RADIO WAVES AND RADAR

Their Postwar Possibilities Excite the Imagination

ASSISTANT PROFESSOR OF PHYSICS

ON THE NIGHT OF NOVEMBER 14, 1942, off Guadalcanal, there lay a Japanese battleship. It was a stormy night. Eight miles away was a ship of our own fleet. With the use of RADAR our ship, with its second salvo, sank the Jap battleship, in the blackness of the night, eight miles away."

Spectacular achievements such as this, quoted from a recent statement by James F. Byrnes, have served to make "Radar" and "Electronics" magic words of today. Indeed, one might be led to believe from reading the popular press that society and our private lives in the future will be largely controlled by electronic devices. This of course is hardly true; yet the basic theory and experimental work which is behind these latest physical developments are of vital importance to Dartmouth students, and will become increasingly so in their study of science after the war.

Radar, which is the key to such battle effectiveness as that described above, makes use of very short radio waves. We refer to these as micro-radio waves, and in order to get an appropriate picture of what is meant by that expression it might be well for us to go to sea. When one is on a ship, and observes the white caps moving along in the ocean, one is seeing the crests of water waves, and ocean waves are measured by the distance between one crest and the next. This length is called one wave-length. Transfer this thought to radio waves. These are electric waves which have white caps or crests similar to water waves. And so when we speak of the wavelength of a radio wave, we speak of the distance between one electric white cap and the next. Micro-radio waves, under which radar can be classified, include those electric waves having wave-lengths from a few feet to a few inches. These are the shortest radio waves which can be produced today. Because they are so short, the apparatus needed to produce them is very different from ordinary radio apparatus. The size of the apparatus is roughly the same size as the waves themselves. Special apparatus for producing, detecting and measuring very short waves has been constructed at the College and is partly shown in the illustrations for this article.

Many are startled by the newness and the tremendous possibilities of these very short radio waves, but few realize that waves of this length were the first radio waves ever discovered. In 1888 the great German physicist Heinrich Hertz pro- duced electric waves from sparks which had wave-lengths of about 12 inches. Had he invented radar then, Germany might have won several wars in the meantime! Hertz was able to detect these waves, measure their length, and transmit them across the large attic of the Physical Institute at the University of Bonn. These experiments by Hertz culminated the great theoretical work of the British physicist James Clerk Maxwell, who predicted the existence of electric waves in 1866, twentytwo years before Hertz.

From that time on, little was done with the practical development of radio till Marconi went to very long waves which enabled him to produce trans-Atlantic radio telegraphy. It might be said that Marconi took a step backward, so far as wave-length is concerned, in order to make the great step forward in radio telegraphy. Long radio waves were necessary at that time in order to produce waves of sufficient energy to travel great distances, a thing which was not possible with Hertz' original apparatus.

Since then physicists have been struggling for many years to get back to the short waves of Hertz. However, in returning to the "wave-length days" of Hertz, we have returned with a tremendous power which Hertz never dreamed of. That power is the modern radio electronic tube. For it was the discovery of the electron by Sir J. J. Thomson in 1897 which made the radio tube possible. Again this is a case of an experimental physicist with no notion of any practical application of his work making a discovery which in the end led to devices of tremendous value to society.

The development of the electron tube was carried out not by one but by many people, based on Thomson's discovery and the later fundamental work of Sir Owen Richardson. These scientists included Fleming in England, DeForest in this country and, last but not least, the great teams in the industrial physics laboratories of the United States. Needless to say, the development of the electron tube and of electronics is still going forward by leaps and bounds, stimulated by the tremendous war effort in which physicists are now engaged.

It is the electron tube rather than Hertz' sparks which today is used to produce micro-radio waves. That is why we are able to do things and to develop devices such as radar which Hertz and Maxwell never envisioned.

Two of the tubes which are used to produce micro-radio waves are shown in Figure 1. In many respects they are very similar to the tubes in one's home radio. However, the reader will note several distinct differences, which are discussed in the caption.

To produce these waves, not only a tube is required, but also a transmitter using, this tube. A receiver is used to detect them. Such apparatus for transmitting and receiving radio waves 20 inches in length is shown in Figure 2. A close examination of the photograph will show a small box on top of which a Western Electric 316 A tube is mounted. This tube has attached to it two rods extending to the right. These rods constitute the tuning element of the transmitter. They essentially take the place of the tuning circuit in one's home radio which one adjusts by turning a knob to pick up different stations. However, in the case of 20-inch waves we accomplish the tuning by changing the length of the rods instead of by turning a knob. Near the end of these rods and extending to the right along the length of the picture are two copper wires attached to a wooden frame. This is a wave meter and is used to measure the wave-length of the radio waves. In the background, near the transmitter is a vertical pair of wires which are bent out horizontally at the top. This is the transmitting antenna for 20-inch waves. At the far right, the receiving antenna of similar structure is shown. Both these horizontal antennas are about 10 inches long—half the length of the wave. Compare these in length with the usual home radio antenna and the reader has an idea of the relative length of broadcast waves and micro-radio waves. The receiving antenna is provided with a crystal for detecting the 20-inch waves. Again, this is a return to old days of radio and "crystal" sets; for in the region of micro-radio waves the crystal detector is usually found to be more adequate than radio tubes.

