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).

The MF (band 6, medium frequency, 300kHz-3MHz) band comes next, and its major use is for the medium wave radio service (does anyone still listen to MW?). MF also contains the 160m amateur radio band, and there are also a few navigation and global distress beacon applications. Next comes HF (band 7, high frequency, 3-30MHz), which many people think of as “shortwave” radio. Both broadcast radio stations and amateurs use this band, as well as military and aircraft-to-ground communications.

Due to the way HF propagates – by reflecting or, more accurately, refracting off the ionosphere and bouncing back to Earth – this band is also used in over-the-horizon radars. The crude resolution of such radar makes it useless for targeting, but it still beats modern satellite wizardry for defence early warning systems.

The US military is alleged to have a weapon that fires a high-power, directional beam of 3mm radiation, which is reported to cause an extremely painful burning sensation

After HF comes VHF (band 8, very high frequency, 30-300MHz), which is employed for FM radio, amateur radio, air-traffic control and instrument landing systems. TV used to operate here, too, but was moved in the 1980s to make room for our woefully inadequate DAB radio system. That DAB appears here is significant, though: it shows we’re entering the part of the spectrum best suited to data communication, the so-called “digital sweetspot”.

A major chunk of that sweetspot is occupied by UHF (band 9, ultra-high frequency, 300MHz-3GHz). It’s there we find current digital TV broadcasts, mobile phone signals (GSM, 3G and most of the 4G flavours), good old-fashioned Wi-Fi, the TETRA trunked radio system used by the emergency services, DECT cordless phones, Bluetooth, wireless sensors for equipment such as weather stations and energy monitors, plus a few amateur radio bands. We start to encroach on the microwave spectrum at the top end of this band. Most of the signals crammed into this very crowded spectrum are digital nowadays, which enables much more stuff to be packed into the available bandwidth.

All the kit I write about – phones, Wi-Fi and so on – operates within the UHF band, but having come this far I might as well complete the trip; next is the SHF (ITU band 10, super-high frequency, 3-30GHz) band. Here we find 5GHz Wi-Fi and satellite TV downlink signals. Almost all modern radar systems employ SHF, and a massive chunk (almost a third) of the band will be used by wireless USB as it becomes more widespread.

This band is great for directional, short-range data communication, and recent developments in microwave integrated circuits mean the signal processing can now happen directly in silicon, rather than a processed signal having to be mixed with a high-frequency carrier. So while UHF is the band for now, SHF looks set to be the band of the future, with more and more of our data signalling moving into this spectrum.

The last but one of the official ITU bands – and the last really usable one – is EHF (band 11, extremely high frequency, 30-300GHz) with wavelengths between one and ten millimetres. Such signals suffer extreme attenuation by the atmosphere, so the band isn’t suitable for long-range communication. The attenuation is caused because these signals stimulate the resonant frequencies of particular atmospheric molecules – oxygen, for example, has a huge absorption peak at around 60GHz – although that does mean that windows exist in the attenuation spectrum where no molecular culprit lives.

Next-generation Wi-Fi

The upcoming Wi-Fi standard 802.11ad is actually designed to work at 60GHz, because oxygen absorption isn’t too much of a problem over LAN-scale distances, and in fact becomes a benefit in that it means 60GHz can be used only for short-distance links and you don’t need to worry about interference at longer ranges (at least not for terrestrial applications). That in turn means the same frequencies can be safely re-used nearby, and some countries allow unlicensed use of 60GHz.

Move slightly away from the oxygen absorption peak and the attenuation quickly drops off, and such frequencies are starting to be deployed for very-high-bandwidth communication links. The high frequency means you can pack in much more data than you could with a longer wavelength carrier. Those famous airport scanners that can see through your clothes also work in the EHF band, but more worrying than that is a reported use of this band as a weapon.

The US military is alleged to have a weapon that fires a high-power, directional beam of 3mm radiation, which is reported to cause an extremely painful burning sensation – as if the victim were on fire – even though no physical damage is caused. I used to work in the defence industry (defence is really a euphemism for offence), and I find such stuff very offensive. No physical damage, perhaps, but imagine the long-term psychological damage if you’d been subjected to it.

Finally, we arrive at THF (ITU band 12, tremendously high frequency, 300GHz-3THz), which is almost into the light spectrum since THF sits just below infrared. This band is used mostly for medical imaging, and although there has been a proof of concept experiment to transmit data at these frequencies, real-world applications are decades away.

