POST OFFICE WIRELESS SERVICES

An extract from
The Post Office Electrical Engineers' Journal
Volume 19, Part 1 - April 1926

THE POST OFFICE WIRELESS SERVICES
 

The completion of the high-power station at Rugby, with its many novel features largely due to the resource and initiative of Post Office engineers, and its success as a telegraph and telephone transmitting station have received generous notice in the press of the day, and have called attention to the important and increasing wireless services conducted by the Post Office by means of its point-to-point and coast stations. A short account of the stations and the organisation by which these services are operated will be of interest to readers of the Journal.

The point-to-point stations are under the control of the Engineer-in-Chief for all purposes. The coast stations are controlled by the Inspector of Wireless Telegraphy, and his staff of Wireless Overseers and operators perform day-to-day maintenance work in addition to operating, although the major maintenance of these stations is in the hands of the Superintending Engineers of the Districts in which the stations are situated. The Engineering Department took over the point-to-point stations from the Inspector of Wireless Telegraphy on 1st April, 1925. This change was brought about by the rapid increase in the size, power and complexity of the stations intended for long distance working, with the consequent necessity for the employment of trained engineering staff. The general adoption of the land line control method of operating such stations, by which the staff of telegraphists is concentrated at the Central Office in London, also contributed to the change of policy, as the point-to-point stations have in fact become relay stations at which the retention of operating telegraphists is unnecessary.

The stations transferred to the Engineering Department under the new arrangements are as follows:-
Leafield, with Banbury Receiving Station, collectively known as "Oxford Radio."
Northolt.
Stonehaven.
Devizes.
Radio Relay Station, London.
Caister.

Cairo, the sister station of Leafield, remains under the Inspector of Wireless Telegraphy and is manned by his staff, including some engineering officers seconded for service at the station.

Leafield and Banbury ("Oxford Radio") form one group under the charge of an Executive Engineer, with the necessary staff of Shift Officers, arc attendants, turbine drivers, etc. Banbury is still staffed by overseers and operators seconded from the commercial side, who are mainly employed in the reception of inward Cairo traffic. The necessity for retaining Banbury as a receiving station arises from the unsuitability of the Radio Relay Station in London for the reception of long distance and difficult signals, owing to local interference from power plant in the neighbourhood. A new receiving station - for all services is in course of construction at St. Albans, and it is anticipated that this station will be able to relay Cairo's signals to the Central Radio Office, as is already done in the case of Continental Services.

Leafield and its complementary station, Cairo, were the first and only stations of the Imperial Wireless "Chain" to be completed. The necessity for the chain disappeared as the range of the high-power stations, of which Rugby is the latest example, increased, until it became world wide. They were originally designed in 1913 as 300 K.W. spark installations, with crystal reception, and it is interesting to speculate upon the probable measure of success which they would have attained under these conditions. The installations were commenced but the outbreak of war interrupted operations, and the subsequent development of other methods rendered it necessary to reconsider the design of the stations. In 1919 it was decided to proceed with the installation of 250 K.W. Elwell-Poulson Arc transmitters, and Leafield and Cairo were erected by the Post Office Engineering Department and completed in 192 l and 1922 respectively.

Leafield is situated on a plateau 600ft. above sea level, approximately 20 miles N.W. of Oxford. No power supply being available, a complete steam generating plant was installed. Two Babcock & Willcox water tube boilers, each capable of evaporating 10,000lbs. per hour and fitted with super-heaters, provide the steam required. The prime movers consist of two 250 kilowatt Allens de Laval type steam turbines, each coupled to two 500 volts D.C. generators in series, thus giving the woo volt D.C. supply required for the arc generators. Two 60 K.W. turbo generators, of the same type, supply the power necessary for auxiliary purposes at a pressure of 220 volts D.C.

The water supply for the Station is obtained from a well situated in the valley some two miles distant, at which a pumping station has been erected. A reservoir to contain 50,000 gallons was built at the top of the wireless site, and also a cooling pond, measuring 200 ft. x 200 ft., to hold 1,000,000 gallons of water for the condenser circulating system.

The arc generators are designed for an input of 250 K.W. and are each capable of delivering a current to the aerial of 250 amperes at a frequency of 24.3 kilocycles, corresponding to a wave-length of 12,350 metres. In order to eliminate harmonics and mush, emitted by the arcs, a coupled circuit has recently been installed. The four condensers used in the primary circuit, each of which have a capacity of .025u.f., are constructed of aluminium plates supported in steel tanks. The dielectric used is oil.

The keying is carried out by means of a Creed pneumatic key having eight pairs of contacts, which short-circuit coils coupled to an inductance fitted in the primary circuit, thus causing marking and spacing waves to be emitted, the difference in wave-length between the marking and spacing waves being 40 metres. The keys are capable of a speed of 80 words per minute.

The aerial, which is of the inverted L type, is supported by ten tubular steel masts, each 305 ft. in height. It is constructed with 7/19 phosphor bronze wire and is approximately 1000 yards long by 200 yards wide.

