A beginner’s guide to radio
Listen very carefully. Can you hear that noise? Can you hear the radio? No, I don’t mean the FM radio booming from the car driving past, nor the mediocre sound of DAB wafting from the kitchen. I’m talking about all of the other radio signals buzzing around your head.
Of course you can’t hear them – not if you’re mentally stable, anyway, which I prefer to assume you are. However, you can’t even hear “normal” radio without some kind of receiver. The right apparatus allows you to watch and listen to broadcast stations, and exactly the same is true for all of the other wireless signals in the air – you need the right equipment to pick them up.
In order of increasing frequency, the electromagnetic spectrum is as follows: radio, microwaves, infrared, visible light, ultra-violet, X-rays, gamma rays. My old physics teacher taught me a good way to remember this – Rabbits Mate In Very Unusual eXpensive Gardens. I say good way, but whenever I try to remember this I’m never sure whether it’s “very unusual expensive gardens” or “very expensive unusual gardens”. Perhaps I’ve spent too much time visiting National Trust properties.
My old physics teacher taught me a good way to remember this – Rabbits Mate In Very Unusual eXpensive Gardens
It’s the radio part that we’re really interested in, and that’s generally accepted as running from 3kHz through to 300GHz, although the International Telecommunications Union (ITU) – the UN agency responsible for information and communication technologies – splits this space into 12 bands stretching all the way up to 3THz (or 3,000GHz). Each band is an extra zero wide (so 3kHz-30kHz, 300MHz-3GHz, and so on), which is simple enough.
The first three ITU-defined bands – ELF, SLF and ULF (for extremely, super and ultra low frequency) can be mostly ignored as they’re mainly generated by natural phenomena such as lightning and earthquakes. ELF has been used for submarine communications because the signal penetrates a fair distance through salt water: it can take hours to send a simple message – we’ll see why in a moment – but it’s delivered to boats operating hundreds of metres below the surface.
The logistics are complex, since the wavelength will typically be around a tenth of the circumference of the planet! Obviously, nobody is going to build an antenna that big (nor even a quarter-wave dipole), so instead these systems use parts of the Earth itself as the antenna.
Huge poles are sunk tens of miles apart in areas of low ground conductivity, so that the current penetrates deep into the Earth. It’s mind-boggling engineering, and only the Americans and Russians have ever built such systems (Britain once planned one in Scotland, but it was abandoned). Since the transmitters required are so huge, it’s a one-way system – there’s no way submarines can transmit back.
Very low frequency
The first band you might think of as normal “radio” is VLF (band 4, very low frequency, 3-30kHz), which has such a low frequency it can’t be used for voice communications, since the carrier wave frequency must always be higher than any signal you need it to carry – regardless of whether modulation is by amplitude (AM), frequency (FM), or whether you’re dealing with analogue or digital signals (it is possible to bend the rules slightly by compressing digital data before transmission, however). As a result VLF is only really usable for slow, low-bandwidth data transmission.
Next comes LF (band 5, low frequency, 30-300kHz), whose main use is for aircraft beacons and weather systems, although the good old long wave, which sits at the top end of this band, will be familiar to those who follow cricket matches or church services. Remember that low frequency and long wavelength go together: as one number goes down the other goes up. Visualise some kids making standing waves in a skipping rope: the faster they wiggle their hands (higher frequency) the more wiggles they can fit in, so the peaks are closer together (shorter wavelength).