The radio spectrum is a complicated and varied thing.

Although some people seem to separate "radio" from "microwaves", I tend to lump them together, so I'm going to consider "the radio spectrum" to be the section of the electromagnetic spectrum from the very lowest frequencies we can usefully use (a few Hertz) up to frequencies that we'd start to count as "infrared light" (several terahertz).

Those bounds are based on practicality - for very low frequencies, the antennas required to efficiently transmit become impractically large and the bandwidth available to communicate gets ridiculously low; at very high frequencies, our electronics cannot process the signals and we have to switch to optical methods, which is a whole different ballgame.

I'm going to give you a whirlwind tour of the radio spectrum - but with an agenda: I'm just setting the scene so I can talk to you about amateur radio...

First, a little theory.

Radio waves travel at about 300 million metres per second in empty space, or air, or stuff like that; a little slower in denser materials. They are "waves"; I'll spare you the physics, but they're a lot like sound waves, or ripples spreading across the surface of a lake (except in 3D). You can read up on Poynting vectors if you want the full lowdown!

They have three interesting properties that distinguish them from each other; the direction they're going in, how big they are in terms of "up and down" (or their strength) which is know as "amplitude", and how big they are in terms of the length from one wave-peak to the next with a wave-trough inbetween, which is the "wavelength". Well, there's another interesting property called "polarisation", but that's not relevant to my points here so we'll ignore it for now.

The amplitude of the wave is really just a measure of how strong it is. Waves are at their strongest where they're being generated, because as soon as they're made they spread out, and spreading them out means less energy in any given volume, so they're weaker. This doesn't always simply hold when the waves get reflected off of concave surfaces - when weak signals from a distant transmitter hit a big satellite dish, its curvature focusses them down on the receiver on the end of the prong, and the waves start getting stronger again as they all squash together as they get towards the focus. So waves of a given kind get weaker as they spread out in general; this is interesting for people who want to make sure their transmitter is emitting powerful enough waves in the right direction to reach the receiver, but that's not what I'm going to be talking about here, so we'll leave direction and amplitude and focus on the property that makes different kinds of radio waves behave so interestingly differently: wavelength.

Because radio waves in a given medium all travel at the same speed, the wavelength can be measured another way just as well: the "frequency", which is how many complete cycles from peak to peak will pass a fixed point in a second. If each cycle is L metres long, and the wave is going at 300 million metres per second, it stands to reason that 300,000,000 divided by L cycles will pass a fixed point in a second. Frequency is measured in "Hertz", which just means "per second"; but a lot of interesting frequencies happen to be in the millions of Hertz, so to keep the numbers nicer, we often deal in megahertz; and so 300 divided by L is a frequency in megahertz (and, of course, 300 divided by a frequency in megahertz is a wavelength in metres).

It's usually easier to talk about frequency rather than wavelength, for reasons I won't go into here; but we still use both, because one area where wavelength is more relevant than frequency is in antenna design. Most (but not all) antennas tend to be most efficient at transmitting and receiving frequencies in a relatively narrow band, perhaps only a few megahertz across; and that band is centered around the frequency where some conductive part of the antenna is the same length as (depending on the design) a quarter, a half, or the entire wavelength long. For instance, the optimum frequency range for the kind of antenna that's just a wire sticking up - as often found on the tops of cars to pick up FM radio - is around where the antenna length is a quarter of the wavelength.

FM broadcast radio is around 100MHz; 300 divided by the frequency in MHz (100) gives us 3m for the wavelength. A quarter of that is 75cm; many car radio antennas are somewhat shorter than that for cosmetic or not-getting-caught-in-trees reasons, using a trick called a "loading coil" to make it still work OK while being shorted (at a cost in efficiency, but FM broadcast signals are strong so you still pick them up well enough), but you'll find the longest ones are all about 75cm long.

But as you can imagine, lower frequencies (and, thus, long wavelengths) mean longer antennas, and that's often impractical, so most "consumer" radio systems use frequencies above 100Mhz.

