Article

Pulling Answers Out of the Air

June 1953

Pulling Answers Out of the Air June 1953

Article

June 1953

lonosphere research at Thayer School produces a solution to a problem in long-distance radio

As an outcome of bouncing controlled radio waves off the ionosphere 150 miles or more overhead, in a research project that has been going on at Dartmouth for the past four and a half years, a research team at the Thayer School of Engineering has found a way to cancel out some of the echoes that have long been a problem in radio communication.

This answer, promising practical benefits to long-distance communications and radio, has been provided by Dr. Millett G. Morgan, Director of Research and Assistant Professor of Electrical Engineering at the Thayer School. His present coworkers in the research project are Blanchard Pratt and W. Cutting Johnson '33, both members of the Thayer School staff. Willis N. Rayton, Professor of Physics, is also participating in this ionospheric research that has been financed for three and a half years by the Office of Naval Re-BELOW, search and for the last year by the U. S. Bureau of Ships.

Most of the research work directed by Professor Morgan centers in the field station maintained on a hill top in Etna, five miles from the campus. As shown in the photograph on the opposite page, this consists of a 100-foot tower, from which high-frequency pulse signals are sent vertically into the sky, and a large trailer laboratory filled with equipment for controlling the transmitting of radio waves and recording those reflected or echoed back from the ionosphere. The signals are transmitted and reflected "at vertical incidence," meaning straight up and down, and the speed at which they travel is indicated by the fact that they rise to a height of 150 miles or more and return to the earth in about one and a half thousandths of a second.

The ionosphere, a region of the earth's atmosphere that starts about sixty miles up, is variable and a nuisance in some ways, but it is indispensable in longdistance radio, because if it did not act as a gigantic reflector, radio waves would just keep going straight out into space. The ionosphere reflects the waves and enables them to return to the earth's surface. But it is like a mirror with a wavy surface —in fact, like two wavy mirrors, because it sends back two principal reflections or descending sky waves, and may reflect even more. The big problem at the receiving end is that these "echoes" often overlap, creating quite a jumble.

If a sender transmits a "dot" in international Morse code, it may be received as a series of dots thousands of miles away, depending on how the ionosphere is reflecting at the time. Trouble begins when the next signal, perhaps a "dash," arrives at the receiving set at the same time or before the last "dot" echo does. Then the message is mixed up and unreadable at the receiving end.

Transmitters can easily send code faster than the echoes arrive, so they are literally slowed down to a pace dictated by the atmospheric conditions. Only by sending slowly enough can the characters be separated at the distant point. This is often a speed that is frustratingly slow.

A method that would screen out the extra echoes would eliminate this problem, as well as the problem of most radio fading, which results from mixtures of these extra echoes. This is what Professor Morgan's work has accomplished in initial form.

The Dartmouth researchers have found that by manipulating the characteristics of the transmitted radio signal they can cancel out echoes. "Echoes of any given kind can always be completely cancelled by adjusting the polarization of the transmitted signal through the use of two antennas and 'matching it' to the ionosphere," Professor Morgan explains. "Our problem was to learn whether the two descending sky waves produced by the earth's magnetic field when a radio wave is bounced off the ionosphere were in any way coupled to each other. If they were coupled, then sending up a signal that matched one of them would still produce the other echo, but if they were not, then we could eliminate the second wave. It was found that the two waves were independent and that a polarization adjustment would eliminate the echo."

But having learned this much, Professor Morgan also found that he had to keep adjusting the polarization of the wave because the surface of the ionospheric mirror changes rapidly. "For complete elimination of an echo," he says, "the position and axis ratio of the transmitted polarization ellipse must be adjusted with extreme care. Under the most stable conditions, cancellation can be maintained for many minutes without readjustment, but the conditions most often encountered require readjustment every half minute or so."

More work remains to be done, but the results so far point the way to practical advances in long-distance radio communication. One interesting prospect has to do with automatic sending and receiving. Automatic transmitters and receivers can operate, in the relatively static-free region of high-frequency radio, at almost unlimited speed, except for the limitations imposed by the ionosphere. Dots and dashes that represent letters can be punched on tapes and fed at high speed into an automatic transmitter so that they emerge on the air as a series of signals so rapid the human ear cannot decipher them. At the distant point, however, an automatic receiver can store the signals on another tape, as fast as they are sent. Then the message can be automatically produced on an electric typewriter at a speed far higher than one operated by hand.