The fact that we have these short radio waves running along wires and being radiated from wire antennas (although these antennas may be very short) appears to be quite conventional. However, when we shorten the waves to say 8 inches, it becomes inconvenient to use wires. Consequently, we resort to pipes for propagating the shorter radio waves. And as we go to even shorter waves of 4 inches, pipes become still more satisfactory for conducting them.

Transmitters for 8-inch and 4-inch waves (shown in Figure 3) consist to all appearances largely of pipe. However, inside the pipes are Western Electric 368 A tubes which produce these waves. The "8-inch" transmitter in the background is made from 5-inch stove pipe, to the end of which is attached a horn. This horn is the antenna for 8-inch waves, strange as it may seem. It acts as a megaphone for these radio waves. The "4-inch" transmitter is the small bit of pipe in the foreground. Plumbing is becoming these days an important branch of physics! For it is possible to run these waves through stove or drain pipe around bends and-elbows with some of the plumbing fixtures that can be obtained in Hanover hardware stores.

Two types of "8-inch" receivers are also pictured in Figure 3. One of these is in some respects the exact converse of the "8-inch" transmitter, in that it consists of a horn for an antenna, and a piece of stove pipe—the stove pipe in this case containing a piston and a crystal detector. The receiver is tuned by moving the piston back and forth. This makes the receiver in essence a trombone for radio waves, although some musicians might object to the analogy. In front of the "trombone receiver" is the more conventional wire antenna provided with a crystal detector and attached to a wooden stick. The total length of the antenna is about 4 inches, again half the wave-length. The "trombone receiver" can be much more sharply tuned than the wire antenna.

With this apparatus (which some have referred to as a plumber's nightmare!) one can show many of the properties of microradio waves and the principles upon which radar operates. For RADAR—"RAdio Detecting And Ranging"—is merely the application of micro-wave principles and techniques to the problem of locating objects.

One of the first properties of these very short radio waves which can be demonstrated with the apparatus is what might be termed "the searchlight principle": namely, that these radio waves travel in straight lines like a beam from a searchlight, and will not bend around corners as will ordinary longer "broadcast" waves. For example, if the "8-inch" transmitter is put on one side of a fairly large sheet of copper and the receiver on the other, none of the waves get around the edges of the copper sheet. However, if the receiver is placed alongside of the transmitter, the waves can be reflected from the copper sheet back into the receiver, in the same way that a mirror reflects a beam of light.

One other fact of importance is that the horn antenna can be made highly directive for these very short waves. In particular the horn of the "8-inch" transmitter is directive, just as a megaphone is directive for a person's voice. Consequently it is possible to obtain a "radio beam" much like a searchlight beam, which can be swung about at the will of the operator.

It is true that a "radio beam" can be produced for longer radio waves, even in the broadcast range, but this would require apparatus enormous in its physical size, which could not be swung about easily as can that for very short waves.

Another important property is the fact that radio waves in general will penetrate fog, and clouds, and mist, while a searchlight beam will not. Furthermore radio waves travel with the speed of light, 186,000 miles per second, which means that the time interval between a transmitted and received signal is extremely small.

Now what benefit can society expect to derive from these very short waves? Perhaps their primary significance to us is that they have opened up an entirely new field in communications which will bring people closer together than ever before. Radio telephony, and point-to-point communication or what might be termed "personalized walkie-talkie service" is now possible by the use of these waves.

For instance, after the war, it should be possible for a person in his automobile or sail boat or helicopter to phone his wife at home and tell her that he is again going to be late for dinner. This conversation could be accomplished by having a beamed antenna on top of his car and a beamed receiver in his home. Of course it cannot be as simple as pictured, for evidently some means would have to be provided for lining up the antennas on the beam.

The other great advantage of these waves is that. many conversations can be sent on the same beam, because of the greater band-width which these very short waves have in comparison with broadcast waves. It is conceivable that several thousand conversations could be sent on the same beam at one time. Consequently, our trans-continental telephony might be achieved by placing towers, say, every 50 miles across the country (60 towers in all) with beamed antennas on top. The radio beam would then go from the first tower to the next, and would be repeated from this to the third, and so on, completely across the country. This would eliminate some of the difficulties encountered with wire communication.

In all probability it might also prove to be less expensive to the public because of the large number of conversations which can be sent simultaneously on one beam compared to the few which can be sent on wires. This particular type of communication is generally known as point-to-point communication. Of course, eavesdropping is possible, but so is tapping of wires, and besides, there are ways of providing secrecy.