# Hold onto your hats for a whirlwind tour of the radio spectrum!

So now we understand the units of wavelength and frequency, let's start exploring...

## Crazy low frequencies: 3Hz-30kHz / 100km-10km

As you can see from the wavelengths, these aren't frequencies anybody without nation-state resources are going to be making efficient antennas for. They are still used, however, because these low frequencies are able to penetrate things nothing else can - rock and the ocean. So they're mainly used for communications with submarines, or ground-penetrating radar, and things like that. They tend to be used with the biggest antennas their users can afford and fit on their land, but even so, the antennas are far shorter than required to be efficient, and waste a massive proportion of the power put into them, so need to be coupled with similarly massive transmitters (all the wasted energy ends up as heat in the antenna, the wires, and the transmitter's output stages, in case you were wondering what happens to it).

Where these waves propagate through air, they tend to sort of flow along surfaces, so can get around corners - if they hit a mountain range, although some will penetrate the rock due to their very low frequencies, the waves that go over the top of the mountain will also bend down over the top and still be received behind the mountain; and as the Earth curves, they'll follow the curve and travel long distances. However, higher frequencies do this as well and have many other advantages, so this band isn't picked for that reason.

Also, this band is narrow. The whole band is 30kHz wide; a single FM radio station needs a frequency range of at least 50kHz to fit just one station with high-quality stereo sound. A Morse-code "continuous wave" signal takes up a mere 50-100Hz, or less if you go really slowly, so that and similar digital data modes are what you'll find down here, rather than voices.

Strictly speaking, it's divided into bands called Very Low Frequencies (3KHz-30KHz, 100km-10km), Ultra Low Frequencies (300Hz-3kHz, 1,000km-100km), Super Low Frequencies (30Hz-300Hz, 10,000km-1,000km) and Extremely Low Frequencies (3Hz-30Hz, 100,000km-10,000km) but I'm not sure if those terms are used much.

Therefore, users of these frequencies are people with deep pockets and a burning desire to communicate with submarines (the military), or scientists doing science stuff, miners, or hobbyists who want a challenge. More on those later!

## Low frequencies (LF): 30kHz-300kHz, 10km-1km

This band is 270kHz wide, so we finally have some room to do some stuff. It's still quite tight - we could only fit a handful of FM radio stations in the whole band, for instance - but an AM radio station takes up about 20kHz, so we can fit several of them in here. Although the wavelengths still mean that properly efficient antennas are still impractical, we can get a lot closer to resonance with a massive antenna mast that is actually practical to build, meaning that this is the lowest frequency band that really takes advantage of that ability of the waves to curve around the surface of the Earth and ooze around mountains and hills (which is known as "ground wave"). So this band has machine data signals like standard time signals that "radio clocks" pick up, and some AM broadcast stations - known as "Long wave" stations.

## Medium frequencies (MF): 300kHz - 3MHz, 1km-100m

2,700kHz of bandwidth (enough for a hundred AM broadcast stations) to play with, an affordable 25m-tall antenna can actually be a fully resonant antenna for the higher frequencies - now we're cooking! But ground wave propagation is getting weak up at these frequencies; mountains cast serious shadows, and the wave strength drops off rapidly as you get around the curve of the Earth, meaning that practical transmitters tend to reach for hundreds of miles, while low-frequency waves go for a thousand miles or so.

One particularly interesting thing found in this band is navigational beacons for aircraft, transmitting a constant signal that directional receivers on the aircraft can pick up. Pilots can find a beacon on an aviation map, and read the frequency it transmits on and the callsign it transmits; then tune their receiver to that frequency, check they hear the correct callsign, and then read off the direction to that transmitter for navigation. The ground-wave propagation means the beacons can be useful over great distances, and aircraft tend to be at higher altitudes than things like mountains so don't suffer from the reduced efficiency of ground-wave at these frequencies as much as land-based receivers.

