ON WIRELESS ECHOES. 42» 



when listening to distant stations at night. But the individual echoes in that case 

 cannot be recognised by the ear. We require some other form of record in which 

 the effects of direct signal and echo can be separated. As many of you are aware, 

 the most recent application of electrical communication, namely, television, has 

 reached the stage which is sufficiently advanced for broadcasting experiments to 

 begin, and now there are seven half-hour periods of experimental transmission each 

 week conducted by the Baird Television Company through the B.B.C. stations. 



Some months ago it seemed to me of interest to inquire what kind of effect such 

 wireless echoes as we have been considering would have in a broadcast of television. 

 The question to be answered was : would the echoes be sufficiently distinct from the 

 main signal to cause ghost images ? 



To examine this point we must first consider how a television image is made up. 

 Let us consider first the transmitter. Here we find that a spot of light, which may 

 be called the television ' eye,' is made to traverse or look over the whole of the image 



in of a second. The '[eye ' is made to do this by allowing light to pass through the 



2" 



holes of a disc which is revolving uniformly. There are thirty holes in the disc, 

 which is made to rotate anti-clockwise. Moreover, each hole is succeeded by one 

 which is a little nearer to the centre of the disc than itself. Thus, the picture 

 is scanned from right to left and from bottom to top. Now if there are thirty holes 

 and if the speed of rotation is 12.5 per second, it requires only simple arithmetic to 



show that the spot of light runs up a strip in g ~^-^ milliseconds. 



Now at the receiving end there is a similar disc which by an ingenious device, due 

 to Mr. Baird, is made to revolve exactly in step with the disc at the transmitter. 

 Now when the television ' eye ' finds the particular part of the strip it is traversing light 

 or dark, an electrical impulse is sent out by -ndreless informing the receiver, as it were^ 

 whether such a particular point should be light or dark in the reproduced image. 



Now at night, when echoes are prevalent, such a light or dark message can be 

 received both along the ground and after reflection by the upper atmosphere. We 

 thus might expect two light or dark messages to be received, the echo image from 

 the Heaviside layer coming after the main one a fraction of a millisecond later. Since 

 the spot is travelling always up the picture we should expect such echo-signals to 

 cause images situated above the main one. 



Some time ago, through the medium of the Wireless World, I was able to ask 

 amateurs receiving television to look out for such echo-signals. And, appropriately 

 enough, the first set of observations I received were made by an amateur in Bristol. 

 Mr. W. B. Weber, who has been very successful in receiving the Baird transmissions. 

 He sent me a most interesting communication saying — 



1. That he had received echo images by night and not by day. 



2. That the echo image was always above the main one. 



3. That, so far as he could measure, the shift upwards was almost about f of a 

 whole strip. 



From these interesting observations it is possible to calculate the echo-time which 

 I make out to be about J millisecond, corresponding to a path difference of 108 km. 



As the distance between Brookman's Park and Bristol is 100 to 103 miles, the 

 result gives a height of 108 km. for the reflecting region, which is in satisfactory 

 agreement with the value obtained by other methods for this particular wave-length. 



So far we have been dealing with what may be called short-range echoes. But 

 echoes are often experienced in long-distance transmission, in which case transmission 

 straight along the ground no longer takes place. For example, let us consider the 

 case of transmission across the Atlantic. In this case, the first signal to arrive is that 

 due to waves which have been once reflected by the la3'er. But wave-tracks makmg 

 two or more journeys to the layer are also possible. The separation of these 

 signals, which is a matter of milliseconds, cannot, of course, be done by the ear, 

 but they can often be recognised in the transmission of still pictures, because they 

 give rise to multiple images. The Marconi Company, who have developed a very 

 efficient system of facsimile transmission, have experienced such multiple images, 

 and Mr. T. L. Eckersley, of the research staff of that company, has been able to 

 deduce some very interesting results from a study of these records. 



One very interesting fact is brought out by Mr. Eckersley's work on this subject, 

 and that is that the shorter the wave-length used the fewer the echoes. This is 



