55 STAT.] NORTH AMERICAN--REGIONAL BROADCASTING-DEC. 13 , 1937 required for the type of emission employed. With respect to type A-3 emissions (radio-telephony) the transmitter should not be modu- lated in excess of its modulation capability to the extent that inter- fering spurious radiations occur, and, with respect to amplitude modulation, the operating percentage of modulation should not be less than seventy-five (75) percent on peaks of frequent recurrence. Means should be employed to insure that the transmitter is not modu- lated in excess of its modulation capability. E. DETERMINATION OF PRESENCE OF OBJECTIONABLE INTERFERENCE 1. Antenna performance. For the purpose of calculating the pres- ence and the degree of objectionable interference, stations of the several classes shall be assumed to produce effective field, corrected for absorption, for one kilowatt of input power to the antenna, as follows: Class of Station At One Mile At One Kilometer I 225 mv/m 362 mv/m II and III 175 mv/m 282 mv/m IV 150 mv/m 241 mv/m In case a directional antenna is employed, the interfering signal of a broadcasting station will vary in different directions. To deter- mine the interference in any direction, in the absence of actual inter- ference measurements, the horizontal and vertical field intensity patterns of the directional antenna must be calculated and by com- paring the appropriate vectors in the horizontal or vertical pattern with that of a nondirectional with the same effective field, the inter- fering signal toward any other station can be expressed in terms of kilowatts. This rating in kilowatts shall be applied in the use of mileage separation tables or in computing distances from the propa- gation curves or tables. 2. Power. The power of a station shall, for the purposes of notifi- cations required by this Agreement, be determined in one of the following manners: (a) By taking the product of the square of the antenna current and the antenna resistance (antenna input power). (b) By determination of the station's effective field intensity, corrected for absorption, by making sufficient field intensity measure- ments on at least eight radials as nearly equally spaced as practicable and by relating the field intensity thus determined to the effective field intensity of a station having the antenna efficiency stipulated above for its class. 3. Methods of determining the presence of objectionable interference- General. The existence or absence of objectionable interference from stations on the same or adjacent channels shall be determined by one of the following methods: (a) By actual measurements obtained in the method hereinafter prescribed; or, with the mutual consent of the countries concerned: 278941°-42 -PT. II-- -9 1013
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