PMR Radio

From Airsoft Community Wiki

PMR radios are a type of two-way radio (walkie-talkie) used for communications in airsoft skirmishes. They are licence free, meaning that anyone can use them without having to obtain official certification or permisison. Their full designation is 'PMR446'. The 'PMR' part stands for "Personal Mobile Radio", and '446' indicates the frequency band that they operate on: 446MHz.

There is no privacy with PMR radios - unlike mobile phones, anyone who has their radio set to the same channel as yours (and is within range) can hear your transmissions. PMR radios are point-to-point communications tools - they do not rely on a network between one user and another to establish a connection.

Specifications and features

General

The most basic PMR radios have 8 channels, while more sophisitcated versions have 38 sub-channels for each of these, giving a total of 304 channel/sub-channel combinations. Some radios have connection points for an external headset (combined speaker and microphone worn by the user). A headset is useful in airsoft because it helps to keep the noise of incoming transmissions relatively quiet, thus preventing members of the opposing force from hearing them and possibly discovering your location. Headsets often include an external push-to-talk (PTT) switch which can be mounted somewhere convenient like on the user's chest or on their weapon. The use of a separate PTT switch increases ease of use by eliminating the need to remove the radio from its pouch.

Channels and frequencies

The UK PMR446 channels and frequencies (in MHz) are...

Ch.	Frequency
1.	446.00625
2.	446.01875
3.	446.03125
4.	446.04375
5.	446.05625
6.	446.06875
7.	446.08125
8.	446.09375

The 38 sub-channels are implemented by means of a system called CTCSS (Continuous Tone-Coded Squelch System). This works by superimposing a low frequency audio tone onto your voice transmission, and you can choose from 38 such tones. The receiving radio is configured to respond to the same tone that's set on the transmitting radio - when it receives a signal with the configured tone mixed into it, it opens its squelch circuit (a kind of noise suppressor normally used to blank out background hiss), and the transmitted signal can be heard. If it receives a signal with a different CTCSS tone, the squelch circuit remains closed and the transmission is ignored.

Note that CTCSS is not a means of providing privacy - a receiver with no CTCSS can still receive a CTCSS'd signal, and play it through the speaker. What CTCSS does do is allow you to suppress transmissions that you don't want to hear. It's a way of reducing background chatter when there are many people using a particular channel. For example, everyone in a particular team might set their radios to an agreed channel and CTCSS tone, and thus allow themselves to communicate with each other more easily.

Output power and antenna type

The output power of PMR446 radios is limited to 0.5 Watts (or 500mW). The antenna is fixed, and is usually a short helical coil of wire inside a rubber or plastic housing. A helical antenna gives good perfomance when the radio is held at strange angles, but is ultimately an inefficient type of antenna - there are other types (such as a quarter-wave whip) that radiate the radio waves better, but which are more dependent on good positioning.

Range

The range of any radio device is highly dependent on the environment. In particular, it can be affected by the terrain and the atmospheric conditions. PMR radios are usually advertised as having a range from 3-5km (2-3 miles). It should be noted that this 'specification' is nominal - there is no guarantee that a radio will be able to transmit these distances, or that it will be limited to them. It all depends on where you are, how you hold it, and what the weather's like. For example, in a built-up area, range might reduce to 1km or less, while standing on a hilltop might extend the range to several kilometres. PMR radios have been used on mountain tops and have acheived ranges of several tens of kilometres.

Generally speaking, PMR radios have a range that is suitable for airsoft skirmishing, although it is possible that transmissions could become weak if a game is set in a relatively large site, or in a densely built-up area. Transmissions between the interiors of buildings, or where there is a land mass between radios, could be significantly weakened.

Accessories

There are few accessories for PMR radios. They are generally limited to various types of headset, along with basic things like carrying pouches and spare batteries. The radios are quite small (typically about 10cm high, 6cm wide, 3cm deep, with an antenna length of about 5-10cm). They readily fit into almost any pouch that an airsoft player might have on his tactical kit.

Headset

There are several types of headset. There's the basic consumer hands-free type, consisting of an earphone bud and a small clip-on microphone incorporating the PTT switch. There is also a larger version of this using a headphone-style external earpiece which is fitted with a boom microphone, and there are combined versions of these which use a bud type earphone and some lightweight support to hold a small boom mic.

Throat mic

A variation on the headset is the throat mic. This is like a collar which fits around the user's neck, and which has one or two microphones in contact with the throat. There is usually a small bud type earpiece connected to the collar, and a separate PTT switch. The advantage of the throat mic is that it isolates external sounds to a much greater degree than the normal types (the versions with two mics even more so). Some versions are very sensitive and can easily transmit a whisper clearly and legibly - a distinct advantage for close covert operations in airsoft.

Legality of using radios that operate on non-PMR frequencies

It's important to be sure that your radio transmits on the frequencies allocated to the UK PMR446 service. Radios imported from other countries may look similar to PMR radios that can be bought in the UK, and be sold to serve a similar need, but may be built to transmit on other frequencies. In particular, some channels on FRS/GMRS (Family Radio Service / General Mobile Radio Service) sets from the United States use frequencies that, in the UK, are used by the fire service, for communications on large ships, and during special events which include 'safety of life' communications, eg, by motor racing marshalls.

Remember that radio wave propagation is a strange beast - seemingly weak signals from low powered sets can travel huge distances for no obvious reason. Moreover, the fire service frequencies pass through repeaters which retransmit on non-FRS frequencies for miles around - your signals could be picked up by a repeater a couple of miles away, affect a critical situation 20 miles distant (way out of normal PMR range), and you won't even know you're doing it.

If caught using such equipment, especially if such use has resulted in disruption to the emergency services, risk to shipping, or loss of life or serious injury, the penalities are extremely severe: you can be fined up to £5,000 and/or be imprisoned for up to six months. Such use is of questionable value when a radio that transmits on the correct frequencies can be purchased very cheaply.