54 STAT.] INTER-AMERICAN-RADIO COMMUNICATIONS-DEC. 13, 1937 e) that the power radiated by radio aids to air navigation in the authorized frequency bands should ordinarily be confined to a value consistent with the normal required signal intensity within the area in which it is desired to render service in order that interference beyond the service area may be reduced to a minimum. NOTE: See additional material submitted by the U.S .A. for informative purposes, annexed. SECTION 12. SUPPRESSION OF INTERFERENCE CAUSED BY ELECTRICAL APPARATUS. 1. Diathermy apparatus, induction field heaters, carrier call systems, and similar non-radio apparatus which use radio frequency currents as an essential to their operation, may be a serious source of interference to radio communications. 2. The use of such apparatus has an important place in therapeutics, surgery, industry, etc. 3. The radiation of radio energy is not essential to the proper functioning of the apparatus and can be prevented or controlled without impairing the usefulness of the apparatus for its intended purpose. 4. The radiation takes place generally from the output circuit, internal circuits or power supply connection, all of which are essential elements. 5. The extent of the radiation depends upon the operating frequency or frequencies, power, and the design, installation and operation of the apparatus. 6. The radiation through the power supply connection can be prevented by means of the proper line filter. Radiation from the internal circuits can be prevented by means of suitable metallic cases. The radiation from the output circuits can be reduced to a level so as not to cause interference to radio communications by means of suit- able metallic shielding, if the shielding encloses the entire apparatus and is of sufficient dimensions that large eddy currents are not produced in the shield. 7. In many cases it may not be practicable to employ the required shielding. 8. The frequencies used for such apparatus may be any frequency in the useful radio spectrum. However, many moder diathermy units (which cause most long-distance radio interference) operate on frequencies from approximately 10,000 to 20,000 kilocycles. Opera- tions on other frequencies mainly cause interference to local or moderate distance reception. 9. In cases where it is not practicable to shield the entire apparatus to control the radiation, then the only means of operating machines without causing interference would be to use frequencies which are not assigned to any radio services. 2533 Interference from electrical apparatus.
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