There's a lot of more-or-less correct stuff out on the net relating to what will work and what will not for emergency communications. Reading some 'prepper' (or SHTF) websites I've been unimpressed with the quality of information so this is an attempt to provide some accurate and fairly authoritative guidelines written from the perspective of the UK, and with most being relevant to other countries too.
The assumption is that in certain emergency situations you are likely to need to able to communicate with friends and colleagues without relying on existing infrastructure. It's important to note that cellphones in particular may not be reliable. If everyone tries to use a cellphone at the same time, there is simply not the capacity and the GSM network as used in the UK and Europe is specifically designed with a feature that drops non-emergency calls when necessary. Authorised users have special SIM cards for their phones which give them priority in such conditions. You are probably not one of them.
In modern times it's possible almost to forget that radio is the basis for a lot of communications. For most people the Internet comes into their home over wires or fibre, so it's a 'wired' service, but cellphones, car radio, and much TV (terrestrial or satellite though not cable) are carried by radio signals. Radio is carried on electrical waves which travel through space. Though we don't often think of it as a radio signal, light is also a type of radio wave as are X-rays and many other natural phenomena too. Years ago it was common to refer to radio as 'wireless' specifically to emphasise its magical ability to get messages through the air, and even through a vacuum or through solid materials like bricks and concrete too.
It's usual to refer to radio waves when we're talking about how radio works. One of their most important features is their frequency: like sounds, radio waves vary from low frequency (think bass instruments) to very very high (think of the higest note on a piano with a keyboard miles wide). And just like sound waves, radio waves of different frequencies travel differently through obstructions - at sea fog-horns use very low frequencies because those travel further and penetrate fog better, whilst high-pitched sounds are quickly muffled by fog. Low frequency radio waves easily penetrate buildings or go around or over mountains, but high frequencies do not. This matters a lot for emergency communications in built-up or hilly areas, or when communicating over longer distances. Radio frequencies are measured in MHz (megahertz).
There's almost too many options, an attempt to over-simplfy them is one of the reasons for some of the confusing and occasionally inaccurate advice I've read on the internet.
For any survival skills to be ready for instant use, you have to practise them. Setting up radio communications is no different. You need to be prepared with the right equipment installed the right way and you need to be able to operate it competently when the time comes that you need it. Because in every country the law constrains your access to the useful radio frequencies so as to prevent interference with fire, police, ambulance, air, shipping and other radio users, you and your group have to choose whether to obtain a licence or limit yourself to licence-free frequencies. In either case, you are only prepared if you rehearse and practice the routine skills of using your radio on a regular basis.
Licence-free frequencies are very restricted and it will come as no surprise to learn that many of the most useful frequencies are precisely the ones that need a licence. Whilst some people ignore the law and go ahead without a licence, regular training and practice use of your radio equipment may attract the attention of the authorities and may also possibly disrupt legitimate communications or at the thoughtless extreme, endanger life. In the UK the only practical licence-free frequencies are CB radio, and PMR446, both of which will get discussed in detail in subsequent paragraphs.
Frequencies requiring a licence are just about all the others except for CB/PMR446. Most licences are commercial and since they cost money aren't likely to interest anyone looking into this on a personal basis. The non-commercial way is to get an Amateur Radio licence, which requires a certain amount of study and passing an examination. The simplest Amateur Radio licence takes about as much study as learning the Highway Code to pass a driving test, followed by a multiple-choice examination. Once you have the licence it's free to keep it for life with no further testing. Amateur Radio licences permit the use of specific allocations amongst all the interesting radio frequencies, but of course only when you have put in the time and effort to obtain a licence. Amateur licences have clauses which specifically mention emergency communications, though in practice the professional emergency services make no use of this. The volunteer St John's Ambulance does collaborate with some amateur groups in some circumstances, see "Raynet" below.
