August 12, 1922J 



NA TURE 



posed are within or near to the limits of audibility. 

 The possible gamut of wave-lengths is becoming very 

 fully occupied, and although the development, during 

 the last four years, of nearly pure continuous-wave 

 transmitters, and of receivers with vastly improved 

 selective powers has eased the problem, the time will 

 soon arrive when the only way of increasing the number 

 of possible services will be by employing systems 

 having good directional characteristics. 



There are, broadly, two general classes of directional 

 aerial systems ; (a) Those having the general character- 

 istic that their directional power or polar curves are 

 nearly independent of their dimensions. The directional 

 result is obtained by opposing the effects of a number 

 of aerials, or parts of an aerial with suitable phasing 

 adjustments, the degree of opposition being a function 

 of the direction. Systems of this class may be made 

 small compared with the wave-length employed ; for 

 the purposes of position finding, and as receiving 

 systems enabling interference to be eliminated from 

 several directions, they have already been developed 

 to a considerable degree. The simplest example of 

 this class is the well-known frame aerial, (b) Those 

 having the general characteristic that their directional 

 power or polar curves depend on their dimensions 

 relative to the wave-length employed. In this class 

 the directional result is obtained by adding the effect 

 of a number of aerials, or parts of an aerial, when 

 working in the required direction. The underlying 

 principle is that the effects, for the required direction, 

 are integrated over a wide front in proportion to the 

 wave-length. Such systems can, therefore, have small 

 dimensions only when using short waves, and this 

 fact makes their development difficult. 



As examples of such systems may be mentioned — 



(1) Reflector systems in general. 



(2) Systems composed of lines of aerials, at right 



angles to the working direction, correctly ad- 

 justed as regards phase. 



(3) The Beverage long, horizontal receiving aerials. 

 The reflector system was the first tried for wireless 



telegraphy. The use of reflectors of reasonable 

 dimensions, however, implies very short waves of the 

 order of a few metres, and the very high attenuation 

 of such waves over land or sea, and the difficulty of 

 getting much power into them, tended to make early 

 attempts very discouraging. 



The investigation was commenced by Senatore 

 Marconi in Italy in 1916, with the idea of' developing 

 the use of very short waves, combined with reflectors, 

 for certain war purposes. 



The waves used were 2 metres and 3 metres. The 

 only interference experienced with such waves is from 

 motor boats and motor cars, for these machines ap- 

 parently emit waves from near o up to about 40 metres 

 in length. A coupled-circuit spark transmitter was 

 developed, the primary having an air condenser and 

 spark in compressed air. By this means a moderate 

 amount of energy was obtained, and the small spark- 

 gap in compressed air proved to have very low resistance. 

 The decrement of the waves emitted was judged to 

 be of the order of 0-03. The receiver used was a 

 carefully picked crystal, while the reflectors employed 

 were made of a number of strips or wires tuned to the 



NO. 2754, VOL. I IO] 



wave, arranged on a cylindrical parabola with the aerial 

 at the focus. The transmitting system was arranged 

 so that it could be revolved and the effects studied 

 at the receiver. 



Reflectors having apertures up to 3 -J wave-lengths 

 were tested, and the measured polar curves agreed 

 very well indeed with the theoretical curves. The use 

 of two reflectors with apertures of 3J wave-lengths, 

 one at the transmitter and one at the receiver, increased 

 the working range about 3 times. 



These Italian experiments showed that good direc- 

 tional working could be obtained with reflectors 

 properly proportioned with respect to the wave-length. 

 The attenuation over sea for the wave-length used was 

 found to be very high, and with the apparatus available 

 the maximum range obtained was 6 miles. 



The experiments were continued at Carnarvon in 

 1917. With an improved compressed-air spark trans- 

 mitter, a 3-metre wave and a reflector having an 

 aperture of 2 wave-lengths, and a height of 1-5 wave- 

 lengths, a range of over 20 miles was obtained to a 

 receiver without a receiving reflector. The experiments 

 at Carnarvon brought into prominence a property of 

 wave propagation which is not generally known, and 

 the extent of which is not realised, namely, the very 

 rapid increase in the strength of the electric field with 

 height above the ground. The rate of increase appears 

 to be a function of the height divided by wave-length, 

 and while not very noticeable with waves of several 

 hundred metres, is very marked with waves of a few- 

 metres' length. 



It was found that the limiting range at sea level 

 and over sea was 4 miles. When both transmitter 

 and receiver are at a low level the range is very de- 

 pendent on the nature of the intervening country, and 

 is very restricted even over sea ; when, however, both 

 stations are many wave-lengths above the intervening 

 country its nature is of far less importance, and the 

 range is increased many times. These experiments 

 showed that very considerable ranges were possible 

 with very short waves. 



In 1919 experiments were commenced at Carnarvon 

 with valve transmitters, with the idea of producing 

 a directional telephone system. A wave of 15 metres 

 was selected, which while well within the capacity of 

 the power valves available, allowed a simple reflector 

 to be used without too large a structure. After some 

 trials a single valve transmitter was arrived at taking 

 about 200 watts with a 15-metre wave, and giving 1 

 ampere in the centre of a half-wave aerial. A hetero- 

 dyne receiver with supersonic beat-note was employed. 

 Finally, very strong speech was obtained at Holyhead, 

 20 miles away. The strength was such that shadows 

 produced by small hills and buildings were scarcely 

 noticeable unless the stations were close behind 

 them. 



The next point was to test the maximum range, and 

 particularly to find whether such waves would carry 

 over the horizon, and whether there would then be a 

 rapid falling off of strength. Tests were carried out 

 with the Dublin Steam Packet Company's boats running 

 from Kingston to Dublin in June 1920, and speech 

 was received in Kingstown Harbour, 70 nautical miles 

 from Carnarvon, and the point was proved that there 



