High Frequency Radio Research

Propagation of High Frequency Radio Waves

High frequency radio (commonly known as short wave radio) is a means of communication in which a radio wave signal is transmitted from one point to the ionosphere, where it is reflected back down to another point on earth. The ionosphere consists of layers of ionized gas situated a few hundred miles above the earth. The term "high frequency" refers to the number of radio waves transmitted per second, and simply serves to distinguish this type of system from those which transmit at higher or lower frequencies.

For many years, HF radio was the major form of communication in the Arctic. HF radio was considered appropriate for the North because it was a suitable and economic form of long distance communication. HF radio has other characteristics which limit its usefulness for inter-community communication, the most serious of which is that it is very prone to interference from atmospheric and other sources of disturbance. The ionosphere is subject to considerable variability and this variability affects its performance as a reflecting surface (hence affecting the reliability of radio wave propagation). Sun spots are another source of ionospheric variation. Such solar activity is cyclical, which means that any HF system which works well at a given point in time will certainly need frequency modification in about five years to compensate for the solar cycle. Also, HF radio does not permit privacy of conversation. Anyone within range and access to the same frequency may listen to the communication.

While reliability and privacy were important issues, there were certain advantages which made HF an important form of communication. For example, HF can be used for communication over great distances and between points separated by geographic barriers, such as mountains. Also, HF radio systems are generally easy to use, with a very minimal level of training; anyone can make a system work to some degree. In addition, HF served a useful role as an intermediate step between having no communication access and having more advanced technology.

The importance of HF radio to communication in Canada was a significant reason why CRC researchers focused their attention on HF radio predictions. Because HF is subject to interference from atmospheric conditions and sun spot activity, different frequencies are required for day versus night, winter versus summer and for various times within the eleven year solar cycle. CRC research has focused on prediction methods to improve the reliability of HF radio.

An HF radio system consists of three basic components: the transmitter/receiver unit (commonly called the transceiver), the antenna and the power source. The transmitting and receiving functions are usually contained in the one unit; the transceiver includes both a microphone and a loudspeaker to allow either function. The antenna usually consists of a wire or metal rod which is mounted above the ground in a clear space, and is connected to the transceiver by means of a coaxial cable. When the unit is transmitting, the electrical signals it produces travel through the coaxial cable to the antenna, where they are changed into radio waves and radiated. When the unit is receiving, the antenna receives radio waves, translates them into electrical signals which travel through the coaxial cable and are heard through the receiver as voice signals.

In 1954, the Radio Physics Laboratory (RPL) developed a method for Canadian high frequency radio wave prediction. Eventually, this method of prediction was translated into a computer program for a large mainframe computer in the 1960s. The program could produce a table and graph of the predicted optimum working frequencies for any Canadian HF communication circuit. The table listed, for each hour of the day, the monthly median maximum usable frequency for both the E and F layers of the ionosphere and the optimum working frequency for the circuit. The graph was a convenient presentation of the hourly optimum working frequency values. In practice, a radio operator would choose an operating frequency equal to or less than the hourly optimum working frequency. This information was prepared for specific circuits on request. There was no charge for the service and the charts and information were mailed out every three months. In addition, the program also created contour maps based on the following base stations: Vancouver, B.C.; Prince George, B.C.; Resolute Bay; Winnipeg, Manitoba; Prince Albert, B.C.; Frobisher Bay; Alma, Quebec; Trenton, Ontario; Halifax, Nova Scotia and Saint John, N.B. Special arrangements could also be made to supply maps for other base stations in Canada. Usually twelve maps, one for every second hour of the day, were prepared for each base station, each month.

For about twenty years, HF prediction techniques were available for mainframe computer systems. The earliest programs predicted only ionospheric reflection, while later ones permitted the inclusion of system parameters. Over the years, major improvements were incorporated into these programs. However, CRC undertook to create a computer program for HF prediction that required only a microcomputer. In 1984, a contract between CRC and Petrie Telecommunications led to the development of MICROPREDIC. It was designed to operate on a computer with 8086/8088 microprocessors. MICROPREDIC computed both the maximum usable frequency and the E layer screening frequency for any terrestrial HF path. The optimum working frequency and lowest usable frequency, based on these, are displayed on the computer monitor and could also be printed. The program incorporated the following features: a worldwide data base of ionospheric characteristics for four seasons; prediction of E and F2 layer maximum usable frequencies; E layer screening frequency, which provides a first-order estimate of the lowest operating frequency during daytime hours. The program was written in C language to permit portability and easier modification. The accuracy of MICROPREDIC was comparable to that of similar programs running on mainframe computers. The program required approximately four minutes to compute a prediction for the twenty-four hours of the day, for a given path, sunspot number and month. The program was to be later modified to include performance assessment based on power and antenna gain, coefficients for twelve months and an optional graphic output.

High frequency prediction information was vital to radio operators who were trying to determine the best frequency for radio communication. High frequency radio is commonly unreliable, but these methods of prediction helped operators improve the chance of completing a communication. Research continued at CRC to help improve high frequency communication. This involved measuring background noise, antennae research and radio systems research.

HF prediction services have been very important for radio operators and particularly for communication in the north. For a significant part of this century, HF radio was the backbone of communications in the Canadian Arctic. This method of communication was used for a variety of inter-community communications. The HF prediction services offered by CRC were extremely helpful in conducting high frequency radio communication. In fact, without it, HF communication would have been extremely difficult. However, these propagation predictions could not turn HF radio into a highly reliable form of communication. As a result, CRC researchers continued to look for ways to improve the reliability of HF radio communications.

MICROPREDIC was a follow on from CRC's development of a mainframe computer program. RACE (radio automatic channel evaluation) was a HF radio telephone system, designed to automatically choose the best frequency for communication just prior to the initiation. This radio system grew out of research into improving HF communication. Syncompex (synchronized compressor and expander) is related to the development of RACE. Both of these technologies are discussed elsewhere.

Sources

Bhaneja, B., Lyrette, J., Davies, J.W. and Dohoo, R.M. "Technology Transfer by Department of Communications: A Study of Eight Innovations." MOSST Background Paper. Ottawa; Supply and Services, 1980.

DuCharme, E.D. and Thomas, J.L. "HF Predictions Description of Services." CRC Report No. 1228. Ottawa; Department of Communications, November 1971.

Meagher, J.E. Inter-Community Communications in the North: Requirements and Alternatives. Ottawa; Communications Canada, October 1974.

Petrie, L.E., Goudie, G.W., Ross, D.B., Timleck, P.L., and Chow, S.M. "MICROPREDIC: An HF Prediction Program for 8086/8088-based Computers." CRC Report No. 1390. Ottawa; Supply and Services, 1986.