SHORT WAVES AND SEA WAVES

New Zealand Listener, Volume 32, Issue 821, 22 April 1955, Page 22

SHORT WAVES AND SEA WAVES

T is called the Doppler shift -a simple if rather odd name which won’t mean anything to many people. Yet it is the Doppler shift which tells the traffic officer whether you are driving at more than 30 m.p.h. through a built-up area and tells the radio engineer the variations in the distance between here and Hawaii. The Doppler shift can give us interesting information which may be of little scientific value, like the speed at which Tyson bowls (a little over 87 m.p.h.), or it can make a major contribution to a science, like the study of ocean movement which tries to find the relation between wind and waves. It can measure the muzzle velocity of a shell or help find the safest anchorage for a ship. And much of the credit for the development and extension of the technique of speed measurement based on the Doppler shift goes to New Zealand scientists working in the Dominion Physical Laboratory at Gracefield, Lower Hutt. The value of this method of measuring the speed of objects, often small objects moving over short distances, is that instead of measuring the speed indirectly by first measuring the distance travelled and the time taken, the

Doppler shift is directly dependent on the speed. We can take the set-up used recently for measuring the speed of Tyson and Statham as typical. A radio aerial was set up behind the wickets where the keeper usually stands. This: radiated a beam of energy of a constant frequency along the pitch towards the bowlet, who had to send the ball down the beam. Some of the energy radiated from the aerial struck the moving ball and was reflected back to the point of origin. On the return journey, its frequency was the original number of cycles a second, plus an increment depending on the speed of the ball towards. the wicket. The original frequency was then subtracted from the return frequency and the resulting figure was a measure of the ball’s speed. All this can be made clearer by an example. Suppose the original beam was radiated at 10,000 mes. (that is, 10,000,000,000 cycles a second), and the returning beam at 10,000,002,000 cs. Then the difference is 2000 cs.,. which at this frequency is associated with a speed of 70 m.p.h.-at which speed the ball must have been travelling. One of the scientists at Gracefield concerned with the development of the method, mentioned that it would probably be taken over and used by other Scientists; for instance, he said, it could

possibly be used for measuring the speed of birds. We referred this to Dr. Falla, Director of the Dominion Museum, who was interested in the possibilities of the technique fg orni-

thology. Attempts have been made to measure birds’ speeds by pacing them with cars and planes, or by checking the time they took to pass two points-for instance, to pass across the face of the

moon-but the Doppler shift method would certainly provide more accurate results. The Doppler shift is also the basis of a method, evolved at the Gracefield Laboratories, for measuring the roughness of the surface of the sea, and this without anyone so much as looking at a strawberry box, This depends on the fact that a radio beam of certain wavelength being the distance from crest to crest-will be reflected back to its origin only by a sea-wave of exactly half the length of that radio wave. By constructing a transmitter in which the transmitting frequency (which determines the wave-length) can be continuously increased, the length and the velocity of waves for a considerable distance out to sea can be calculated. "This technique is of considerable help in thé general problem of how energy is transmitted from wind to sea," said J. W. Brodie, of the Oceanographic Institute. "Far instance, when you have two conflicting storms in the same area, and in any storm you usually have that to some degree, you will get complex wave-formations which conventional measuring devices cannot adequately cope with. The Doppler technique would be very useful here." All this has im- portant practical results in that the study of wave genesis and structure is important in selecting anchorage sites, building breakwaters, designing ships, and so on. A third problem which the Doppler shift is helping to solve is concerned with radio. Encasing the earth like a shell is the ionosphere, which loses or gains height according to the strength of the sun. This ionosphere will reflect an upward directed beam of high frequency back to the ground, thence upwards again, so that the beam moves across the earth’s surface in hops. A beam of known frequency, transmitted throughout most of the day from a station in Hawaii, is recorded and measured in the Gracefield laboratories. The frequency with which it is received here is usually different from that at which it is transmitted, due to its striking the moving ionosphere which acts just as the cricket ball did. Graphs

are then drawn showing the variations throughout the day and night in the received frequencies, and from these further graphs are compiled, this time showing the variations of the daily variations. From this latter data much information can be gathered. about the ionosphere and related phenomena. For instance, it will enabie scientists to calculate the number of hops a shortwave beam takes between here and Hawaii, a figure which varies according to the altitude of the ionosphere. As a distance is always relative to the means of measuring it, it is quite legitimate to say that radio distance from here to Hawaii varies throughout the day. Of these techniques, the one for measuring the speed of small objectsand, for that matter, big objects like cars-hag been considerably — simplified and made more efficient at the GracefieldLaboratory; that for ionospheric work has been improved and the work on sea-waves ‘has largely been originated by. the scientists there.