Aerials for Short-Wave

Radio Record, Volume IV, Issue 23, 19 December 1930, Page 17

Aerials for Short-Wave

Their Height; Length and Type

(CORRESPONDENTS often discuss the pros and cons of short-wave aerials, and the following article from a contemporary is made timely by a querist this week. . Many readers will no doubt have discovered for themselves that: almost any sort of aerial seems to be effective, from a very high one to a’ length of wire hung: around the room, and ‘that the great care one takes in the ef‘ficient erection and insulation of the aerial, apparently, is not altogether justified by the results. Much depends on the particular locality and other circumstances in which the aerial is erected, and the seeming incongruity between a carefully installed aerial system and a rough and ready one is in reality a compromise between one, or more detrimental characteristics. ‘The aerial employed by the writer consists of a flat-top of about 80 feet between two poles thirty odd feet high, which, with the lead and ground lead (both of which ‘come into the calculation), make an effective length of about 65 feet. Several other ‘acquaintances use a length of wire strung round the picture rail, and find that, although the signal strength drops a little, the noise level is very much,

less and the results apparently average out fairly well. Because of the high frequency of the lower wavelengths, efficient insulation especially of the free end is imperative.. This is obtained by means of a string of five or six insulators, or one having a long leakage path such as a strip of plate glass. Because of this alternatittg current nature of the high frequency currents, cireulating in the aerial, an insulator merely acts as a small capacity and its "insulating" properties depend on.the reactance (or a.c, resistance) of the

capacity formed therein. The reactance for. a given frequency will be higher (and the "insulation" therefore better) the lower we can make the effective capacity. ‘ The effective capacity of several in series will always be less than the capacity of the smallest of them, so that by inserting a string of insulators in series with one another we can reduce the effective capacity of the whole to a low value and thus increase the resistance or insulation property to a high value. A number of insulators in series also increases the leakage path during moist weather and is another desirable feature. Where the lead-in passes close to the building it should also be well in-

sulated and the capacity to earth: (by which signal strength would be lost) kept very low. The earth connection should be short and well insulated, forming as it does part of the actual aerial system. If the earth lead is too long, the tuning coil, across which we need ag much of the aerial current as ‘can be obtained for building up voltage across the grid and filament circuit of the valve, will not be in a favourable position. In any aerial system in which one end is earthed, maximum. current will be found at the eatthed end and maximum. potential, with no current, at the free end. With this in mind, the current and voltage distribution for such an aerial system can be easily plotted for a given wavelength. The earth lead must be insulated for stability of tuning. If it is left uninsulated it may so happen'that during wet weather the receiver will be earthed at a point considerably shorter than the normal’ connection and will materially upset the tuning and general stability of the system, An Ideal Aerial. — "THE ideal aerial should be as high as possible, but height adds length. Length increases the capacity and inductance and reduces,.the number of turns which may be included in the aerial (or loading) coil across which we must devolop the valve operciting voltages. The capacity can be reduced by inserting a small capacity condenser in series with the.aerial, such as. isso commonly used for coupling in a short-wave receiver. By reducing the capacity of the aerial system it is possible to increase, within limits, the inductance, or for a given aerial length for any particular wavelength, the number of turns on the aerial coil.