TRANSOCEANIC RADIO TELEPHONE DEVELOPMENT 565 



times these antennas are actually poorer than are much less sharply- 

 directive systems. Such observations also indicate a wide variation in 

 the performance of antennas as regards selective fading, and the signal 

 distortion accompanying it. 



The result of all this work has been the development of a system 

 based on an entirely new approach to the problem of sharp directivity 

 and of telephone receiving. This system is called a MUSA System, 

 the word MUSA being synthesized from the initial letters of the 

 descriptive words Multiple Unit Steerable Antenna. An outline of 

 the principles and methods is given below. 



By sending short spurts or pulses of short-wave radiation from one 

 side of the Atlantic, and receiving on the other side, it has been observed 

 that each spurt may be received several times in quick succession. 

 But these echoes do not arrive like successive bullets from the same 

 gun, all following the same path. They come slanting down to the 

 receiver from different angles of elevation, these vertical angular 

 directions remaining comparatively stable. While the signal received 

 at each of the individual directions may be subject to fading, the fading 

 is somewhat slower and is not very selective as to frequency. The 

 signal component coming in at a low angle takes less time in its trip from 

 the transmitter than a high angle component. Evidently the low- 

 angle paths are shorter. All these facts fit in well, on the average, with 

 the ideal geometrical picture of waves bouncing back and forth between 

 the ionosphere and the ground and reaching the receiver as several 

 distinct components which started out at different angles, have been 

 reflected at different angles, and have suffered different numbers of 

 bounces. 



The ordinary directive antenna is blunt enough in its vertical receiv- 

 ing characteristic to receive all or nearly all of these signal components 

 at once. Because of their different times of transit the various com- 

 ponents do not mix well but clash and interfere with one another at the 

 receiver. This shows up as the selective fading and distortion which 

 characterize short-wave reception much of the time. The MUSA 

 method remedies this trouble. 



The MUSA provides extremely sharp directivity in the vertical plane. 

 By its use a vertical angular component can be selected individually. 

 It consists of a number of rhombic antennas stretched out in a line 

 toward the transmitter and connected by individual coaxial lines to the 

 receiving apparatus. The apparatus is adjustable so that the vertical 

 angle of reception can be aimed or "steered " to select any desired com- 

 ponent as a telescope is elevated to pick out a star. The antennas re- 

 main mechanically fixed. The steering is done electrically with phase 