## "High" frequencies (HF): 3MHz-30MHz, 100m-10m

I put the word "High" in quotes because, when this band was named in the distant past, people thought these frequencies were pretty damned high; but nowadays we routinely work with frequencies in the thousands of megahertz, and so had to think up a load of silly-sounding names for higher frequencies that we'll all get to roll our eyes at in the next sections.

At "high" frequencies, ground wave really doesn't work much. Your signals will get a bit over the horizon thanks to it, so our ancient ancestors thought this band was going to be pretty useless for long-distance communications (and short-distance communications were already amply catered for by telephones and carrier pigeons and messenger boys), and gave it to hobbyists to experiment with.

How wrong they were. It turns out that radio waves up to around 30MHz (sometimes as high as 50MHz, but it varies) get reflected and refracted by layers of ionised gas in the upper reaches of the Earth's atmosphere, known as the ionosphere. The intensity and altitude of those layers varies, because the thing ionising that gas is the Sun; so the layers vary on a day/night cycle, and vary depending on how much solar wind the sun is putting out, and what time of year it is. This is known as "sky wave" propagation.

This means that, when conditions are good and the ionosphere is nice and shiny and high up, HF radio waves going up into the sky can reflect down onto the ground a long distance away - and can then bounce back up off of it (because they're not crazy low frequencies they reflect rather than penetrating) onto the sky and back down again further away. This means that they can reach all the way around the planet so you hear your own echo, even from a relatively low-powered transmitter that a private individual might be able to buy or own. And with wavelengths under a hundred meters, you can fit efficient antennas into back gardens.

The hobbyists, therefore, had a whale of a time, and "amateur radio" took off; but military and commercial users had also discovered sky wave propagation, so as well as hobbyist communications, this band is also popular for international commercial (or government propaganda) "short-wave" broadcast stations, governments communicating with their spies, over-the-horizon radar, international telephone calls (before undersea cables were quite as widespread as they are now) and communications between ships and aircraft and their ground stations.

(MF and LF did sky wave as well, but because the ground wave propagation worked 24/7, sky wave propagation was considered more of an occasional annoyance that carried signals beyond where they were intended to go, so nobody had really paid it much attention at those frequencies).

And that variable ionospheric skywave propagation isn't just useful for communication; you can monitor the varying propagation to actually measure the changing nature of the ionosphere, in order to better understand the weather and the solar wind.

## "Very high" frequencies (VHF): 30MHz-300MHz, 10m-1m

Sky wave sometimes works up to 50MHz or so, but only sometimes; and ground wave doesn't give you much range beyond the horizon (or behind a hill or a building or a tree...) at these frequencies, so what use is the VHF band?

Well, it turned out, there is a lot of useful stuff you can do with radio waves that don't get much beyond the horizon. If you put a tall mast on a tall hill, the horizon is pretty far away - perhaps a hundred miles. And since the VHF spectrum has a lot of bandwidth compared to HF and below - 270MHz, while all of the bands we've looked at so far including HF add up to just 30MHz - there's a lot of space. Here we have the high-quality broadcast FM radio stations taking up 50kHz each, for instance (and DAB radio too). There's space for thousands of low-quality FM (but still better than AM) voice channels at 25kHz each, and the wavelengths are such that handheld radios with little stubby antennas can work reasonably well, so this band is full of bands set aside for emergency services, military battlefield communications, taxi radios, walkie talkies used by security staff, and that sort of thing. It's also used for short-range communication with aircraft - where "short range" is still a hundred miles or so because aircraft fly high enough that the horizon is a long way away for them, so it's ideal for communicating with local national air traffic control and particularly with the controllers at the airfield you're planning on landing at.

And the really useful thing about VHF not reflecting off of the ionosphere is that it can go into space. Although most satellite stuff happens at much higher frequencies, low-bandwidth satellite communications happen in the VHF band.