Leafield is utilised for commercial purposes to Cairo and Prague and for a press service to Halifax, Nova Scotia. The Halifax and Prague traffic is dealt with at about 40 words per minute as a rule, and the Cairo work usually at from 18 to 25 words per minute.

Until recently the British Official Press, the Long Distance Ships commercial service and the Marconi Company's and Radio Communication Company's press services to ships were transmitted by Leafield, but the desirability of a world wide range for these items has led to their being transferred to Rugby.

The day range of Leafield and Cairo is 2,500 miles, but the night range is very much greater, and during tests made last year Leafield was received at strength 8 (strong signals) in the China Seas.

Northolt Radio Station is situated l l miles W.N.W. of London and was put into commission in August, 1921, using two 25 K.W. Elwell Poulsen Arc transmitters. A 30 K.W. Valve transmitter was installed in 1925 and the two transmitters are now in simultaneous use daily. The power supply is obtained from the Uxbridge Power Supply Company at 6,000 volts, 3-phase, 50 cycles per second, and is transformed down to 415 volts for use on the Station.  Motor generators giving 650 volts and 110 volts D .C. are provided for the main and auxiliary power supplies.

The valve transmitting set is equipped with a valve rectifying unit consisting of six 2.5 K.W. Silica Valves. These are fed from a 3-phase transformer, three-phase full wave rectification being employed. This rectified supply is fed to the oscillating unit, which consists of 3 water-cooled 10 K.W. metal valves.

To ensure constant frequency and wave-length a system was developed and first used practically at Northolt in which a valve sustained tuning fork working at audible frequency provides the control for the main oscillators. A harmonic of this frequency is selected for amplification.

Keying is carried out by the simultaneous interruption of the grid circuits in the second stage of the tuning fork, and also in the grid circuits of intermediate valve and oscillating valves.

The key used for breaking the last two connections is a Creed pneumatic, double contact type. The break in the tuning fork stage is effected by means of a small auxiliary key.

The two 25 K.W. arc transmitters are designed for an input of 40 K.W., and are connected to the aerial through a coupled circuit in order to eliminate undesirable emissions.

Keying is carried out by means of a single contact Creed pneumatic key which short-circuits a small portion of the inductance in the primary circuit, thus emitting marking and spacing waves, the difference between these two wave-lengths being 80 metres.

The aerials are supported from three wooden lattice masts, each 446 t. high, arranged in the form of an equilateral triangle, and one 300ft. tubular steel mast.

The Northolt Valve transmitter is used mainly for European broadcast press services, but, in addition, it maintains commercial services with Buda Pest and Belgrade. The arc transmitter is allocated wholly to Italian traffic, working in conjunction with Milan. The press, which is received at a large number of stations on the Continent, is forwarded at hand speed but the commercial work is dealt with at speeds up to 50 words per minute.

Stonehaven was originally designed and completed in 1916 as a two transmitter station, with 30 K.W. and 5 K.W. spark installations, to serve the dual purpose of coast working with ships and for emergency communication with Aberdeen in the event of
a total breakdown of the land lines connecting that city with the South. The development of continuous wave transmission led to the addition of two 25 K.W. arc sets and later to the conversion of the 30 K.W. spark set into a valve transmitting installation of similar power. The latter task was undertaken by the Post Office Engineering Department after enquiries had shown that satisfactory tenders from outside firms were not forth coming for a valve installation of this size. A large amount of pioneer work was done in connection with this alteration, with a result which more than fulfilled expectations, as the aerial power was doubled as compared with the arc with a smaller expenditure of electrical energy.

A public supply not being· available, generating plant consisting of two 75 B.H.P. Gardner oil engines direct-coupled to 40 K.W. 220 volt, Crompton generators is installed, together with a storage battery of 120 cells of 1070 A.H. capacity. The high tension D.C. supply is derived from Crompton Motor Alternators, the output on the generator side being 30 K.W. at 1000 volts, 350 cycles. This supply is transformed, rectified and smoothed for delivery to the transmitting valves at 12,000 volts.  Rectification is effected by means of two sets of three two-electrode valves, V.T. No. 21, in parallel. The transmitting valves, V.T. No. 19, eight in number, are connected in parallel and are each Capable of dissipating 800 watts from the anode. Current for the valve filaments is obtained from 5 K.W. motor-alternators through a step-clown transformer, the supply being 16 amps. at 16.5 volts. Separate secondary windings are provided on the transformer for the rectifying and oscillating valves. The wave length is 4600 metres.

Keying is effected by means of a two-contact Creed pneumatic key, the method of keying being that of breaking the grid leak and transformer primary circuits simultaneously. An air blast is provided to prevent arcing at the contacts.
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The main services conducted via Stonehaven are those to Warsaw and Hapsal (Esthonia); certain Continental press services are also dealt with. The station is used during breakdowns of  cable and land lines for emergency services between London and Aberdeen and to Berlin and Iceland. Although the Aberdeen requirement was one of the reasons for the selection of Stonehaven as a site it should be noted that this function has seldom been brought into operation.

