ABOUT PRIVATE MOBILE RADIO

• hand-held radios used by a family exploring the countryside, sometimes called 'walkie-talkies'.
• personal radios used within a large factory, and perhaps interfaced to the telephone system
• radios fitted to fleets of vehicles such as taxis and delivery vans.

The following information hopes to answer some of your questions about PMR and explain some of the terms you will find in the product descriptions in our online shop. Please contact us if you require more advice before buying.

Do radios need a licence?

Some short-range radios can be used without a licence on specially allocated frequencies, and are ideal for recreational use and in small business premises such as garden centres. They use a European system called PMR446. Remember, unlike mobile phones, there are no call charges! Other radios require a licence, please click here for more information.

What determines the range of the radio?

The power of the radio is one of the main factors in determining range. Hand-portable radios are limited to 5 Watts ERP (radiated power from the aerial), whereas the power allowed from some mobile (vehicle-mounted) radios is 25 Watts ERP. Height is another important factor, as the higher the antenna, the further the radio waves can travel unobstructed.

Under ideal conditions, a PMR446 radio signal may be received up to 3km away; a professional hand-held up to 5km; and a vehicle-mounted up to several tens of kilometres.

Frequency: VHF or UHF?

Typically PMR users are licensed for VHF (Very High Frequency, normally 133MHz-170MHz) or UHF (Ultra High Frequency, normally 430MHz-470MHz). Generally speaking, the lower the frequency, the greater the range of transmission. This does not mean that VHF is automatically the best choice, because VHF signals do not penetrate buildings as well as UHF, which is therefore more suited to urban areas.

Choosing between UHF and VHF for your system can be very difficult and even the experts get it wrong sometimes. We can help you make a choice and if necessary visit you to try out both systems before you buy.

Avoiding interference: CTCSS/DCS tones

To avoid radio users hearing messages that are not intended for them, the Radiocommunications Agency will allocate specific frequencies to a business when it issues a licence. In addition to this, a system of sub-audible tones can be used to further restrict reception of messages.

CTCSS is a fairly standard feature on all radios, but may be called something different by each manufacturer, e.g. Motorola call it Private Line, Kenwood call it Quiet Tone, etc. (DCS is the digital equivalent.) The CTCSS tone is carried, along with your voice, by the radio signal. The receiving radio is tuned to the same radio frequency as the transmitter, but will only "unmute" its speaker, allowing the message to be heard, if it can detect that the appropriate tone is present.

Typically there are 38 tones to choose from which means on a single frequency (e.g. 462.625Mhz) there could be up to 38 different users or user groups, each assigned to a different tone. Each group is able to have private conversations without the other groups being able to hear what they are saying. However, if any one group is having a conversation, no other group can transmit at the same time without blocking the signal because no matter which CTCSS tone they use, they are still using the same radio frequency.

On some licence-exempt PMR446 radios the tones are user selectable, giving effectively 38 individual channels on each radio frequency. PMR446 radios operate on 8 frequencies which are programmed into the radio when it leaves the factory. With 38 tones per frequency, you have up to 304 different combinations to eliminate interference.

Selcall, 5-Tone

Selective calling is the closest thing to privacy on radio. A 5-tone system will be designed by the radio supplier in conjunction with the customer's requirements. Normally a system enables single users to call other single users, on the same system, without any other users hearing what is being said. You can also define groups and sub-groups and program calls accordingly. Supervisors may have the functionality to call every user individually whereas the shop-floor staff may only have a group call facility.

It works in a similar way to CTCSS but uses 5 tones, sent when the transmit button is pressed, instead of a single continuous tone. The receiving radio is constantly looking for a signal on its frequency. When it sees that signal it checks to see if the CTCSS tone is present and correct, and then if programmed correctly checks to see if its 5 tone id is transmitted before it "unmutes".

5-tone is normally used if the radio operators require extra privacy or if the over-all radio system is quite large encompassing different departments within a company. Each user can have his own ID and can be called if the radio has the correct features. These features may be used either from a contact list or a numeric keypad.

Selcall systems are well-established with plenty of scope: most have a group call function to enable users to call all other users simultaneously, some utilise in-built timers to operate a lone-worker facility. You can call sub-groups, send status messages, use auto acknowledgement from the called radio, the list of features is endless. We can talk you through the details if you like the sound of a more private radio system.

Working with the telephone system: DTMF

DTMF provides the tones you hear when you press a button on most telephones. It is the telephone equivalent of 5-tone signalling. In some organisations radios are required to work alongside and with the telephone system. It is quite simple to integrate a phone patch to a radio system, so that office workers are able to make internal telephone calls to workers who carry radios, and the workers can call the internal phones.

Most radios can receive DTMF to some extent and some can make pre-programmed calls, but only if you have a full numeric keypad can you really take full advantage of this system.

Tel:
01223 426155