Radio Propagation Research
Radio is a general term that refers to the radiation and detection of signals propagated through space as electromagnetic waves to convey information. It encompasses wireless or radio telegraphy, voice radio, microwave relay systems, radio and television broadcasting, radar, radio aids to navigation and satellite communications. The range of areas related to radio is a clear indication of its importance to research undertaken at CRC. Radio studies are at the root of CRC research, just as present day CRC grew out of the Radio Physics Laboratory.
All electromagnetic waves travel at about the speed of light, but their other characteristics are variable. For example, amplitude is the magnitude of the disturbance of a wave; frequency is the number of complete waves propagated past a fixed point in one second (measured in cycles/second or hertz); and wavelength is the distance between one wave and the suceeding or preceding one. The higher the freqency of the wave, the shorter its wavelength will be. At CRC research has been undertaken throughout the radio spectrum. This includes very low and low frequencies (VLF/LF), medium frequencies (MF), high frequencies (HF), very high frequecies and ultra high frequencies (VHF/UHF), as well as the microwave spectrum.
How radio waves travel from a transmitter to a receiver depends upon their frequency. Although radio frequency waves tend to travel in straight lines from the point of generation, certain frequencies are affected by the earth's atmosphere in ways that can be used to expand the uses of radio. For example, very low frequency radio waves generally follow a path that is close to the earth's surface and which follows its curve. These waves, with frequencies below 30 kilohertz, can be received reliably at distances of up to thousands of miles. However, very low frequency radio waves are generally limited to telegraph-type signals, since their information-carrying capacity is limited.
Long distance voice transmission can be achieved with higher frequency sky waves which can be reflected to extend the range of transmission. The ionosphere consists of layers of electromagnetic particles in the earth's atmosphere and is known to reflect radio waves. Sky waves are sent up into space from the transmitter and reflected by the ionosphere back down to the receiving station. The character of the ionosphere varies with the time of day, the season, geographical location and the activity of the sun to such a degree that this type of transmission is not always dependable. Prior to the development of satellites, high frequency radio was used extensively in Canada and elsewhere for various forms of radio communication.
Very high frequency and ultra high frequency waves travel in straight lines. These are used for line-of-sight transmission and reception, where the construction of very tall antennas or a series of relay antennas enables the transmitter and receiver to "see" one another. The transmission of direct waves is not dependent on the unpredictable state of the ionosphere. Because their transmission distance is limited, these direct wave systems tend to be used for local transmissions. These systems are also used over long distances such as in the case of the telephone relay networks which consist of a series of repeater or relay towers that maintain line-of-sight contact across the country. However, there is a major disadvantage to this system related to the cost of building and maintaining the relay stations required to cover any great distance.
Radio propagation research focuses on the interaction between radio waves and the physical media in which they travel - the troposphere and ionosphere - which affect radio waves in a variety of waves. They can absorb, scatter, bend, reflect, depolarize or polarize, retard, rotate or offer several paths to radio waves. These possibilities must be considered in the design of communications systems, as well as in the development of spectrum policy and spectrum management.
Radio communications, or "wireless", was known and was used extensively early in the twentieth century without much definitive knowledge about the propagation of the signals. In Canada a modest beginning to propagation research was made by the National Research Council during the 1930s, but wartime requirements for communications over circuits not previously established and to mobile units at sea and in the air resulted in a substantial expansion and acceleration of that work. At the request of the Royal Canadian Navy, and to support its anti-submarine campaign, an extensive study of the conditions affecting the transmission of radio waves was initiated. An ionospheric sounder was installed at Chelsea, Quebec, in 1941 by F.T. Davies to measure the critical frequency and the height of the reflection of radio waves from the ionosphere. Because the reflective properties change with time of day, season and sunspot cycle, up-to-date measurements were of great value for estimating the best frequencies for medium and high frequency communications. Such information was also of advantage in determining some of the details of transmission from distant hostile stations. Of particular importance was the need to assess and, if possible, forecast intervals of ionospheric disturbances during which communications were disrupted.
The value of this work was quickly recognized. in 1943 additional ionosondes were built and installed at Churchill, Manitoba, and Clyde River, N.W.T. Similar installations were subsequently made at Rupert, B.C., St. John's Nfld., and Portage la Prairie, Man. by 1945 all three military services as well as the NRC and the Department of Transport were involved in the operations, while the overall supervision of the work was done by the Canadian Radio Wave Propagation Committee (CRWPC) which was set up in 1944 under the Navy to coordinate the research of the three services in radio wave studies.
When hostilities terminated in 1945, the importance of propagation research for peacetime communications was recognized, but its close ties to operational needs made a long-term future within NRC unlikely. A small unit, essentially an interservice staff under the CRWPC, known as the Radio Propagation Laboratory (RPL), and housed in Naval Headquarters in Ottawa, continues an active interest in the field, while the DOT took over the operation of the ionosonde stations. In 1947, RPL moved to a building on the Prescott Highway just outside of Ottawa and, following the formation of the Defence Research Board (DRB), became an integral part of the organization. This was an important step in stabilizing RPL's otherwise precarious interest in radio wave propagation and to the subsequent evolutionary development of that establishments research program through a number of distinct phases.
