Wireless technology is all around us. Most of us have a mobile phone or a wireless connection on our computer for Internet access at home. We rely on these to communicate and to exchange information.
We all know how frustrating and inconvenient it can be when one of these services fails to connect or exchange information correctly. But what if your life or the protection of your property relies on wireless communication? You would want to make sure that you had the very best available.
So when you choose a wire-free alarm system what should you look for within the radio link to ensure that alarm system messages are communicated reliably? And how can you be sure that the system will operate every time?
There are two main components of a radio link: a transmitter which sends information to a receiver, which then recovers the information.
Receiving the message
The key factors that define performance are:
1) Receiver Bandwidth – expressed in Kilohertz or as 25 kHz – 25 Kilohertz, with the lower figure the better. Ideally you need your receiver to listen to a very narrow band either side of the exact frequency on which a transmitter sends its information. This eliminates the chance of receiving unwanted transmissions, which cause interference and reduce receiver sensitivity. A narrow band receiver is unlikely to receive a transmission you don’t want it to hear.
2) Receiver Sensitivity – expressed in micro volts (muV) or decibel milliwatts (dBm). You will often find figures quoted as better than 1muV or >1muV – the lower the figure, the greater the sensitivity of the receiver. Alternatively you may find figures quoted as better than -107dBm or >-107dBm.) or decibel milliwatts (dBm). Again, the lower the figure the more sensitive the receiver. As a general rule consider receivers with at least the sensitivity quoted here, as a sensitive receiver is usually a well designed receiver.
3) Modulation Type – these generally fall into one of two categories, either Amplitude Modulation – often referred to as AM, ASK or Amplitude Shift Keying, or Frequency Modulation – often referred to as FM, ASK or Frequency Shift Keying.
Selection here is easy, Frequency Modulation (FM) types are less prone to interference than Amplitude Modulation (AM) types and will provide a more reliable radio link.
4) Operating Frequency – quoted in Hertz. Radio frequencies are typically quoted in kHz (Kilohertz), MHz (Megahertz) or GHz (Gigahertz). Hertz is a measure of cycles per second; so 1 Hertz = 1 cycle per second, 1000 Hertz = 1 kHz, 1000 kHz = 1 MHz and 1000MHz = 1GHz.
Sending the message
When considering transmission, all the factors we have considered already apply. But in addition there other factors to consider:
1) Deviation
Deviation is the volume of frequency found either side of the centre frequency which an FM transmitter uses to send its data. Often expressed as 6 kHz or + or – 3 kHz, the exact deviation will vary depending on the application. For data applications and wire free alarm systems low kHz values are preferable. For audio transmissions, 50 kHz or more is optimum. The deviation of the transmitter must always be within the bandwidth of the receiver.
2) ERP or Effective Radiated Power
ERP is the amount of power actually radiated by an aerial connected to a transmitter. This is measured in watts. Generally, the more power radiated, the further a signal will travel.
For wire-free alarm systems this is measured in fractions of a watt or milliwatt, as signals are only required to travel over short distances. All that is required is a signal strong enough to be reliably decoded at the receiver – any more results in unwanted interference to neighbouring alarm system. The actual amount of power needed will depend on the frequency used.
As an example, 1mW of power at VHF frequency will travel 100 metres in free space and decode reliably into a receiver. A 1 mW transmitter at UHF frequency will travel 50 meters and decode reliably into an identical receiver.
Connecting the message
To make transmitters and receivers work effectively together, aerials are required. Good aerial design and positioning is critical to system performance. As a general rule, for maximum performance:
– Keep as much ‘free air’ around aerials as practically possible. This maximises the aerial’s performance.
– Position aerials as high up as practical. This allows the detection devices to use the path of least resistance;
– Keep aerials a good distance from large metal objects. Be aware of metal hidden in walls, above doors and in/around windows;
– Always consider the building materials in use when positioning. For example thin floorboards offer a better radio path than a thick stone wall; and
– In situations where remote aerials can be used, always use aerials specified by the equipment manufacturer.
So always choose the correct aerial for your application, and always think carefully about the installation.
Frequency Selection
All radio frequencies in the UK are allocated by OFCOM – previously the Radio Communications Agency. By carefully planning the radio frequency spectrum, and allocating each user type a frequency range, the number of users working within a frequency slot without interference can be maximised.
Some radio frequencies are allocated exclusively to specific user groups, whilst others allow general access. Of course a frequency dedicated to a specific application will carry less traffic than a shared frequency. As a general rule the less traffic on a frequency, the more reliable the communication.
Different radio frequencies travel in different ways. Lower radio frequencies, often used for AM/FM radios, tend to travel more easily through solid objects. Higher frequencies are typically used for Microwave links and mobile phones.
So a mobile phone may show a good signal outside a building but fail to operate indoors, whilst an AM/FM radio gives a consistent performance. The radio is probably operating at a frequency between 160 kHz and 108 MHz, but the mobile phone will typically operate between 870MHz and 3GHz.
For the best performance, you should find the lowest available frequency on which you are permitted to operate. There are generally national frequency allocations as well as international agreements/agreements between adjacent countries.
When considering frequency allocation, check whether there are exclusive allocations or general purpose/shared allocations available. Generally exclusive allocations carry less traffic, and are therefore more reliable in use.
Most alarm systems in the UK operate on either 173.225MHz, 418MHz* (only for alarm systems manufactured before 2003), 433.92MHz or 868.6MHz frequency.
Don’t just consider price – lowest cost generally does not equal best performance! Do your research and ask questions before you buy. Quality manufacturers will happily tell you about their product specifications.