All of the practical options have features and limitations which have to be borne in mind:
The jargon term for how radio waves get from you to your listener is 'propagation'. This largely determines how far apart you and your counterpart can be. A more powerful signal, like a brighter torch, may travel a bit further than a weaker one but no matter how bright a torch is, you can't get the light through a wall. The same goes for radios. More powerful radios use more electric power and in fact there is rarely much to be gained from using a lot of power. Typical power levels are in the 1 - 10 Watt range except at the extremes and these are enough for anything other than brute-force very long distance use. By far the most important factor is the choice of frequency and the quality of the installation, in particular the antenna (known in times gone by as an aerial). If you need to communicate through walls, or buildings, or past mountains, or over long distances, you need to pick a radio frequency which does that.
The typical radio installation consists of the actual radio unit, a power source (battery or generator or mains), then a cable from the radio to the antenna. Handheld radios, like cellphones, integrate all of those, but they are the exception. Any modern commercial radio unit will be of high quality and interchangeable with any other similar radio: these do not make the difference. The real derminant of how well your set-up will work is
There are a few basic factors affecting the choice of frequencies depending on what your options are and how far you need to communicate:
You will further be constrained by the type of installation and people you want to talk to. Are you thinking of a fixed point, say in a house or similar premises, or a moving vehicle, or something that can be hand-held?
Since the selection of frequency is all-important, I've listed the practical choices below based on frequency, because that also determines the all-important practical range. In every case I've tried to be realistic about what's achievable by non-professionals without exceptional cost and effort.
A licence is required for these, equipment costs in the £200-500 per set range, with some technical skill required to install and operate it. The reliable range is up to 400 miles or more but affected by atmospheric conditions.
These frequencies are in the Amateur 1.8MHz, 3.5Mhz and 7Mhz allocations. They are greatly affected by the action of the sun on the upper layers of the atmosphere: the reflected wave and the ground wave mentioned above both play a strong part. Reliable communications between two points will need to choose the right frequency for the time of day, usually moving down in frequency as night falls. After dark extreme ranges are possible of 2,000 miles or more and there is liable to be congestion of frequencies with many foreign stations also being involved. One odd daytime effect can be that the ground wave gives good coverage for a certain distance, then the reflected wave arrives further out, leaving an intermediate 'skip zone' where no contact is possible on that specific frequency. Lower frequencies are sometimes badly affected by thundery weather ('static') especially in summer months.
Vehicle installations are for the enthusiast and barely practical unless the need is extreme: they should not be entirely ruled out, but the antennas will be large and obvious. Handheld operation is not realistic. Effective field-stations can rapidly be established with an effective antenna as simple as a piece of wire between two tall trees, or trailing from a kite.
CB is licence-free. The amateur 10m band can use the same antenna and cable. CB equipment is cheap (say from £50 new) and readily available, antennas tend to be large-ish chunks of aluminium tubing (about 5m long) and a little cumbersome to erect high-up where they are most effective. Reliable range is highly variable but say 20 miles or more between fixed installations.
CB uses a frequency that was initially allocated in 1940s as being too high at that time to be of use for commercial or military purposes. Being licence-free it's a very popular local-range option. There is no real ground wave at this frequency so it's predominantly line-of-sight but not entirely, the frequency being low enough to get around some obstacles. Modern CB radios work on the 40 UK channels and also on a further 40 European channels, though not simultaneously. In emergency situations there could possibly be channel congestion, as many people have equipment that could be put into use if needed even though they don't use it much at other times.
Certain amateur equipment can be programmed to operate on both CB and the Amateur 10m band. The Amateur band could be used in emergency situations to provide a further 200 channels and may well be an interesting choice for those who are licensed, giving them access to both CB and 10m from the same installation. Strictly interpreting the law, the 10m equipment should not be used on CB frequencies but the authorities are most unlikely to be paying attention when the emergency occurs.
Vehicle installation is popular. Small antennas mean that handheld operation is barely useful, so handheld equipment is less easy to find and relatively expensive due to low demand. Vehicle to vehicle range is limited to 1-3 miles or so, dependent on terrain. Vehicle to fixed station may achieve 5-10 miles or more. In certain circumstances considerably more may be possible.
