Canada Puts Limit on R.F. Interference
February 1960 Electronics World

February 1960 Electronics World   Table of Contents  Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

If you think the ISM (Industrial, Scientific, and Medical) unlicensed bands were a relatively new spectrum allocation, you might find this 1960 Electronics World news piece interesting. Individual countries generally acknowledge the ISM emissions specifications set forth by the International Telecommunication Union (ITU), which created the bands in 1947. The 900 MHz, 2.4 GHz, and 5.8 GHz WiFi bands are well known to most people. 24 GHz is gaining traction as current spectrum gets more and more crowded and high bandwidth data channels are needed. Interestingly, the first few ISM bands are integer harmonics of the lowest (6.78 MHz, center of band 1). To wit:

6.78 MHz (band 1)
2 * 6.78 = 13.56 MHz (band 2),
4 * 6.78 = 2 * 13.56 = 27.12 MHz (band 3),
6 * 6.78 = 2 * 13.56 = 40.68 MHz (band 4),
64 * 6.78 = 32 * 13.56 = 16 * 27.12 = 433.92 MHz (band 5).

According to some sources, the harmonic band allocations are to accommodate use of higher powers on the lower bands that generate harmonics which will fall within other ISM bands. RF heating ovens are an example of such applications.

Canada Puts Limit on R.F. Interference

By J. E. Kitchin

Inspector, Telecommunications Department of Transport

Audio noises have been well tabulated in the past but how does one set a tolerable limit for radio-frequency noise? A recent Canadian Department of Transport regulation (known officially as SOR 59-116 by the Queen's Printer, Ottawa, Canada) has defined radio-frequency noise as any electrical disturbance "produced by any mechanical, electrical, or other device" and capable of being received on a radio receiver.

The regulation refers specifically to r.f. generators in the ISM industrial bands, in which the amount of r.f. energy produced is without limitation. However, any r.f. generator in those bands must not have any emission outside of the band in excess of 25 microvolts-per-meter at a distance greater than one-thousand feet from the r.f. generator. A provision is also made that if r.f. generators are operated on any frequency outside the ISM bands the limit shall be 15 microvolts-per-meter at 1000 feet.

Allowance is also made for higher power industrial equipment (over 5 kw.) and high-frequency arc welding equipment where it is not practicable to apply the limits mentioned above. In these cases the field intensity is restricted to ten microvolts-per-meter at a distance of one mile from the industrial equipment.

It is also Interesting to note that these measurements must be made on field strength meters of a make and type approved by the Department of Transport, or by meters having an equivalent standard of measurements. Field-strength meters must be adjusted to read the "quasi peak" values of field strength and the prescribed antenna system is a horizontally polarized dipole seven and a half feet high for measuring the frequencies above 25 mc.

Any records of measurements made to ensure compliance with the regulation must be kept for three years and be available to Inspectors of the Department of Transport. It must also be remembered that, as usual in such regulations, the provision is made that in certain cases or circumstances suppression may be required by the Department at values lower than those specified in the regulation.

Posted June 19, 2018