KNOW YOUR AERIAL

Radio Record, Volume I, Issue 26, 13 January 1928, Page 13

KNOW YOUR AERIAL

CAPACITY AND WAVELIINGTH. _ ‘ It is very interesting, wiien carrying out experiments with various sets and circuits, to know the capacity of your aerial to ground (writes Sydney P. O’Rouke, in the New York ‘Radio News’’). ‘here are various coniplicated formulas by which the capacity and inductance may be calculated fairly accurately; but these involve accurate measurements of the length and height of the aerial which are not very practicable for the average experimenter. A much simpler method, and one which gives very accurate results, if conducted properly, is to tune in your local station, with the aerial and ground connected in the usual way and the aerial tuning condenser in parallel with the coil. for instance, supposing WGY is tuned in at 80 degrees; now disconnect the aerial and again tune in the same station. It should appear now at about 120 degrees; in the ’phones, of

course, There is, therefore, a difference of 90 degrees between the twa readings,’ whicli in the case of a .0005-inf. condenser, corresponds to .00025-mf. "Thus .00025-mf, is the capacity of the aerial to ground. It is essential, of course, to use an old-type variable condenser with semi-circular plates, as this is the only type which gives a straight-line-capacity reading from O degrees to 180 degrees. Most experimenters, however, will | have hidden away in their junk box one of these ancient variables, which may be connected up temporarily for the above calculations. It is a big point in short-wave work to keep the aerial capacity as low as possible, and the above method may serve as a measurement of any aerial-ground improvements that may be effected. ‘The usual method of determining the natural wavelength of the aerial system is by the well-known formula: Wavelength equals 1885 times the

square root of the microfarads multiplied by the microhenries. Another method is described below which, in the writer’s opinion, is very much more practical. It should appeal to the non-mathematical reader, since it involves no algebraical calculations or formulas. Disconnect the aerial from the set and tune in your local station with the ground connected to its usual terminal. You should receive it at fairly good strength with an ordinarily efficient set. Take note of the acrial-tuning condenser’s dial reading. Let us suppose in a particular case that this is 130 degrees. Now connect the acrial lead and tune in any station you can, nearest to 180 degrees on this dial. Supposirig in the first instance WGY is received; now, when you have connected up the aerial, WEAI tunes in at 130 degrees or thereabouts. Subtracting WGY’s official wavelength from that of WISAI, we have, 491.5 less 879.5 equals 112.0 meters. 112.0 meters then is the natural wavelength of the aerial in question. Finally, a word about the receiving set. In order to find the wavelengt! accurately it is absolutely necessary to have a method of regeneration which, when varied, will produce no change in the wave to which the set is tuned; since less regeneration will be required when the aerial is disconnected. receiver of the Hartley or Reinartz type is most suitable.