Radio Soup

Paul Ockenden explores a world of wireless signals.

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 do I mean 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 – I like to assume that we don’t have any readers from the tinfoil hat brigade. 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 – with the right kind of kit you can start to see and hear them.

In order of increasing frequency, the electromagnetic spectrum goes Radio, Microwaves, Infra-Red, 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. Well, I say good way, 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.

It’s the radio bit that we’re really interested in, and that’s generally thought of as sitting between 3kHz and stretching up to 300GHz, although the ITU (International Telecommunication Union – the UN agency responsible for information and communication technologies) splits the 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, etc.), which keeps things nice and simple.

The first three ITU defined bands ELF, SLF and ULF, Extremely super and ultra low frequency can be pretty much ignored, as they are mainly generated by natural phenomena such as lightening 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 gets delivered to boats operating hundreds of meters below the surface. The logistics are very complex though – the wavelength will typically be around a tenth of the circumference of the planet!

Obviously nobody is going to build an antenna that bit (or even a ¼ wave dipole), so instead these systems use parts of the earth itself as an antenna. Huge poles are sunk tens of miles apart in areas of low ground conductivity, so the current penetrates deep into the earth. It’s really mind boggling engineering, and only the US and Russians have ever built such systems (Britain once planned its own system in Scotland, but it was abandoned). Oh, and because the transmitters are so huge it’s a one-way system – there’s no way that submarines can transmit back.

The first band that you might think of as ‘radio’ is VLF (band 4, very low frequency, 3-30kHz) which has such a low frequency that it can’t be used for voice based communications as the carrier always has to be higher than any frequency that you want it to hold, true whatever kind of modulation (amplitude modulation AM and frequency modulation FM being the most common, although there are others too). The same “carrier must be a higher frequency than the message” rule holds true whether we’re dealing with analogue or digital signals, although of course it’s possible to bend the rule slightly by compressing digital data before transmission. Because of this rule, though, VLF is only really suitable for slow, low bandwidth data signals.

Next we find LF (band 5, low frequency, 30-300kHz) whose main use is for aircraft beacon signals and weather systems, although you’ll also find good old long wave radio (familiar to those who like cricket or church services, neither of which are particular favourites of mine) sitting at the top end of the band. Remember, low frequency and long wavelength go together – as one number goes down the other goes up. Just visualise kids creating standing waves in a skipping rope – as they wiggle their hands faster (higher frequency) an additional wave is introduced, so the distance between the peaks is reduced (shorter wavelength).

The MF (ITU band 6, medium frequency, 300kHz-3MHz) band comes next. Its main use is for medium wave radio (does anyone still listen to MW?). MF also contains the 160m amateur radio band, and there are also a few navigation and global distress system uses. Next up is HF (ITU band 7, high frequency, 3-30MHz), and this is what many people think of as shortwave radio. You’ll find both broadcast radio stations and amateurs using the band, as well as military uses and aircraft to ground communication. Also, because of the way HF propagates (it reflects, or more accurately refracts off the ionosphere and bounces back to earth) the band is also used for over the horizon radar systems. Although the crude resolution of these systems makes them useless for targeting, they are still used (despite all of our modern satellite wizardry) for defence early warning systems.

After HF comes VHF (ITU band 8, very high frequency, 30-300MHz). You’ll find the FM radio band here, alongside amateur radio bands, air traffic control and instrument landing systems. And of course we used to have TV in this band too, but that moved in the 1980s freeing up the frequencies now used by our woefully inadequate DAB radio system. Actually, DAB appearing at the top end of the VHF band is important, as it shows that we’re now getting to the part of the spectrum which is most useful for data communication. The so-called ‘digital sweetspot’.

A major part of that sweetspot is the UHF (ITU band 9, ultra high frequency, 300MHz-3GHz) band, and it’s here that you’ll find TV broadcasts (now fully migrated to digital, of course), mobile phone signals (GSM, 3G and most of the 4G flavours), good old fashioned wi-fi, the TERTA trunked radio system used by the emergency services, DECT cordless phones, bluetooth, wireless sensors for things like weather stations and energy monitors, and a few amateur radio bands. We’re starting to get into the microwave spectrum at the top end of this band. It’s a very crowded space, but as you can see, most of it is digital signals these days, which makes it so much easier to pack more stuff into the available bandwidth. These are the radio frequencies that usually concern the things I write about in this column.

But anyway, onwards and upwards, we might as well be complete. Next comes the SHF (ITU band 10, super high frequency, 3-30GHz) band. You’ll find 5GHz wi-fi here, and satellite TV downlink signals too. Almost all modern radar systems use SHF, and a massive chunk (almost a third) of the band will be used by wireless USB, as it becomes widespread. The band is great for very directional low range data, and recent developments in microwave integrated circuits mean that the signal processing can happen directly in silicon, rather than a processed signal having to be mixed with a high frequency carrier. Where UHF is the band for ‘now’, I expect that SHF will very much be the band of the future, with more and more of our data signalling moving into this space.

The last but one of the official ITU bands, and the last really usable one, is EHF (band 11, extremely high frequency, 30-300GHz). The wavelengths in this band are between one and ten millimetres, and the signals suffer extreme attenuation in the atmosphere, so the band isn’t generally considered suitable for long range communication. There are some holes in this attenuation though – the problem is caused because we’re starting to hit the resonant frequencies of particular molecules. Oxygen, for example, has a huge absorption peak at around 60GHz. Despite that, the upcoming Wi-Fi standard 802.11ad is actually designed to work at 60GHz, because at LAN scale distances the oxygen absorption is less of an issue.

In fact, the attenuation is a benefit because it means that 60GHz can only be used for short distance links, and so we don’t have to worry about interference – at least, not with terrestrial applications. The same frequencies can be re-used nearby. As a result some countries allow unlicensed use of 60GHz.

Move slightly away from the Oxygen absorption peak and the attenuation quickly drops off. These frequencies are starting to be deployed for very high bandwidth communication links. Because the frequency is so high it’s possible to pack much more data in than you could with a longer wavelength carrier.

Those famous airport scanners that see through your clothes also work in the EHF band, but more worrying that that is a reported use of this band as a weapon. The US is alleged to have a weapon which fires a directional bean of 3mm radiation at high power. This is reported to cause an extremely painful burning sensation, as if they were on fire, and yet no physical damage is caused. I used to work in the defence industry, but defence is really a euphemism - it’s really offence, and I found stuff like this American weapon very offensive. No physical damage maybe, but just imagine the long term psychological damage if you’d been subjected to it. Sorry, rant over!

Finally we come to THF (ITU band 12, tremendously high frequency, 300GHz-3THz). We’re almost getting into the light spectrum here – just above THF sits infra-red (remember Rabbits Mate In…) THF is used mainly for medical imaging, and although there has been a ‘proof of concept’ experiment to transmit data in this band, any real world application will be decades away, if not longer.