RPL's first few years were largely devoted to consolidating what had gone before and to defining a proper base to build on. The propagation studies to meet the practical requirements of the wartime communications had opened the door to many problems and research areas that could only be pursued under more stable circumstances. In addition there were wartime activities that continued to operate largely on inertia, and there were technological advances that made new applications possible. During this phase additional ionosonde stations were built at Baker Lake, N.W.T., Resolute Bay, N/W/T/ and Fort Chimo, Quebec, and a mobile ionospheric observatory was operated on the railway between Portage la Prairie and Churchill, Manitoba. The Defence Research Electronics Laboratory began work on communications equipment problems and soon grew into a sister laboratory for RPL (which by then was renamed the Radio Physics Laboratory because of its increasing interest in the fundamentals of propagation), and the two became identified as the Defence Research Telecommunications Establishment. The Stormy Weather Group, headed by Dr. Robert C. Langille, which had been studying the effects of meteorological factors on radar with the Army Operational Research Group, was included in RPL for a time, then later was transferred to McGill University where it continued to work with DRB funding. As a further consolidation measure, and to accommodate an anticipated growth, a new laboratory was planned and built at Shirley Bay, into which the RPL staff moved late in 1952.
The DRTE was now actively engaged in a broadly based and well planned research program. Its direction was conditioned, in part, by the earlier work on ionospheric propagation, by the recognition of future needs for communications in the vast Canadian regions if they were to be developed, and by considerations associated with the then unstable international situation and the need for continental defence. Propagation research was undertaken in several different regions of the radio spectrum, and pursued at different locations in the country. High Frequency radio communications in Canada are plagues by difficulties arising from ionospheric disturbances that are peculiar to the auroral and polar latitudes. DRTE embarked on research on the aurora, on the physics of the upper atmosphere, on high frequency propagation and the prediction of the most suitable frequencies for use over Canadian radio circuits at any time, and on low frequencies as an alternate mode of communications since they are less influenced by auroral disturbances. At the same time, DRTE was involved in radar developments at VHF and UHF and undertook studies of the propagation of such frequencies over several types of Canadian terrain. These propagation studies were extended to include the reflection and scattering from auroral ionization, from ionization irregularities, and from ionized meteor trails. they also included studies of diffraction over hills and ridges, of ionospheric absorption at VHF, and of solar radio emissions as an aid to forecasting ionospheric disturbances. Eventually these activities came to include studies of the nature and formation of the lowest ionospheric regions and of the regions above the peak density of the ionosphere, as well as theoretical work on the morphology of magnetospheric and ionospheric disturbances. Propagation studies were also initiated at UHF and microwave frequencies to investigate the influence of various factors in the troposphere, though some of this word was subsequently transferred to the University of Western Ontario where it continued with DRB financing.
This research program had as its primary objective the understanding of radio wave propagation, and of the factors that influenced it. The research continued, albeit with decreasing emphasis in the later years, throughout the life of DRTE.
As the emphasis on ionospheric propagation decreased, there was a growing interest in factors affecting microwave propagation, particularly as might be applied to satellite communications. During the latter half of 1962, studies were carried out to determine those areas of satellite communications in which further research work at DRTE might be profitable. The assumption was made that satellite communications would be of interest to the military, as well as to civilian, agencies. The intent was to determine areas of interest that might provide the necessary background for future satellite communications systems.
Since it was unlikely that, in the near future, Canada would be in a position to launch a system of satellites for communications purposes, the problems of satellite development and reliability were largely overlooked, even though experience in these problems was built up during the development of the topside ionospheric sounder satellites, Alouette and ISIS. The studies did result, however, in a proposal to carry out a program of research on propagation at frequencies between about 8 and 16 GHz. It was recognized that the most suitable frequencies for a satellite communications system were in the range from about 1 to 8 GHz. These frequencies are high enough that most ionospheric effects are negligible, and low enough that tropospheric effects such as absorption by rainfall are also minimal. However, this desirable spectrum was at the time fully allocated. If additional spectrum space for communications would be required, the only place to find it was above 8 GHz. The immediate problem to be solved was that of rain attenuation, since the relation between attenuation and rainfall rate at different frequencies was uncertain, as was the extent and frequency of rainfall occurrence in any particular region. Additional problems, such as the possible limitation of coherent bandwidth and rain scatter interference were also recognized. These considerations are all discussed in DRTE Report 1122*.
This Report, and the studies that preceded it, resulted in a number of significant initiatives at DRTE. A research program in microwave propagation research was started, first at 8 and 15 GHz on a terrestrial link running from DRTE to Kingsmere, Quebec. The 9.1m earth terminal was acquired and installed at DRTE, with its official opening in 1966. This terminal was initially used for microwave propagation studies but in later years has been used for many systems developments and applications.
In the years since 1966, microwave propagation studies have continued at DRTE/CRC, on both terrestrial and earth-space links, studying a range of phenomena including rain attenuation and depolarization, multipath fading, scatter interference between stations, and 'low angle fading' on earth-space paths. Climatological atlases for Canada were developed, detailing rainfall and the radio refractivity of the troposphere for many locations throughout the country. In addition, a great deal of effort was expended on developing a prediction program for VHF/UHF propagation, based on a knowledge of terrain profiles and terrain cover along the propagation path. This work, in addition to being made available to industry for system design, has been used extensively to support the Canadian positions at Administrative Radio Conferences of the ITU, and has been used in the development of spectrum policy for Canada.
Much of this material was extracted from a report prepared by staff of the Radio Research Branch, 1976.
Barrington, R.E. "An Overview of Propagation Research at CRC." Paper presented in March 1976.
Babaian, Sharon. Radio Communication in Canada: An Historical and Technological Survey. Ottawa; National Museum of Science and Technology, 1992.
*Blevis, B.C., J.W.B Day, R.M Dohoo, and O.S. Roscoe, "Some Considerations in Satellite Communications Systems", DRTE Report No. 1122, January 1964.
Page created on August 18, 1997 by Cynthia Boyko
Last updated on October 27, 2001 by Stu McCormick
Copyright © Friends of CRC, 1997.