Recently, the Federal Communication Commission assigned new bands to be available in the future. It seems significant that the wave-lengths suggested extend not only in the ordinary broadcast band, through frequency modulation and television wave-lengths, but also extend down into the micro-radio wave region as far as waves about 1/2 inch long. This would indicate that much can be expected from the region of very short waves in the future of communications, public and private.

Of course the application which has been so striking during the war has been that of radar. Although much of radar is still secret, nevertheless the principles upon which radar must be based are old.

Before the war, the Bell Telephone Laboratories developed the absolute altimeter for measuring the height of an airplane above the ground. This consists of a very short wave radio transmitter and receiver located in the airplane as indicated in Figure 4. A beam is sent out from the transmitting antenna, which upon striking the ground is reflected back to the receiving antenna in the airplane. The time which it takes the radio beam to go to the ground and return is measured by means of appropriate apparatus, and since radio waves travel 186,000 miles per second, the height of the airplane above the ground can be calculated.

Evidently if this altimeter were placed on the ground and an airplane flew overhead and intercepted the beam from the transmitter, the plane would reflect some of the beam back to the altimeter receiver on the ground. In this case, the "altimeter" has found the height or the range of the airplane, and one has, in essence, a radar.

The altimeter is known to operate satisfactorily through rain, fog, and clouds because of the ability of radio waves to penetrate these elements.

It seems that devices of this type will be installed on every ship at sea, and probably on every tug and ferry boat in New York harbor after the war. They should prevent collisions in fog and accidents with icebergs, and permit a ship to sail safely into any harbor in the world irrespective of the weather. Similarly airplanes should be able to fly over mountain ranges in storms without mishap—for their position above the mountains will be known at all times with a radio locator. By its use, airplanes should be able to effect "blind landings" at any airport in bad weather day or night, and should be able to avoid collisions with other aircraft.

It has been said that the greatest service which radar has rendered in this war was in the Battle of Britain when the Royal Air Force knocked out the Luftwaffe. The British were well equipped with radar stations which located the German bombers as soon as they took off from the airfields of France, and mapped their positions. This enabled the Royal Air Force to be at the proper place at the proper time when the German bombers arrived. The history from this point on is well known—it was the turning point of the war.

"Since then radar has stood guard at many danger points along United Nations frontiers and at sea, warning of the coming of aerial and sea-borne enemy forces, and contributing toward victory in combat. The new science has played a vital part in helping first to stem and then to turn the tide of Axis conquest." This I quote from an official Army-Navy statement.

From this, and the tremendous peacetime possibilities of micro-radio waves, it is self-evident that these waves have come to stay. It is one of the new scientific developments in which Dartmouth students will be interested.

THE AUTHOR of this article, Prof. Gordon Ferrie Hull Jr. '33, shown in his office in Wilder Hall, where he has ingeniously set up the micro-wave apparatus partially described and illustrated here.

FIGURE 1. Two electronic tubes which are used to produce micro-radio waves, the Western Electric 316 A tube (left) and the Western Electric 368 A tube (right). The prongs in these tubes come directly through the glass envelope instead of being mounted on a base as in ordinary radio tubes. Another difference between these and ordinary tubes is that the spacing between the pieces of metal which form the tube elements is very much less.

UNKNOWN ARTIST'S CONCEPTION of a physicist engaged in micro-radio wave work. (Courtesy Bell Telephone Laboratories, Inc.)

FIGURE 2. A micro-radio wave transmitting and receiving system for 20-inch waves. The transmitter and its antenna are on the left, the receiving antenna is on the right, and a wave meter is in the foreground.

FIGURE 3. Shown in the photograph above to the right are micro-radio wave transmitters for 8-inch and 4-inch waves. The "8-inch" transmitter in the back is equipped with a horn antenna. The "4-inch" transmitter in front of it is made of three-inch brass tubing and can be seen by comparison to be quite a b.t smaller Shown in the left-hand photograph are two receivers for 8-inch waves. One consists of two short wires mounted at the end of a stick, while the other is made of stove pipe with a horn for the antenna. The diagrams below illustrate the arrangement of the Western Electric 368 A tube in the 8-inch and 4-inch transmitters (right) and the arrangement of the piston and crystal detector in the "8-inch" receiver (left).

FIGURE 4. Diagram illustrating the operation of the Western Electric absolute altimeter which is used for finding the height of an airplane above the ground.

Gordon Ferrie Hull Jr., son of Dartmouth's noted Appleton Professor of Physics Emeritus, joined the College faculty last November, after serving for seven years as a member of the technical staff of the Bell Telephone Laboratories. He took his A.M. at Dartmouth the year following graduation and his Ph.D. in 1937 at Yale, where he was Assistant in Physics during three years of graduate work. He has done a great deal of research in the field of short radio waves, some of it of a secret nature for the government, and in recent months he has appeared at a number of New England colleges lecturing on the subject covered only briefly, and non-technically, in this article.