ULTRA-SHORT-WAVE TRANSMISSION 511 



disturbance zone is effectively moved nearer the transmitter and the 

 probability of increase in fading amplitude is enhanced. The usual 

 increase of fading with decrease in wave-length is thus explained. 

 When the wave-length difference is small, on the other hand, the fading 

 type should be much the same on both wave-lengths, as was generally 

 found. The lack of coincidence would arise from the fact that the path 

 difference being a considerable number of wave-lengths, a small wave- 

 length change can introduce a marked randomness in fading without 

 appreciably affecting the type. 



As has been mentioned earlier, a multiple of reflecting boundaries is 

 the normal condition, rather than that of a single boundary. This 

 circumstance, without invalidating the explanations already given, 

 makes a further elaboration of the theory possible. The "roller" 

 type or component of fading, in particular, requires explanation. In 

 addition to the smooth signal modulation, from which the name has 

 been derived, this type of fading is characterized by showing more or 

 less frequent deep minima or drop-outs and these are often distinctively 

 twinned. Further, the roller component is that component of fading 

 which shows coincidence, in spite of wave-length or polarization dif- 

 ferences. Such coincidence indicates small path difference and this 

 is what we have when a double boundary or stratum exists. Such a 

 stratum would give two "A" components and, if of variable thickness, 

 would, as it was carried along by the prevailing air currents, give the 

 steep, deep, minima at phase opposition thickness. Further, if the 

 stratum contour were that of a hump, thick enough to carry the second 

 "A" component past phase opposition to the first one, the twinned 

 minima would result as the hump entered and left the reflection zone. 

 Occasionally the two "A " components would add properly, with the 

 residual "B" component, to give complete extinction, a result less likely 

 from the phase addition of a single "^" and the "5" component. 



This explanation of "roller" fading assumes, tacitly, that the "B" 

 component is, at the time, relatively subdued, that is, the disturbance 

 zone has moved inwards due to an increase in the reflection coefficient 

 of the layers or to a decreased "effective" earth radius. The fine 

 structure often appearing at the bottom of a prolonged roller minimum 

 corroborates this, the mutual cancellation of the two "A" components 

 having uncovered, so to speak, the weaker "B" component with its 

 much shorter traversed path. 



With the "roller" condition characteristic of high "A" component 

 signal amplitude, the "scintillation" condition would be characteristic 

 of low "A" component signal amplitude, the relatively steady "B" 



