The best way of giving an idea of the present state of large scale wireless engineering is to describe some typical modern sta tions. The Rugby station is designed for simultaneous trans mission to all parts of the Empire and to ships on any sea, and for telephony to America. Its antenna is 82o feet high and carries a current of 700 amperes at a frequency of 16.7 kilocycles (18,000 metres). It occupies a site about II- miles long by i mile wide 4 miles south-east of Rugby. The antenna is supported on 12 insulated stayed steel masts 82o feet high, a quarter of a mile apart, and has a capacity of 0.045 microfarad, but can be divided so that when the portion on 8 masts is used the capacity is 0.033 microfarad. The primary source of oscillations is a small tuning fork maintained in vibration by a triode valve by the method of Eccles and Jordan ; the nine-fold harmonic of the fork current is selected, filtered and amplified by three stages of low voltage triode valves, until about roo watts of high fre quency current is obtained. This current is amplified by a bank of high voltage valves of 2 kilowatts output, then by a bank of 3o kilowatts output, and finally by a bank of 54o kilowatts out put, whence the current passes to a closed circuit which is coupled to the antenna. All stages are separated by metal screens to prevent retroaction that might lead to the generation of unwanted oscillations. The last three stages are fed with a direct current high-voltage dynamo set capable of delivering up to 1,500 kilo watts at 18,00o volts. About so kilowatts of direct current power is consumed in heating the filaments.
All the power for the station is taken from a public electricity supply at 12,000 volts so cycles. The intermediate circuit be tween the final bank of valves and the antenna consists of two condensers in series, values Fos and 0.25 microfarad, and an inductance coil variable from 400 to 600 microhenries, the last being coupled to an aerial coil of 4o microhenries which has a series tuning-coil variable from 90o to 4,000 microhenries. The antenna is a continuous conductor strung across the tops of the masts, passing from each mast to the next in a flat festoon; this conductor is a cylindrical cage of eight 7/14 S.W.G. silicon bronze wires stiffened by spiders 12 feet in diameter every 140 feet. The tension is such that the pull at the top of any mast never exceeds io tons ; if this is exceeded the steel rope holding the insulators and the antenna is paid out automatically by a slipping brake until the tension is io tons. The insulators will withstand a pull of 20 tons and a high frequency voltage of about a quarter million volts. The earth system is a broad band of buried copper wires running round the site under the masts and the aerial. The transmitting key is operated at the Central Telegraph Office in London. For details a paper by E. H. Shaughnessy should be consulted.
As an example of a short-wave beam station we take the Post Office station at Bodmin. This has one reflecting antenna directed to Canada, the other to South Africa. A similar station at Grimsby transmits to India and Australia. The receiving station corre sponding to Bodmin is at Bridgewater, that corresponding to Grimsby is at Skegness. The transmission to Canada is on 16.57 metres by day and 32.4 metres by night; to South Africa on 16.15 and 34.01 metres respectively. The primary source of oscillations is a back coupled master oscillator of the Arco and Meissner (Telefunken) type carefully screened by heavy metal casing. The high frequency current from this (about 8o watts) is amplified by a triode valve taking 2,000 volts, and this in turn by two more valves taking 6,000 volts, and finally by two ten kilowatt water-cooled valves operated at half their rated voltage of is poo. The high frequency current is then taken along tubular
feeders to the antenna to be supplied.
Two complete outfits like the above are needed for Canada and two for South Africa in order to supply the four wave-lengths. The Canada antenna is supported on five masts, the wires in two bays being suitable for the day wave-length and those in the other two bays for the night wave-length. The masts are steel struc tures 287 feet high with cross arms 90 feet long. The antenna and reflector wires hang vertically from steel rope triatics joining the cross arms. In the Canada aerial there are 24 antenna wires 19 feet apart and 48 reflector wires, the distance between the antenna plane and the reflector plane being 4o feet for the shorter wave and 24 feet for the longer. The reflector wires are divided in 8 insulated portions and the antenna wires are loaded with two spaced inductances. Power for the whole station is derived from three 92 kilowatt dynamos, which supply direct current for driv ing motor-alternators and auxiliary machinery. The direct current for the anodes is obtained by rectifying the transformed current from the alternators. The receiving station at Bridgewater has an antenna system very like that at Bodmin, namely a line of masts broadside to Canada, a line broadside to South Africa, each line comprising two bays for the 16 metre wave and two bays for the 32 metre wave. Actually the wave-lengths received are, from Canada 16.50 and 32.13 metres, from South Africa 16.08 and 33.71 metres. It will be seen that the most original feature of the station is the unidirectional antenna due to C. S. Franklin who overcame many difficulties in carrying a great enter prise to a successful issue.
The Radio Corporation of America has installed numerous short-wave transmitters for supplementing the transoceanic work of their long-wave high power stations. Most of those in com mercial use are operating without reflectors and can therefore communicate in any direction. The first two stations were erected in 1923, and five others, with wave-lengths ranging from 95 to 43 metres, in the spring of 1924. Several others, using various wave lengths down to 14 metres followed in 1925. The Californian station may be taken as typical of modern practice. At this station the primary source of oscillations is a quartz crystal main tained in vibration by a triode valve by the same principle as is the fork at Rugby, but the frequency of mechanical vibration of the crystal is nearly 800,000 per second while that of the fork is less than 2,000. From the current in the crystal circuit is selected the triple harmonic, and this is amplified to 30o watts. The current is now passed in turn through two amplifiers and selectors of increasing size, the triple frequency being selected at each step. The total multiplication of frequency is 27 and the final frequency therefore 21 millions per second (14 metres wave length). Finally this current is amplified again by water cooled triode valves for delivery to the antenna. The antenna is a vertical or sloping wire about 20 feet long excited through a few turns at its middle which are placed in inductive relation with the tuned circuit of the last amplifier. Even so small an an tenna can radiate in kilowatts at this frequency.