The continental traffic is handled at speeds up to 60 words per minute and the station is a very efficient one, in spite of the serious drawback of its being controlled by a land line nearly 500 miles in length with three repeaters in circuit, and containing long sections of over ground line.

Devizes Relay Station, which is controlled from the Burnham Receiving Station, deals with C.W. ship traffic and has a guaranteed range of 1500 miles. It is situated 4.5 miles from Devizes and in the absence of a local power supply is equipped with prime movers. Its equipment consists of two 30 H.P. Robey Engines driving 15 K.W. D.C. generators, which are used for operating the transmitters and charging the 450 A.H. battery.  The transmitting equipment consists of two 6 K.W. valve transmitters working on 2100 and 2400 metres respectively. The transmitters, including their running machines, are remote controlled from Burnham, two transmissions being carried on simultaneously.

The first transmitter was installed in 1920 by the Marconi Company and the second was designed and installed by the Department in 1925. The second transmitter is separately excited by means of the Tuning Fork Control System and contains one novel feature. Owing to the relatively high frequency of transmission a double multiplication of frequency is made in the Tuning Fork Unit. The wave generated at the fork frequency (2000 cycles) is distorted to enlarge the harmonics and the 9th harmonic is picked out, filtered and amplified. This 18,000 cycle wave is then distorted and, by selecting and amplifying the 7th or 8th harmonic, frequencies of 126,000 and 144,000 cycles are obtainable, corresponding to wave-lengths of 2380 and 2080 metres respectively.

The aerial systems are suspended from three 300 ft. masts arranged in a straight line and under working conditions currents up to 15 amperes are obtained on each set.

Caister is a 1.5 K.W. Valve Station, which is retained for emergency communication with Holland during cable break downs.

Rugby and the short wave Beam Stations now in course of erection will be dealt with in future articles. The Beam Station at Bodmin, Cornwall, with its receiving· station at Bridgwater, Somerset, is intended for communication with South Africa and Canada. A second group of stations, Grimsby and Skegness (receiving), are to be utilised for working to India and Australia.

Similar stations are being erected in the Dominions named. The guaranteed periods of effective transmission per day by these stations, of comparatively low power, is Canada 18 hours, South Africa l l hours, India 12 hours and Australia 7 hours.

The Coast Stations were taken over from the Marconi Co. and Lloyds Corporation in 1909: the stations taken over were: Caister, North Foreland, Niton, Lizard, Seaforth, Holyhead, Malin Head and Crookhaven. The Cullercoats Station was taken over from the Poulsen Company in 1912. All these stations were remodelled or replaced by other stations, and equipped with rotary disc discharge transmitting plant in the years 1913-14. After the war, stations at Wick, Grimsby and Port Patrick were taken over from the Admiralty. The stations at present comprising the coast stations are :-Wick, Cullercoats, Grimsby, North Foreland, Niton, Lands End, Fishguard, Seaforth, Port Patrick, Malin Head, and Valencia: the two Irish stations, Malin Head and Valencia, although still under the control of the British Post Office, are manned and maintained by the Irish Free State on behalf of the British Government.

The Coast Stations have been further improved from time to time to bring them into line with the latest practice: in 1920 valve receivers replaced crystal receivers at all stations. The power of the coast stations varies from 1.5 to 5 Kilo-watts.

Direction finding equipment has been installed in the Niton and Cullercoats stations for giving bearings to ships. This service is of great utility and facilities are to be extended by making similar provision at other coast stations, either in their present positions, or on fresh sites more favourable to direction finding requirements.

The Lands End station has been equipped with direction finding equipment, but this is used only to assist ordinary reception in the face of jamming: the site is not suitable for a direction finding service to ships.

The near future will see the complete remodelling of the Coast Stations: valve transmitters are to be installed and these will be arranged for tonic train transmission, the equivalent of spark transmission with valve equipment. This provision will minimise interference to other services, inherent to the spark system, and will provide immediately for continuous wave transmission if and when it is adopted generally for ship working. The trend of development is that spark working shall be ultimately eliminated.

Experimental short wave (non directive) l.5 K.W. stations have been erected at Dollis Hill, the Post Office Experimental Station near London, and at Leafield. During the past nine months, Dollis Hill has dealt with traffic for Cairo and Halifax, Nova Scotia, from late afternoon onwards into the night very efficiently. The Leafield short-wave set has just been completed and also promises well. It is the intention to install short-wave transmitters at Rugby and Devizes in the near future. These sets will be used in substitution for or in addition to the main transmitters when conditions are favourable for short-wave transmission.

The Central Radio Office and the Radio Relay Station are dealt with in another article in this issue. Both these offices are in the Central Telegraph Office building in London. The whole of the wireless point-to-point traffic is dealt with in these offices, with the exception of inward Cairo traffic as previously mentioned, outward by land line control of the various stations, and inward by reception in the R.R.S. and relaying to the C.R.O.