This frequency is very much affected by the 11-year sunspot cycle and other unpredictable atmospheric events. Propagation characteristics can suddenly change to give very long range communications, 1,000 miles not being unusual even from vehicles. This may be exciting for enthusiasts but has the drawback of suddenly hearing far-off taxi drivers speaking their native language and drowning out the local people you are trying to reach.
There are two interesting Amateur freqency allocations and one licence-free one here; the Amateur 144MHz and 430MHz bands plus the Public Mobile Radio (PMR446, 446MHz) allocation of 8 channels. Ranges for all will depend very much on direct line-of-sight with very little over-horizon possibility and with buildings, hills or other obstructions casting radio shadows. To get around obstructions it may be necessary to relay messages. Typical Amateur equipment ranges would be 20 miles between fixed stations, 5 miles between vehicles. Equipment costs are intermediate, £100-200 per set or less, some handhelds as little as £40, antennas are small, easy to source commercially and very effective if you can get them up to chimney height. With no obstructions and chimney-mounted antennas, fixed stations in good locations may reliably be able to communicate over 50 miles or further.
PMR446 is specifically authorised only to provide 8 channels for small handheld radios using low power. You cannot legally have a powerful fixed station, though it's possible to find amateur equipment for 430MHz that can be programmed also to operate illegally on the PMR446 channels. Between mountain tops, PMR446 may achieve ranges of tens of miles, but in built-up areas you may only manage a few hundred metres. The biggest drawback is the very limited number of channels and the many people who may try to use them in emergencies (I've seen packs of four or six of these things on sale for £50 or so).
There is little to choose between the amateur 144Mhz and 430Mhz frequencies. The lower frequencies are more forgiving of cheap cable and connectors, requiring somewhat larger antennas than the higher ones. In earlier times 430MHz was pushing the limits of what could be done cheaply but nowdays it's routine so equipment costs are similar. 144MHz gives slightly greater range with simple antennas but commercial antennas of equivalent size are more effective at 430MHz thus reducing the difference. Dual-band antennas are widely available commercially as are dual-band radios, so it would be almost perverse to choose only the one. Being historically more popular 144MHz will have more potential users in an emergency, though still offers a large number of potential channels (say 160), 430MHz is vast with five times as many channels possible. Unusual atmospheric conditions sometimes open up greater ranges and potential long-distance interference but the situation is not common.
Well, of course, it all depends. How big is your group? How much effort do you want to put in? Are you fixed or mobile? What range do you need? The simplest and least-effort approach for a group just getting started is obviously to select CB augmented with PMR446 handhelds. Because there is no such thing as 100% reliable communications, your group needs to train occasionally, getting used not only to the disciplines of running a net, but also when necessary relaying messages from member to member to cater for times when not everyone can reach everyone else. If you don't train regularly and check your equipment from time to time, you aren't prepared.
For greater range and more reliability, some of your group may choose to add in some licensed communications too. This doesn't mean everyone has to get an amateur licence, but those who are more interested might do so. Nothing says that one precludes the other and the Amateur licence explicitly permits the conveying of disaster and emergency traffic.
In the UK a properly organised group might consider contacting their local branch of Raynet. Raynet is a long-established organisation of licensed Amateurs specifically focused on the purpose of emergency communications. Raynet groups are all volunteers and as a consequence range from the almost-moribund and inactive to very well organised and almost professional operations. There has been an unfortuate tendency, perhaps now dying out, of senior and highly skilled amateur radio types taking a dim view of newcomers and presenting a forbidding and unwelcoming aspect. Some have a very dismissive opinion of CB users. If you encounter that, ignore it and try talking to a different member of the group. I'm not aware yet of any Raynet group welcoming 'outsiders' who are using CB, but perhaps it's time for them to change? Within any Raynet group you are likely to find one or two members who will indeed be welcoming and who will offer help and advice.
Raynet groups sometimes have good contacts with local emergency planning officers. At public events like shows, exhibitions, long distance walks, bike rides and similar, many local Raynet groups collaborate with St John's Ambulance groups to provide them with communications facilities; since St John's is also a volunteer effort they tend to rub along well together.