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Denmark State Police Radio System Case Study (1965-1967)

The extent to which vital security forces throughout the world are placing confidence in the radiotelephone for essential communication links is demonstrated by the Denmark State Police nation-wide radiotelephone scheme installed by Pye Telecommunications Ltd starting in 1965. This scheme, which puts every one of 1000 mobile units under radio control from its base no matter where it may be operating in the country, constitutes one of the largest export orders ever placed for Police radio equipment.


TX-RX rack  
Transmitter-receiver rack

Denmark has 72 local Police centres each with its own fleet of radio-controlled vehicles. These local centres are grouped in five regions, each directed by a regional control.

The Pye VHF radio scheme provides communication, (a) between mobiles and their local centres, (b) between mobiles and regional controls and (c) between local centres. In most cases, the regional control also incorporates the local radio station for that district.


Regional Coverage

Each regional control exercises radio coverage over an area of the order of 150 kilometres in diameter (sometimes considerably more). The map (to the right) shows how this is attained by carefully siting up to four transmitting/receiving stations within the region, all operated remotely over land lines from the regional control centre.

Typical control console  
Typical Denmark Police regional control console




A novel receiver ‘voting’ system prevents confusion which could result from speech signals being received at the control centre from two or more receivers at the same time. The voting circuits select and pass only the best signal being received at any one time, completely muting the remainder.

Lamp indicators on the control console show which receivers are picking up the signal and also which of the receivers is being voted. In replying, the controller selects the transmitter which will best cover the area of the receiver concerned.



Nine channels are used in the system - seven of them two-frequency channels and the other are two single-frequency. Simplex operation (one way transmission at a time) is the only mode of operation provided. The channels are designated as follows:-

Channel Purpose
1Local channel - mobiles to their local centres
2As channel 1, operating in Copenhagen only
3Car-telephone service - Copenhagen only
4Car-telephone service - Copenhagen only
5Regional controls to mobiles
6Nation-wide mobile channel
7Reserved for special services
8Reserved for special services


The frequency range covered by the whole system is less than 1 MHz.

Copenhagen Operators console

All the equipment is of special design. The mobile units, each of them capable of operating anywhere in the country, are 8-channel FM ‘Cambridge’ equipments. They are all of the ‘Universal’ type (i.e. providing for remote control of the transmitter receiver unit). Channel spacing is 25kHz throughout. All mobile equipment is ruggedized and weatherproof, with full compatibility for use either in vehicles or on motor cycles. They will all operate from 6 or 12 volt DC supplies.

Fixed stations use 50 Watt FM transmitters and fully transistorised, highly sensitive receivers. All control stations, both local and regional, operate their radiotelephones over land-lines from console-type remote control desks at the control centre.

These are built up from individual modules using the customary Pye Telecom ‘building-brick’ method. The control functions selected at the control console send coded pulse trains along the line to the radio equipment. Here they operate Ledex electro-mechanical switching elements which adjust the controls accordingly.

Copenhagen Supervisors console

The Copenhagen centre is unique. It combines the functions of a large and sophisticated local centre for the city with those of a regional control for a very wide area - the largest region, in fact, in the country.

The very wide regional coverage from Copenhagen is effected from five transmitting/receiving sites at strategic points in the region. Four of these have receiver voting and one is in an area where voting facilities are not required.

Additional stations provide the local (city area) coverage; two transmitter sites, which between them cover all eight channels, with standby facilities, and four receiving sites each with a receiver for each channel. Thus four voted receivers cover each channel.

1/4 racks  
About one quarter of the Copenhagen equipment racks



All control functions at Copenhagen, both local and regional, are centered in a supervisor’s console.

The supervisor allocates the channels, at will, to five operators. A tone squelch system protects the network against interference by or with other systems (in neighbouring countries, for instance) using the same frequencies. The transmitted signal carries, in addition to the speech intelligence, a sub-audio modulation which is separated at the receiver and used to lift the squelch muting. In the absence of the sub-audio component the receiver remains muted.

The voting equipment includes cold-cathode switching and pulse generating equipment. Received signals give rise to trains of digital pulses, the repetition rates of which vary according to the quality of the signal. The better the signal the higher the pulse repetition rate. The ‘voting’ circuits select the receiver having the signal producing the highest pulse rate and present an indication of the receiver selected to the control operator.


System assembly and factory test

1. System assembly  
1. System assembly
  2. System assembly  
2. System assembly
  3. System assembly  
3. System assembly
4. System assembly  
4. System assembly
  5. System assembly  
5. System assembly
  6. System assembly  
6. System assembly

Sources: 1. Pye Telecom description of Denmark State Police System 1965, Issue 2.   2. Pye Telecom Historic Collection photographic archives