these so called VHF (Very High Frequency), UHF (Ultra High Frequency), and Microwaves can be used for what we line-of-sight or terrestrial transmissions between places on earth that can see each other. The transmission antenna is designed to send the waves in a beam (exactly like a torch beam) to the receive point. (A torch beam is a beam of radio energy and is only different in that it is in the frequency band that human eyes can see. Eyes are actually radio receiving antennae tuned to the extremely high frequency, GHz (Gigahertz) range. Visible waves are between 400 and 700 nm (nano-metres) in length.)
So we wanted to send some music through the air but we could not and instead we sent a steady beam of radio waves that had no direct meaning for us humans. Well, here’s the secret of transmitting the music or other intelligence we want to receive. The basic beam of high frequency radio energy that we can transmit is called the “carrier-wave”. But we modulate the carrier wave with the sound waves we want to transmit. Modulation means that we change the shape of the carrier in some way so that when it reaches the receiver, we can detect these changes, discard the carrier and retrieve the original intelligence. Modulation can be done in any way that will change the shape of the carrier in a recognisable manner. In AM radio (Amplitude Modulated Radio), we cause the carrier to get weaker or stronger directly in proportion to the strength of the modulating signal. i.e. the radio
program. In this diagram that I lifted from the internet, you will see that the outer shape of the top part of the modulated carrier (the upper sideband) has the same form as the original modulating soundwave. Actually the bottom shape of the modulated carrier also carries the same information upside-down and that is known as the lower sideband. In practice we delete the carrier and the lower sideband in the average AM receiver and are left with just one copy of the original modulating sound wave.
So much for AM but what and why do we consider FM to be a better system? Well in FM (Frequency Modulation), we have a carrier as before and we change its shape with the modulating signal as before. It’s only that we use a different method of modulation. Instead of varying the amplitude of the carrier, we vary the frequency of the carrier from moment to moment in proportion to the amplitude of the modulating signal. So a loud sound makes the carrier vibrate faster and a soft sound makes it vibrate slower. For complex reasons that I won’t try to describe here, this can only be done if the carrier frequency is very high. That is why the FM radio band tunes from 88 to 108 Mhz (Million cycles/second), many times higher in frequency that the medium-wave band and even more so than the short-wave band. The reason we use FM is that AM transmissions get interfered with by all sort of things. A car spark plug or a fluorescent light or a switched-mode electronic power supply or an electric drill etc. all radiate signals into space and these are almost all AM type signals. When these interfering signals reach your receiver at the same time as the transmission you want to hear, your radio cannot discriminate and receives both. The result is that AM signals are greatly troubled by interfering noise sources.
None of the devices mentioned above generate FM type radiation however so when we are looking for frequency variations in the receiver we can ignore all the amplitude varying interference and that means that FM radio is quiet and noise free. Here’s a graphic illustration of how FM modulation works.
So, back to Namibia and FM. In 1969, Namibia could only receive Shortwave radio from Johannesburg or overseas and shortwave does not generally produce good quality. It was decided to extend the growing FM transmitter network from South Africa to Namibia (Then called South-West Africa). Well that was all very well, but how could the radio programs be sent to Windhoek so that they could be put onto the FM transmitters? Within South Africa, there was a network of telephone lines that carried the radio station signals to each transmitter on each hill so that it could be broadcast. But at that stage, there were no telephone lines between South Africa and Namibia. To solve the problem, the SABC built a short-wave receiver station in Windhoek. A receiver hall was filled with highly sophisticated German-built receiving equipment fed off highly sophisticated aerials that could make a reasonable job of receiving the South African radio stations from the Johannesburg short-wave transmitters. The quality was very much better than any domestic short-wave receiver could manage, and whilst it was not high fidelity music quality, it was pretty good. FM transmitter stations were then built on the mountain outside Windhoek and at Oshakati on the Ovamboland border and the signals received at the Windhoek receiver station were fed by local post office lines to these transmitters. The SABC then also built radio studios in Windhoek to produce local radio programs in English, Afrikaans, Ovambo, Herero and the Damara-nama languages.
And so FM broadcasting came to Namibia and I’m pleased that I was there and could be part of getting it all going.