There are a few other kinds of propagation that will work in VHF to get you some extra range, but they're very weather-dependent and tend to only occur rarely; you can, occasionally, get signals a thousand miles away if they curve just right through the atmosphere. And although the ionosphere isn't shiny enough to reflect most of the VHF spectrum, locally highly-ionized regions of it such as the streaks left by meteorites burning up in the atmosphere will - as will the fuselages of high-altitude aircraft, not to mention the Moon (yes, people do communicate by bouncing signals off of the moon using VHF and above); so hobbyists looking for a challenge have a lot of fun playing with those kinds of things.

There's so much bandwidth here that some broadcast TV systems work in the VHF spectrum, too - an analogue TV signal takes up 6MHz or so of bandwidth, which would dominate the HF spectrum with just a couple of channels!

## "Ultra high" frequencies (UHF): 300MHz-3GHz, 1m-10cm

UHF is very similar to VHF; the propagation is line-of-sight only, although the shorter wavelengths make it a bit better at getting into buildings through windows and doors, and between trees, and so on. Longer wavelengths are better at penetrating through materials, while shorter wavelengths are better at getting through gaps in things - and many holes in things in our world are sized somewhere between VHF and UHF wavelengths, so the dominant mechanism of getting into things changes between the two!

The main differences between VHF and UHF are, again, an order of magnitude more bandwidth, meaning that this band is where most TV broadcast systems happen - and that the wavelengths are now getting so small that really small antennas can be really efficient. UHF is where the first practical mobile phone networks took off. Other than the huge slabs of bandwidth set aside for television, UHF is largely used for the same sorts of things as VHF - with mobile networks and 2.4GHz wifi/bluetooth hovering around the top end, where it gets a bit microwavey: tonnes of bandwidth, over short ranges.

## Microwaves: 3GHz-3THz 10cm-0.1mm.

And, finally, we get into the microwaves, which are basically "like UHF but more so". They're really bad at penetrating solid materials (the higher ones are even bad at penetrating wet air, and don't work so well when it's raining), but are really good at getting in through small holes. The short wavelengths mean that non-enormous dish antennas (dishes need to be several wavelengths across) become practical, and they're really good at picking up weak signals from a non-moving source - so microwaves are very popular for satellite communications. All satellite TV systems work in the microwave spectrum, because it works with a dish you can actually mount on your wall. Those super-directional antennas are also used for terrestrial purposes to make point-to-point links: radio tower masts often sport a couple of round dish antennas (often encased in cylindrical drums) pointing horizontally. Somewhere in the distance, there's another mast that they're pointing at, with a similar antenna pointing back.

Strictly speaking, the microwave spectrum is divided up into Super High Frequencies (3GHz-30GHz, 10cm-1cm), Extremely High Frequencies (30GHz-300GHz, 1cm-1mm) and Tremendously High Frequencies (300GHz-3THz, 1mm-0.1mm) because, as previously mentioned, we used to think that 3MHz-30MHz was a High Frequency and the name stuck. Oops.

Radar systems are mainly in the microwave band; the exceptions being over-the-horizon radar that has to use HF to get sky wave propagation, and some specialist weather radar stuff that I gather happens in UHF. Microwaves are best for radar, precisely because they don't ooze around obstructions in the way required for ground-wave propagation; radar works by sending a pulse in a direction and then listening for reflected echoes to work out what's in that direction. This works best if your signal goes in a straight line. If it can ooze around a mountain, then things behind that mountain will return radar echoes to pulses sent to either side of the mountain or over the top - which makes it kinda hard to tell what direction the actual thing is in. In addition, objects smaller than the wavelength will tend to not return an echo at all, as the signal just oozes around them and keeps on going. Over-the-horizon radars aren't great at telling you exactly what direction stuff is in, and can't detect small things; microwave radars can track small objects very precisely!

As I hope the above has made it clear, different frequencies of radio bands have very different characteristics when it comes to how far they can travel under what conditions, and what kinds of antennas are best. This makes messing with radios attractive to the sorts of people who like technology and like a challenge. The radio spectrum is this invisible medium surrounding us, capable of connecting us with distant people; there's something magical and mysterious about it. It is a force of nature, for us to study and tame. You can home-build a device that transmits a watt or two of power (the same amount as a fairly dim lamp) into the sky, and can be received thousands of miles away (imagine trying to do that with a torch, illuminating clouds so that their reflections in the ocean illuminate other clouds, so somebody with a telescope in a distant country sees it and picks up your message!)...

