This extract from an old Police Journal provides a history of experiments in two-way wireless communication by Liverpool City Police, which began during the month of May 1932. The Journal reported that, three years later, the force had established an efficient system of wireless communication between their main transmitting station and all motor patrols in the city. The article from the mid-thirties reflects the importance of this crucial breakthrough in Police communication; here's what it said…

To begin with, a transmitter, together with a three-valve receiver, was built and installed at the Headquarters of the Motor Patrol and Transport Department. The transmitter had a maximum input of 60 watts, and was capable of operating on continuous wave (CW) morse only. A crystal controlled portable transmitter, having a maximum input of 50 watts, also operating on morse only, and a receiver similar to that in the main station, were built and fitted in a car. By this time, it had been found that a number of members of the department were capable of passing the GPO. morse test, and these formed the nucleus of the Wireless Section.

Tests with this equipment showed that a reliable two-way range of about two miles only could be obtained. This poor reception was due to the fact that the main station was located in a low part of the city under a hill, and as the district was also industrial, containing many electrical plants, the interference was so great that the signals from the car were obliterated at distances over the two miles. With the object of finding a location suitable for the main transmitter, which would be fairly free from interference, portable transmitters and receivers were installed in two patrol cars, and tests were made at various points in the City by using one car as a fixed station. Finally a site was chosen at a divisional station, approximately in the geographical centre of the city area, and standing at 150 feet above sea-level.

The main transmitter was rebuilt to be capable of an input of 100 watts, a four-valve receiver for the station was made, and the whole equipment transferred to the new location.

Tests showed a considerable increase in the two-way range. Four-valve receivers were fitted to the cars in place of the existing three-valve receivers, and two-way working could then be obtained over the greater part of the city.

A test was then arranged under actual working condition by broadcasting, to three cars patrolling different districts, the description of a car which was touring the city. The test proved highly successful and the car was stopped by all three patrol cars in a comparatively short period.

Attention was then turned to the force's motorcycle combinations. Owing to the limited space available for an aerial, it was found that a five-valve receiver was required to give reception comparable with that obtained by the cars. Four such receivers were built and fitted to combinations, and regular working was commenced.

After a short period of regular working, it was found that the use of continuous wave morse was not ideal for the purpose. The main transmitter was then adapted so that a modulated morse note could be emitted, and this helped to overcome a considerable amount of interference.

To overcome problems associated with the time taken to read morse messages speech transmission from the main station was then introduced, and was an immediate success. The number of calls asking for messages to be repeated was very few, and the time required to broadcast a message was shortened considerably.

In order to ensure privacy for messages for police only, a simple code was evolved, covering all normal requirements. As an example of the working of the code, a message could be broadcast as follows:

This might appear formidable to the uninitiated, but is in fact simple. The message decoded reads:

A copy of the code is carried by each man on motor patrol duty, and it is only a matter of a few seconds to decode any message. Anything not covered by the code can, of course, be transmitted in morse.

Tests of a commercial six-valve super-heterodyne portable receiver showed that its capabilities greatly exceeded those of the police built receivers, and in consequence four cars and five combinations were fitted with commercial sets.

Five cars are now fitted with transmitters, and two-way communication is maintained between the main station and the cars throughout the city. The only fading that is observed is at two spots in the centre of the city, covering about 25 yards each, and this peculiarity is probably due to local absorption.

Telescopic mast aerials (copper tube) are fitted to the sidecars of the motorcycle combinations and these have proved satisfactory for the receivers in the combinations. This type of aerial was tried on the patrol cars, but was found to be inefficient for transmitting when two-way communication was worked. A frame aerial, in box form, and fitted to the rear luggage carrier, was then tried as was also one in the roof of the car. The latter has proved most successful.

The maximum reliable two-way range of those cars (saloons) is approximately 10 miles in all directions, except to the east where it extends to about 15 miles, this district being fairly flat. With the same equipment in a touring car, carrying an aerial in the hood, these distances are nearly doubled.

The mobile receivers operate off the car or motor-cycle lighting batteries, while the mobile transmitters use two separate batteries, one 6 volt to light the valves, and one 12 volt to drive the generator, which delivers 600 volts to their anodes.

Two of the patrol cars are now fitted with a transmitter capable of speech transmission, in addition to morse, and is still in the nature of an experiment, but good quality speech can be received at the main station from these cars.

The main station is manned by trained operators from 7am to 11pm daily, and two-way working used. From 11pm to 7am the transmitter can be operated by remote control, on speech only, from the station Charge Room, and messages of importance can immediately be sent to the night car patrol. One-way working only is used during the latter period.

The advantages of two-way working are shown by the number of messages received from the cars, such as reports of traffic accidents; requests for an ambulance or patrol wagon; motor vehicles stolen or recovered, etc.

The latest type of patrol car added to the fleet is the 1 1⁄2 litre "Le Mans" Singer, with sports body specially adapted for carrying wireless equipment.

A miniature receiver has been developed, suitable for use on a pedal cycle. It uses two valves only, and these with an aerial and high and low tension batteries, are contained in a case measuring 6in by 4in by 3in. The weight is 3lb. The receiver is carried in the usual cycle bag, fitted inside the frame of the machine, and a socket is fitted to take a connection for the loud speaking earphone, which is clipped to the handlebars of the cycle.

To call the cyclist a note is transmitted from the wireless station for 30 seconds. The man riding the machine can hear it while in motion in any part of the city, and the note continuing for 30 seconds allows him, if necessary, to go off the main road into a quiet side street to receive the message.

A long distance test was made with this receiver in a car, using only the aerial inside the receiver, and good reception of speech was obtained up to 27 miles, and morse up to 31 miles. This was with an input of 100 watts to the main transmitter. An increase of power would naturally result in greater ranges being obtained; for instance at night time clear reception of speech transmitted from the Brighton Police wireless station can be obtained in this city without difficulty.

For more on Police wireless see Nottinghamshire Constabulary Personal Radio, Brighton Borough Police Radio and Oxford City Police Radio