(b) Fundamental frequency between 550 and 1,500 kc.-The field intensity of any harmonic radiation shall not be greater than 0.05 per cent of the fundamental field intensity and not exceed 250 microvolts per meter at one mile from the station. (c) Fundamental frequency above 1,500 kc.-No station shall deliver to the antenna in any harmonic more than 0.01 per cent of the fundamental power and in no case shall such power be in excess of one watt.

Exceptions to (a) and (c) may be made in the case of a maritime mobile station which, because of the nature of its service, can operate without causing interference at its harmonic frequencies.

A maximum permissible value in addition to a ratio is given in order not to make it necessary to impose an undue hardship on small stations in order to limit large stations. It is recognized that it is necessary to impose more stringent requirements on larger stations than on smaller ones. On the other hand, even the small stations should be required to take reasonable precautions to reduce harmonic radiations.

The harmonic reduction requirement covering the high-frequency portion of the spectrum (c) is defined in terms of power delivered to the antenna instead of in field intensity because of the difficulty at the present stage of the art of taking account of the directional characteristics of the antenna.

Any superfluous radiation at frequencies not close to the communication band can reasonably be expected to fall within the same limitations as are recommended above for harmonic radiation. The suppression of superfluous radiation at frequencies close to the communication band is discussed in Section IV. 1.

2. Power Rating of Radio Transmitters1

The object of specifying the power radiated from a radio transmitter is ordinarily to indicate (1) the service which the transmitting station is able to give, and (2) the radiation against which discrimination must be provided in order to avoid interference in other radio circuits. It is recommended that, as a nominal power rating, the specification consist of a statement of the power in the antenna circuit (estimated, if necessary, from the power supplied to the circuit which delivers power into the antenna), and, as a second and more refined rating, where desired, a radiation rating based on measurements of field intensity made reasonably near the

'It is appreciated that this recommendation is not entirely in harmony with the provision of Aarticle 13, Paragraph 4 (e), of the International Radio Regu lations of Washington, 1927. Any replacement of "meter-amperes" in the nomenclature issued by the International Bureau will, presumably, have to wait until the next International Radio Conference.

transmitting antenna. These two methods of rating are discussed below:

(1) Nominal power rating (power in the antenna circuit).—The power in a transmitting-antenna circuit may be determined from measurements at the transmitting-station and is given by the formula P=RI 2

where P is the power in watts, R is the antenna resistance in ohms, measured at the point where the current value is observed, and I is the antenna current in amperes.

In the absence of measurements of antenna resistance and current, the power in the antenna may be estimated from the power in the transmitter circuit which feeds the antenna. Such an estimate should be based upon the most reliable measurements or data available for the type of transmitter in question. This gives a nominal power rating for the station.

The power which is actually radiated is dependent upon the design of the antenna, its location and the nature of its surroundings. (2) Radiation rating.'-In the case of certain services, for example, broadcasting, it is frequently desirable to determine the radio field-intensity produced by the transmitter in the area which it serves and to utilize this as a basis of station rating.

The radio field-intensity is ordinarily expressed in millivolts per meter or microvolts per meter. In present practice this method is most applicable at frequencies below about 3,000 kc. For higher frequencies this method is probably impracticable as a basis for a general rating because of directional, high-angle, and abnormally polarized radiation, and because of extremely rapid attenuation of ground waves.

To obtain a suitable average value of the field-intensity, measurements need to be made at several points, not less than two wavelengths from the transmitter and in not less than five directions approximately equally spaced around the transmitter.

Field-intensity measurements may be used as a basis for computing the power radiated from a transmitting antenna at a given frequency, by the use of the formula

P=44. (10)-° Ad2 F2

where P is the radiated power in kilowatts, d is the distance in kilometers from the antenna to the point at which the field-intensity measurement is made, F is the measured field-intensity in millivolts per meter, and A is a factor which may have a value between:

1, where the ground in the vicinity of the transmitting-station is of very low conductivity, or where the transmitting-station is far above the ground; and

14, where the ground in the vicinity of the transmitting-station is of approximately perfect conductivity.