In the early days of radio, there were two main drivers for development: long distance commercial/military communications (eg, with ships and distant military forces), and tinkerers. To avoid interference between different users, regulations were soon agreed to carve up the spectrum into little bands, with different rules about who could do what in each. Some frequencies were dedicated to single users, such as a broadcast radio station obtaining a licence to use a frequency that they could transmit constantly on; some frequencies and bands were designated as shared between multiple transmitters who would need some way to take turns.

So nowadays, every country has a spectrum regulator (ours in the UK is Ofcom) who set the rules in that country, and issue licences. Here in the UK, for example, there are some bits of the spectrum that you can transmit on without a licence:

• The CB "Citizen's Band", which is in the HF region.
• The PMR channels used for licence-free walkie-talkies, which are in the UHF region.
• The "ISM" bands, where NFC, bluetooth and wifi sit (generally in the UHF/microwave spectrum, but there's a few at lower frequencies too)

The cost of licence-free operation is that you're legally constrained by quite meagre power restrictions, designed to limit your opportunity to cause interference over a wide range; and that you have no legal recourse if others interfere with your usage, as long as they're within the power limits; and CB and PMR have restrictions on what kind of equipment and signalling you can use.

Of course, most of us own a mobile phone that routinely transmits somewhere in the UHF/microwave bands, albeit at a low power level - but our mobile provider holds the licence to permit our phones to do that (conferred through the medium of your SIM card).

Those restrictions are too stifling for radio amateurs, who want to push the limits of what the technology can do - for its own sake, or in order to learn skills they can apply to commercial uses of radio (without the cost of having to obtain a business radio licence!). So UK radio regulations allow one to apply for a licence to transmit in certain agreed frequency ranges - all over the spectrum - with few restrictions on what you transmit beyond power levels, and that you need to identify your transmissions with a callsign so problems can be tracked back to you - sort of like having a number plate on your car. The one area where the legal restrictions are more stringent is that you can't make money from it or transmit messages for other people (with a few exceptions for emergency situations), basically to ensure that commercial uses of radio go and get their own kinds of licence and don't flood the amateur bands with taxi firm traffic and the like.

For many years, amateur radio was the haunt of deeply technical people; at first, you needed to design and build all your equipment from scratch. But as mass-produced consumer electronics became cheaper it became more and more accessible to people of lesser finances, drive, and starting skill level.

In the 1980s, CB radio (at the time requiring a licence, but one that was trivial to obtain) became legal; and with the equipment required being affordable, it became wildly popular. In an age before mobile phones, mobile radio communication was a novelty; and the fact that radio let you find people to talk to based purely on them being physically near you provided a social element that telephones lacked.

CB radio gave people a taste for radio, and many then "moved up" to take the examinations required to get an amateur radio licence, allowing access to much greater communications ranges. Off-the-shelf transceivers for the amateur HF bands can be had for quite reasonable prices (especially if you're willing to learn Morse, thereby avoiding the need for the electronics required to modulate and demodulate voice signals), capable of communicating halfway around the Earth; and VHF transceivers costing £20 let you communicate locally in much the same way as CB - while also allowing access to repeaters on hilltops or on board satellites to reach much further.

However, in the 2000s, mobile phones and the Internet (and, before long, the combination of the two) made it possible to talk to other people all over the planet from a hand-held device, and stole a lot of amateur radio's charm (and CB, for that matter). People seeking the social angle drifted away, leaving a smaller hard core of people who are in it for the technical challenge once more.

At the time of writing (2019), there are many claims that amateur radio is dying.

There is still a tradition of social usage, where you transmit a "CQ" message requesting anybody who hears and wants a conversation to reply, but many people who came to radio for that approach are complaining that it's shrinking.