Professor Morgan is conducting his research on "home grounds" in a double sense. The field station is located on his own property in Etna, and he is a native of Hanover, the son of Dr. Frank M. Morgan, for many years headmaster of Clark School. He was graduated from Cornell with honors in 1937 and took his Ph.D. at Stanford in 1946. The following year he was named Assistant Professor sor of Electrical Engineering at the Thayer School. His association with Dartmouth's engineering school actually began in 1941 but he immediately went on leave to fill a series of important research assignments during the war. After a period of teaching radar to naval officers at M.I.T., he served for two years as design engineer with the Submarine Signal Company in Boston, and then in 1944 went to the California Institute, of Technology as research engineer. In 1946 he joined the staff of the U. S. Navy Electronics Research Laboratory at the University of California, working there for one year before returning to Dartmouth.

Professor Morgan is one of the country's leading researchers in the field of ionospheric radio propagation, and is becoming increasingly busy in international activities in this field. For the Institute of Radio Engineers, he is chairman of the ionospheric subcommittee of the Committee on Wave Propagation. Last summer he was a delegate of the U. S. National Research Council to the general assembly of the International Scientific Radio Union (URSI) at Sydney, Australia, where he served as reporter for the U. S. Commission of URSI on lonospheric Radio Propagation. Recently he was appointed secretary of that Commission and chairman of a committee within the Commission to nominate the U. S. delegates and prepare the agenda to be recommended to URSI for the next international assembly in Amsterdam in 1954. In April, at the National Bureau of Standards in Washington, the results of his ionospheric research at Dartmouth constituted one of the leading reports made before the joint meeting of the U. S. National Committee of URSI and the Institute of Radio Engineers.

Professor Morgan's investigations have uncovered some discrepancies with the classic magneto-ionic theory, and in order to get continuous 24-hour data for the further study of these Professor Rayton is constructing an automatic polarization recorder to be used at the field station. The "vertical incidence" research will be carried forward- this summer, but along with it Professor Morgan and his associates will expand their investigations to include radio waves transmitted and reflected in an oblique path.

In connection with the latter, the Dartmouth researchers will be working in cooperation with the Canadian Defence Research Board. They will make echo studies in the transmission of radio signals through the auroral zone to Fort Chimo. And since Hanover is on an extension of the line from Saskatoon to Ottawa, a path over which the Canadian Defence Research Board is conducting investigations, Professor Morgan will also direct a study of radio signals received at Hanover from Saskatoon. Assisting in these studies will be Pratt, Johnson, Professor Ray ton, and probably one other faculty member.

Prof. Millett G. Morgan (left), director of research at Thayer School, who heads the ionosphere project, shown at the field station with Cutting Johnson '33, one of his two chief assistants. Johnson spends long, and sometimes strange, hours at the station operating the transmitter and keeping records on the reflected waves.

BELOW, left, is a close-up view of the large van parked at the base of the transmitting tower. Right, an interior view of the mobile laboratory shows some of the equipment that fills it. Morgan is at the receiver while Johnson adjusts a camera used in making a photographic record of ionospheric echo patterns. A half-hour 16 mm. film has also been made for showing at scientific meetings.

The field station for ionosphere research sits atop a hill five miles from the Dartmouth campus. The 100-foot tower, which offers a fine view of Moosilauke, supports two delta antennas aligned north-south and east-west. The antennas will be shifted 45 degrees this summer to see if results are consistent. To set up the field station, Morgan and his co-workers, with the design help of Thayer's civil engineers, built the tower footings themselves; and also constructed a road to the top of the hill. Hanover's biggest snow tractor pulled the van up the hill. The station solved one of its problems with the help of the local electric company which ran in special power lines. Compared with the first years, all is now smooth sailing, even in the middle of winter.

Blanchard Pratt (right), research associate at Thayer School, shown with Professor Morgan, is responsible for the designing and building of equipment needed in the ionosphere research project. The transmitter exciter in the picture is a good example of the complicated and ingenious equipment he creates. Simpler and better models are constantly sought, the transmitter exciter above having been designed to replace the version below right. Left, below, is one of the cathode-raytube indicators, with its power supply, also built by Pratt for use at the field station.

Readings on the three caihode-ray-tube indicators in the van show (right) polarization of the transmitted pulse, (left) polarization of the echo, with the gap in the ellipse indicating north to west direction, and (center) a radar-type pattern showing at left the transmiited pulse and at right two echoes. The vertical markers on the middle indicator each represent 100 kilometers of height. Polarization adjustment will reduce the middle pattern to a single echo.