Since, in the cases where the above formula applies, transmitting power is thus a function solely of the product of observed fieldintensity and distance, the calculation of power may be obviated by rating stations directly in product of field-intensity and distance. This can be further simplified by arbitrarily agreeing upon a standard distance and defining as the "radiation rating" the fieldintensity at this distance. The distance of 10 kilometers is proposed for this, and a "radiation rating" defined as the field-intensity at 10 kilometers is proposed. Using the symbol F1o for the radiation rating, it is related to any observed field-intensity F (where the distances are not excessive) by the relation

d F=10 F10

3. Power Limitation

10

In international communication services the power of any station should not exceed an amount necessary to carry on efficient and economical communication for the class of service for which the station is licensed. The operating agency is equipped to determine when sufficient power is used to carry on efficient and economical communication, and general restrictions should not be placed on the amount of power to be used. Each communication circuit should be considered individually and power adjusted to meet the needs of the circuit. It is not efficient, for example, in a radiotelegraph circuit, to use a low power which is just enough to get through with slow speed, when the use of a higher power would permit the same traffic to be carried on in much less time by high speed. On the other hand, an operating agency will not use power sufficient for transmission over great distances when operating over short distances.

The use of directive transmitters for high frequencies makes it possible to confine most of the power used into a sector and thus it is possible to have the effect of a large amount of power concentrated in a useful direction. This not only serves to reduce the amount of power actually used but also to reduce interference in all directions except in the direction of the transmitter radiation.

The use of highly directive and selective receiving systems reduces the effect of atmospheric disturbances and other interference, and makes possible more efficient communication.

In broadcasting, the consideration is not of a complete communication circuit, and the situation is somewhat different. There are relatively few station-bands and all classes of receivers are used by inexperienced operators. Power should not be limited on any broadcast channel occupied by a single station. The use of increased power on these channels, if stations are properly located with respect to populous areas, will give improved service to listeners. Experiments with increased power will make it possible to determine the optimum power which it is desirable to use for this class of service.

4. Local Conditions as Affecting Transmitter and Receiver Location

While this is not ordinarily an international problem, the following recommendation is believed to be the best requirement which can be formulated at the present state of the art concerning the location of broadcasting stations.

It should be appreciated at the outset that from an engineering standpoint every case should be considered on its individual merits. Among the technical factors which enter such a consideration, the following may be listed: (1) Field intensity pattern due to a transmitter at the proposed station site; (2) location of and coverage provided by other stations in the same area; (3) frequency assignments of all stations in this area including the proposed station; (4) distribution and density of population in this area; (5) performance of the average radio receiver in this area; (6) arbitrary definition of the radio service which the average listener may reasonably expect in this area.

Every broadcasting station of 5 kilowatts or more should be located at such a place that the radio field-intensity at the nearest boundary of a populous center shall not be more than 100 millivolts per meter.

Roughly, this means that stations of the following power must be located at distances not less than those shown beyond the city limits:

It is noted that no definition of the term "boundary of a populous center" is given in this discussion. A universally applicable definition of this term for radio purposes is believed to be impossible. In applying this recommendation to specific cases, one important consideration may be taken into account. In determining the extent to which the 100 millivolt or blanketing area may encroach upon a

large community the significant figure is not so much the density of population in a blanketed region as it is the ratio of total population in the blanketing interference area of the proposed station to the total population in the service area of that station. It must be recognized that a certain amount of such interference from any broadcasting station is unavoidable at the present state of the engi neering art.

It is probable that in certain situations a field-intensity greater than 100 millivolts per meter can be allowed, so far as interference is concerned. This statement recognizes the performance of certain existing stations and the performance of modern radio receivers. According to present engineering practice interference should be considered unreasonable when it is produced, in modern radio receivers, by a station separated 50 kc or more from the desired station, where the desired station normally gives consistently good service at the point of observation in the absence of such interference.

Before determining definitely upon the location for a high-power broadcast transmitter it is important to give consideration to the following:

(1) Location and nominal power of all other broadcasting stations which the applicant considers to be serving the same area as the proposed station.

(2) Approximate number of inhabitants within the three areas defined by circles centered on the proposed site and having radii equal respectively to 1 mile, 2 miles and 5 miles. (3) A survey of the field intensities established by an experimental transmitter situated at the proposed site. This transmitter may be of low power, but the numerical values of fieldintensity as submitted should be corrected to correspond to a transmitter power equal to the power proposed for the station. The assumptions made with respect to the antenna in making this correction should be stated. The survey should embrace at least an area bounded by the contour of 100 millivolts per meter for the corrected field-intensities.

For any station other than broadcasting, the question is a very local one, and a suitable location is usually selected by mutual agreement of all operating agencies in the vicinity, taking special care to avoid interference to broadcast listeners.

VIII. RELATION OF C. C. I. RADIO TO OTHER SCIENTIFIC

ORGANIZATIONS

In regard to the suggested organization of studies of fundamental radio transmission phenomena, it is recommended that duplication of the activities of existing scientific and technical agencies be