There is a strong tradition of building your own transmitters and antennas, and then going on the air either to communicate with a pre-arranged listener or to find anybody who can hear you, to test your equipment, for the thrill of pushing technical boundaries. This seems to be most of what happens in the extreme microwave regions, where off-the-shelf equipment is nonexistant!

There's also a very strong tradition of "contesting", a sport where you earn points by making contacts with people as far away as possible or from as many different parts of the world/country/etc as possible. For people who really just want to earn the points, they often use entirely digital communications technology that is better at working with weak distant signals, and communicates nothing more than confirmation that the signal got through. People who prefer the social stuff are often the most vocal in complaining about the large amounts of digital contest traffic.

There is a tradition of tradition for its own sake - restoring and operating vintage valve radios to respect the history behind them, including the role they played in the world wars, and because big bakelite knobs and glowing valves look nice.

And there is a tradition of accumulating radio equipment and practising its use with a view to having it available in case some emergency cuts us off from the mobile and landline networks we depend upon - ranging from individual "preppers" who want to be able to communicate with their community, to formal organisations such as RAYNET who plan and train with local government and emergency services to spring into action in the event of a disaster.

And there is a lot of fear that the hobby is dominated by old men, who are slowly dying off, and a lot of worry that the next generation won't get involved because they have mobile phones these days.

But I feel it's at a turning point; a change in how it is valuable, and we're just seeing the effects of that change meeting the inertia of institutions built around how things were before. After all, the Maker movement has exploded over the past decade; cheap tools and components and the ability to share knowledge via the Internet have tapped into a well of creativity and desire to explore, desperate to escape the repression of a culture of homogenized mass-consumption.

As a Scout leader, I find the kids are really interested in communications, even thought they all have phones, because:

• They just seem to find new media fun to explore. I rigged up a field intercom system through the local woods (with military surplus field telephones and a long cable) and set them a challenge using it, but the bit they liked most was mucking around with the telephones.
• They're fascinated by communications systems that don't rely on centralised infrastructure, that they can set up themselves and that will work in an emergency. Kids love emergencies.
• Some (not all, but many) are technically minded, and want to know how it works.
• They enjoyed it when I did an introduction to Morse code with them, too - I set them a Morse challenge and handed out sheets with the code on, and most of the kids asked if they could take the sheets home.

I think that amateur radio is starting to rise again, but by appealing to a new demographic; and so one of the things holding it back is the "image problem" of it being all about old men rambling on about their health problems on HF. We don't need to get rid of those guys, though - they're full of knowledge and are in my experience all incredibly helpful and willing to help people use radio in new ways; I only hear second-hand tales of people being grumpy about "noobs ruining it all". I'm quietly hopeful that the rise in radio will happen soon enough for the next generation to get to meet and talk to the old guard before they're all gone, because we as a society are too quick to shove our old people into retirement homes and forget about them. So we shouldn't try to get rid of them, or push them and their needs aside - we just need to stop presenting them as the standard!

What can we all do to help?

• Research, and focus our growth initiatives on, the things that attract young (by which I mean "10-30 or so" in this context) people to radio, rather than on trying to convince recreate the glory days.
• Get into the maker community. Do amateur radio things at your local hackspaces/makerspaces. Run licence courses and exams there.
• Promote cheap, hackable, radios. Cheap mass-produced radios to get you started, hackable kits for people to move up to. There will still be a market for top-of-the-line transceivers but it will be a small one, and right now fancy base station radios costing thousands of pounds are in all the pictures you see while reading up on getting into amateur radio. People need to not think that's the norm and that they won't be able to afford to get involved.
• A lot of introductory material rambles on about the breadth of things you can do with amateur radio (this blog post included); that's OK for whetting people's appetites, but you need to follow on with task-based HOWTOs on achieving interesting goals starting from nothing (I'd love to write some) so people can see what they need to buy and do to get to a particular milestone that appeals to them.

# Colophon

I make no apologies for explaining refraction of electromagnetic waves as them "oozing around things", it's a great metaphor!