A MULTIPLE UNIT STEERABLE ANTENNA 383 



must be capable of adjustment to meet the various transmission 

 conditions. 



Quality Improvement with the MUSA 

 The distortion of speech and musical quality which characterizes 

 short-wave circuits is due entirely to the interference of differently 

 delayed waves each of which individually is fundamentally free from 

 all kinds of distortion except non-selective fading. This conclusion is 

 almost self-evident and is corroborated by the results of several years 

 of pulse investigation ^ made in cooperation with the British Post 

 Office. 



A MUSA system can be expected to select one out of several multiple 

 reflections. However, these reflections suffer more or less scattering 

 with the result that they appear as bundles of waves of various degrees 

 of compactness. These bundles possess a small spread of both angle 

 and delay. The delay interval included in a bundle of waves is rarely 

 less than 100 microseconds. Double refraction or " magnetoionic 

 splitting" occurring in the ionosphere doubtless accounts for the 

 existence of a small minimum delay. A delay interval of fifty micro- 

 seconds or so may be detected even in the unusually compact bundles 

 represented by the pulses of Fig. 21. Transmission from Halifax 

 appears to include a delay interval of this order, also. With trans- 

 atlantic propagation it is not uncommon to have a bundle containing 

 numerous weaker components extending over several hundred micro- 

 seconds. On rare occasions these have extended over two milli- 

 seconds, masking any multiple reflections which may have been 

 present. 



The quality associated with one MUSA branch which selects one out 

 of several bundles of waves is thus not perfect. The effect of a delay 

 interval of a few hundred microseconds is scarcely noticeable, however, 

 except during deep carrier fades. Therefore, if diversity action 

 between two branches steered at the low and high angle parts of the 

 same bundle is employed, deep fades are avoided to a large extent, 

 and the quality is almost perfect. When more than one wave bundle 

 is present diversity action between branches steered at the principal 

 bundles accomplishes this escape from deep fades. It is desirable to 

 utilize all of the principal bundles in diversity in order to preserve the 

 discrimination of the MUSA. For, one of the bundles, if not provided 

 with a branch to receive it, would cross talk into the other branches 

 when it momentarily became strong and those provided with branch 

 receivers became weak. Signal-to-noise ratio considerations discussed 

 in Section V constitute an equally important (and related) reason for 

 utilizing all principal bundles. 