So how can we detect the various signals buzzing around our heads in a typical office or home environment? In a few cases, it’s easy: just set your phone to manual network selection and it will show you the various 2G, 3G and possibly 4G signals available. Turn the dial on your analog radio and you’ll hear the various broadcast stations (and amateur stations if your set offers shortwave). Select Autostore on your digital TV and you’ll probably see a bar graph as it scans the broadcast TV bands and finds the stations.

For the Wi-Fi spectrum I’ve written here before about some of the wonderful Wi-Spy device, which provide a nice visual display of what’s happening around the 2.4 and 5GHz bands, showing not only wireless networks but also other devices that pump out such radiation, including microwave ovens.

I stumbled across a brilliant little device called the RF Explorer

It would be nice to see the same kind of visualisation across the wider radio spectrum, wouldn’t it? You can, but the necessary equipment is particularly expensive. Recently, though, I stumbled across a brilliant little device called the RF Explorer, which is far more cost-effective – I bought one for £185 from UK distributor Cool Components.

This gadget was designed by Ariel Rocholl, and was originally intended to help pilots of radio-controlled models see what radio frequencies are in use locally. In fact, it’s very much like those Wi-Spy devices, but aimed at a different audience. Initial versions of his device operated in narrow bands – 433MHz, 868MHz and 915MHz – but the latest model, the one sold by Cool Components, covers all the way from 15MHz to 2.7GHz, which is from which is from the HF band all the way through the various ISM bands, broadcast radio and TV and well past Wi-Fi.

The device’s main display shows the selected range as a full spectrum analysis, with a graph showing the various peaks and troughs alongside the frequency and amplitude of the strongest signal detected. Since it’s sometimes difficult to see rapidly changing signals, the device offers various display modes such as “peak hold” and “averaging” that can freeze them.

The unit comes with two internal (switchable) receivers, a WSUB1G unit covering 240-960MHz (the base receiver for the unit), and a WSUB3G receiver fitted as an extension card – it’s this that covers the whole 15MHz-2.7GHz range. You can flip between receivers using the menu and front-panel controls. The average noise level is about 10dBm lower with WSUB1G and the dynamic range is better, so it’s useful to be able to switch when working with sub-gigahertz signals.

On the top are two antennas, both of which screw into SMA connectors. There’s a Nagoya NA-773 wideband telescopic antenna, which is for all sub-gigahertz frequencies, and a whip or helical aerial for the 2.4GHz band. If you’re mainly working in one particular band (868MHz, say), you can buy a third-party antenna optimised for that frequency, so long as it has the requisite SMA connector.

Scanning the signals

At its most basic, RF Explorer is great just for seeing what signals are out there, but there’s so much more it can do besides, from determining the best antenna orientation for your wireless router for optimum reception around your office or home, through to actually being able to see the digital data encoded within a signal in some cases.

Although the RF Explorer is a portable device, with a fantastic battery life of typically 16 hours, it also has a USB port and so can be connected to a computer to extend its functionality. In particular, when connected to a PC you’ll get a far more detailed display. Ariel provides open source Windows software and Dirk-Willem van Gulik has also ported it to OS X.

The device is so sensitive that its designer suggests removing its antennas and screwing in SMA 50ohm dummy load attenuators, which you can pick up for around £5, if you know that you’ll be travelling through areas of high radiation or strong electromagnetic fields (and this applies whether the device is switched on or off). I’d certainly advise this before taking it through an elderly airport security scanner in a third-world country.

Likewise, if you’ll actually be working with high-powered transmissions (perhaps close to a mobile phone basestation), you can use SMA attenuators to reduce the signal before it hits the RF Explorer. There are options in the device menus to set an “Offset dB” value, so that even with an attenuator attached you’ll see the correct signal strength displayed on the screen.

The RF Explorer device is open source, and you could even build one yourself if you so desire. You’ll find the schematics at the RF Explorer website, although I can’t see why you’d want to build your own given the reasonable price of the ready-made device.

Rocholl is still working on upgrades to RF Explorer: he’s currently looking to extend its top end from 2.7GHz to 5GHz, and there’s also a back-burner project to enable the device to detect and display the digital data contained within a modulated signal. I think this would be cool: not only would you see the signal peak every time your wireless outdoor thermometer sent a reading back to its display, but you’d actually be able to see the reading. Okay, wireless thermometers aren’t that exciting – but there’s much more interesting digital data flying around in the radio soup nowadays.