There are currently several approved frequency allowed for the use of radio alarms.

These frequencies or channels shares a spectrum area occupied by low power telemetry users and are located in the very high frequency (VHF 173 mhz) and (UHF 433 and 458 mhz) bands. Because of the channel congestion within these bands, rigid control of the equipment used is necessary to avoid interference between users. Equipment to be used on these channels are required to be approved to the specifications laid down by the Radio Regulatory body of the Department of Trade and Industry. This department is also responsible for the control and licensing of approved equipment. Type approval procedures are mainly concerned with ensuring the radio frequencies generated by the equipment do not produce spurious signals which can interfere with users sharing adjacent channels.

There are currently available several low cost non-approved radio devices. These initially may appear to be financially attractive, but the consequences of using such equipment could be disastrous in terms of reliability and false alarms. The frequency bands used by these devices are illegal and as a consequence are liable to severe interference from legitimate users.

Coding and Modulation

A central feature of any radio system is the radio frequency wave generated by the transmitter; this is used to carry information, whether it be music or speech from a broadcast station or coded data from an alarm transmitter. The intelligence to be transmitted is used to vary the amplitude or frequency of the generated wave. This process is called modulation and the most widely used methods are Amplitude Modulation (AM) and Frequency Modulation (FM). The two types of modulation have advantages and drawbacks, FM is less susceptible to impulse and atmospheric noise, whereas AM is less complex in design and performs marginally better at low signal levels.

In order to identify specific alarm transmitters a unique code is assigned to each device. This information is usually of a binary form, a series of '1' and 'O'. These groups are usually 4, 8 or 16 bits in length and are termed binary words.

Transmitter

Radio alarms can be designed with varying levels of complexity. Operation of the system requires the sending of a coded message every time the transmitter's alarm sensors are activated. This message eventually being received and decoded by the receiver before driving the appropriate alarms and indicators.

It is important, even in the most simple of systems, to ensure correct operation of every component part if false alarms or failure are to be avoided. Poor performance by any part of the alarm will inevitably degrade the whole system.

Considering the operation of the alarm transmitter. A carrier frequency is derived from a crystal controlled oscillator,or ceramic resonator  because of their ability to produce stable oscillations. These oscillations are influenced by the modulator which varies its amplitude or frequency in sympathy with the encoding information which is determined by the code switches. This waveform then passes to an amplifier which performs the dual role of multiplying the initial crystal oscillations to the required frequency, and ensuring optimum matching between the oscillator and output stage. This output stage is combined with a filter which ensures that only the required frequency is presented to the aerial, all other spurious products being rejected.

Aerial

The aerial though only a simple length of wire plays a vital role in the efficiency of the transmitter and receiver. It can be considered a simple transducer which converts the electrical energy produced by the transmitter into electromagnetic energy. This energy is then radiated in a similar manner to light from an electric light bulb. The analogy of light from a bulb is particularly relevant since the shadows and reflections produced by it can equally apply to the propagation of radio waves. In order to operate at maximum efficiency both the receiver and transmitter aerial have to be 'tuned' to the particular frequency in use. This is achieved by cutting the aerial to a preset length usually corresponding to a quarter wavelength of the frequency in use { 430 mm for 173 mhz and 165 mm for 433mhz } These dimensions obviously introduce constraints when transmitters are used in PA or covert applications. A compromise is usually sought in these situations between effective range and aerial size.