The main object in employing a number of aerial wires or antenna has been to obtain increased capacity wherein to store electrical energy to be radiated as electric waves, the vertical wires being virtually one plate of a condenser, the earth the other plate and the air the insulating medium. The effective ea pacity of closely adjacent parallel wires is not, however, proportional to the number of such wires, but is equal roughly, to the square root of the number of wires. The capacity of a vertical wire .1 inch in diameter and 100 feet in length is .0002 micro-farad (Fleming).
For ordinary service the grounded antenna has been found to operate satisfactorily, but for tuned circuits the direct earth connection possesses some disadvantages. Thus variations ;n the resistance of the earth connections have introduced difficulties in the way of maintain ing exact tuning or syntony. (Described sub sequently).
In shipboard practice the aerials A are frequently extended in a sort of cage ar rangement C from the heads of the fore- and as outlined in Fig. 9. The cage con sists of wires held apart by insulated hoops or spreaders. This cage arrangement is also used in land or shore stations. In this case the cage is about 20 feet in length and consists of six or eight wires. The upper end of the cage is attached in the usual way to the top of the mast or tower; a single wire suspended more or less obliquely connects the cage with the wireless apparatus.
On shipboard when the available masts are low the number of wires must be increased to obtain capacity in the radiating system A type of aerial much used on ships and at shore stations is that known as the T or L aerial. An L aerial is indicated in Fig. 10, in which between two masts six horizontal wires W are supported and from which 6 vertical wires W' held apart by wooden spreaders S are dropped to a point near the deck where they converge and after being cabled are led into an operating room C. In a T aerial the vertical wires are attached to the horizontal wires at X. In the L arrangement the horizontal wires, which pos sibly give a directive tendency to the aerial waves, are connected at the far end. The horizontal wires are supported by corrugated insulators R, about two feet in length, and are attached to the masthead M by suitable tackle T, which upholds the spar P 15 feet in lett and to which the insulators are fastened.
horizontal wires are two and five feet apart.
Condensers,— The condensers used in trans mitting circuits for installations up to one or two kilowatts are generally of the Leyden jar type. Such jars, 16 inches high by 5.25 inches
in diameter, have a capacity of .004 micro farad. For more powerful installations, large plate-glass condensers immersed in oil, con Mined in insulated tanks, have been used by Marconi, De Forest, Fessenden and others.
Generators of Electric Oscillations.—The need of greater radiating power in long dis tance wireless transmission than is obtainable from the ordinary induction coil, as well as the unreliability of the interrupters of such coils, when used with large currents, which soon wear out the contact points, has led to the adoption of special types of transformers. In stead of battery power also, alternating rent generators are employed as the source bf electromotive force for these transforq0s. The power of the generators may Yarn'' 1 kilowatt to 300 or more rate of alternations of the transformer depends ',on the frequency of the generator. The electro motive force in the primary circuit of the trans former may be from 50 to 200 volts, which is greatly increased at the secondary terminals of the transformer ,• in some cases to 20,000 or 50,000 volts. In the case of the New Bruns wick, N. J., transatlantic station a 60-cycle 24,000-volt current in the secondary circuit of the transformers is employed to charge the large glass condensers, which in (ilscharging produce the high frequency oscillations thrown upon the aerial wire.
In addition to the spark gap method of generating high frequency' oscillation in wire less telegraphy, as described in the foregoing, by means of induction coils, several other methods are now employed, namely, the Poul sen arc generator, the high frequency machine alternator and the three-electrode vacuum tube oscillator, to which reference will be found in article on (Wtavtr%s TELEPHONY (q.v.). Consult Anderson and Elliott, (Poulsen Arc' (Electric Work, 30 Aug. 1919) • lantic Radio Communications ((Proceedings,' American Institute of Electrical Engineers, 1 Oct. 1919). Marconi has also developed a suc cessful method of obtaining sustained oscilla tions by means of rapidly rotating metal discs which by suitable arrangements of capacities and inductances set up rapid charges in an oscillation circuit. For a description of this cscHlator consult (Transactions,' Royal Institu tion (London, March 1908) or Mayer, (Wireless Telegraphy and Telephony) (Part 2, page 9)..