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Metropolitan Police wireless experiments prior to 1930
On June the 9th 1921 the Metropolitan Police requested a permit to install a receiving station at New Scotland yard in order to be able to monitor the communication between an aeroplane and a ground station that was going to be established to monitor traffic for the 1921 Epsom Derby. The requirement was for a 75 foot single wire aerial mounted on the roof of New Scotland Yard. The receiver to be used was of a 1 valve regenerative design typical of the time.
The post office granted permission for the installation on the 10th of June 1921 on the proviso that the valve caused no interference to other stations and that if it was required to be used after the Epsom Derby was completed that further permission would need to be obtained or that confirmation that the equipment had been dismantled would be provided.
This was acknowledged on the 20th of June 1921 with a further comment that should the experiment prove successful continued use on reception would be needed and application for transmission would be made in due course.
During the Epsom Derby the Airship R33 Call sign G-FAAG was used to report traffic information by radio on a wavelength of 800mtrs (375Kcs) to a Marconi wireless car which transmitted back to the Airship on 900mtrs (333Kcs). The usefulness of wireless in traffic control was proven sufficiently to justify the Metropolitan Police to consider further experiments. On the 4th of July permission was given by the post office for continued use of the receiving station in anticipation of an application for a formal licence.
The Metropolitan Police initially wanted 900mtrs(333Kcs) and 1000mtrs(300Kcs) allocated to them because this was the wavelength used by the Paris Police and The Metropolitan police had already liaised with the Paris police regarding the use of radio in police work and it was thought given the results the Paris Police were having that these wavelengths would be suitable for London. But 900mtrs was not available and 1000mtrs had caused interference when used by amateurs to the in flight communications of aircraft from Croydon aerodrome.
The next step forward was on the 31st of August 1922 when a test was conducted between Marconi House London (2LO) and a Metropolitan Police, Crossley Tender (5CN) fitted out with a Marconi AD1 aircraft transmitter receiver. With 2LO transmitting on 260mtrs (1153kcs) and 5CN transmitting between 700 and 750mtrs (400 and 430kcs).
Fig2 Crossley tender
The Marconi AD1 was a transmitter set using two valves which could be combined with a receiver to make what was for the time a small transceiver set it was also known as a CAIRPHONE, It operated on telephony but could also be used for interrupted continuous wave. The method of modulation was choke control where the anode feed current of the oscillator or power valve is varied in response to the acoustic frequencies imposed on it by another valve and choke. The standard aeroplane fit used a propeller driven generator. which was replaced in the vehicle installation with a DC motor driven generator.
Transmissions from Marconi house were received well but transmissions from the Crossley tender were not received at Marconi House. Following this transmitting equipment was installed at New Scotland yard to operate on 320 mtrs (937Kcs) and in early 1923 further trials were carried out between New Scotland yard and the Crossley tender. However these trials were picked up by the Post Office who were not happy, because not only did New Scotland yard make an unauthorised move off their allotted wavelength due to problems with interference from French shipping and the Boulogne Coast station. They also used a made up call sign this being NSY, whilst figuratively appropriate, it was however a call sign allocated to the United States of America. The Metropolitan Police had, with their permit to transmit application to the Post office asked for NSY or CO (Commissioners Office) to be allocated, however for some reason while permission was given to transmit from New Scotland yard no call sign had been issued. However this matter was soon resolved and the call sign 6SY was allocated for New Scotland yard for a permanent wireless installation with strict instructions regarding staying on the allotted wavelength.
The trials that had taken place paved the way for further use of wireless for traffic control during the 1923 Epsom Derby. With 6SY at New Scotland yard, 5CN being a mobile Crossley Tender, 6MP being a fixed station at Epsom race course and the aircraft call sign being the registered letters of the aircraft G-EBBL. Permission was granted to temporarily use 265 mtrs (1131Kcs) and 730 mtrs (410kcs) due to the interference issues experienced on 320 mtrs (937kcs).
The frequency plan for the Epsom Derby
This worked well with only minor issues. Telephony was used for this event and there was a problem with 6MP receiving clear speech from 5CN when it was close to Epsom however the messages were received by 6SY and relayed to 6MP. Also present during this event to witness the use of Wireless was the Chief Constable of Lancashire County Police, a force later to become a leader in the development of wireless for use by Police.
With the successful use of wireless at the Epsom Derby the Metropolitan Police approached the Post Office and the Wireless Board of the Imperial Communications Committee to have 265mtrs(1131Kcs) and 730mtrs(410kcs) permanently allocated for use by police. A further request was made for the allocation of the call sign NSY or CO because it was felt that 6SY had too much sibilance. Call signs considered by the Post office to meet this request were 2CO which was already allocated, 5CO again already allocated. 6CO artificial antenna licence which could be allocated, 2SY which was already allocated and 5SY artificial licence which could be allocated.
Following an exchange of correspondence between the Metropolitan police and the Post Office it became clear that the call signs allocated were in fact experimental call signs and that the conditions of use were the same as those of other amateur experimenters and that they operated on a secondary basis to fully licensed stations. As a result of this it was proposed that the police use of wireless no longer be considered experimental that they be fully licensed and suitable call signs issued from the internationally allocated Gxx series. the following call signs were offered GSY, GSZ, GPA, GPB, GPC, GPN, GCO. With GSY selected for New Scotland yard and GCN for the Crossley tender.
For the next Epsom Derby in 1924 another Crossley tender was fitted for wireless and call signs GPA for the tender and GPC for the Epsom fixed station allocated. These two new stations were only licensed for the Epsom Derby week but due to the original Crossley tender GCN not being available on occasions, the Metropolitan police started to use the GPA vehicle as a back up and then as an additional unit to GCN, these vehicles, being used to good effect by the Flying squad AKA The Sweeney.
In 1926 proposals to change some of the BBC transmissions to 260 mtrs(1153Kcs) and the increased interference on 265mtrs(1131Kcs) from foreign stations prompted the Metropolitan police to look to experiment on shorter wavelengths of 150mtrs(2000kcs) and between 20 and 50 mtrs(6000Kcs and 14000Kcs). Experiments were authorised to take place on 40mtrs(7500Kcs) between New Scotland yard and the assistant police engineers house in Sydenham and one of the radio operators houses in Croydon, these proved to be satisfactory and the Metropolitan police was then given temporary authorisation to use 40 mtrs(7500Kcs). The use of 40mtrs(7500Kcs) was for a transmitter at New Scotland yard and for a number of receivers to be fitted to Lea Francis motorcars. These stations were unusual for receiving only stations, as they were allocated call signs by the Post Office. GSZ1-6 with GSZ7 being a collective call sign for all the units and GSZ8 being allocated when a further car was procured a little later. Further flying squad vans were fitted with wireless and given the call signs GQC, GRP, QUF and GYV with a Flying Squad collective call sign of GXX. Most of the operating was by morse code with the operators having to be able to read at least 12wpm.
Following the Washington international telegraphic conference in 1927 international frequency allocations were decided upon and from 1st January 1929 they came into force and the UK reserved 145-150mtrs for Police use. With the Metropolitan Police allocated 2060Kcs but licensed to use all the channels if needed.
New police frequency plan January 1929
HO 341/1 Home office police call signs
Fig1 Post 33/661 from BT archives used under creative commons license
Fig2 Fig3 Fig4 Marconi Pamphlet Post 33/661 from BT archives used under creative commons license
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D. J. MCKAY G1JWG 2016