1 See footnote 3, codification note, page 6434.

NOTE: The Commission has ordered a hearing with respect to what changes should be made in the Standards concerning the assignment of two stations with the same general groundwave service area for operation with a frequency separation of 30 kc or less. Until a decision is rendered with respect to this matter, the provisions in the Standards in effect on and prior to February 7, 1947 shall govern, including any relevant material contained in the Standards in 47 CFR, 1945 Supp., pages 4350-4355.

From the above, it is apparent that in many cases stations operating on channels 10 and 20 kilocycles apart may be operated with antenna systems side by side or otherwise in proximity without any indications of interference if the interference is defined only in terms of permissible ratios hereinbefore listed in the table headed Interference Ratios. As a practical matter, serious interfer

The Commission will not authorize a directional antenna for a Class IV station assigned to a local channel.

ence problems may arise when two or more stations with the same general service area are operated on channels 10, 20 and 30 kilocycles apart.

Accordingly, no station will be licensed for operation with less than 40 kilocycles separation from another station, if the area enclosed by the 25 mv/m groundwave contours of the two stations overlap. Frequency separation of 20 kilocycles and 10 kilocycles are considered inappropriate for stations with the same general urban coverage and therefore no station will be licensed for operation with less than 30 kilocycles frequency separation if the area enclosed by the 25 mv/m groundwave contour of either one overlaps the area enclosed by the 2 mv/m groundwave contour of the other.

Two stations, one with a frequency twice that of the other, should not be assigned in the same groundwave service area unless special precautions are taken to avoid interference from the second harmonic of the lower frequency. In selecting a frequency, consideration should be given to the fact that occasionally the frequency assignment of two stations in the same area may bear such a relation to the intermediate frequency of some broadcast receivers as to cause so-called "image" interference. However, since this can usually be rectified by readjustment of the intermediate frequency of such receivers, the Commission in general will not take this kind of interference into consideration in allocation problems.

Two stations operating with synchronized carriers' and carrying the identical program will have their groundwave service subject to some distortion in areas where the signals from the two stations are of comparable intensity. For the purpose of estimating coverage of such stations areas in which the signal ratio is between 1 to 2 and 2 to 1 will not be considered as having satisfactory service.

CODIFICATION: The text consisting of the paragraphs and tables beginning with "The night separation tables " immediately following Table IV, and ending with Table VIII-H, inclusive (47 CFR, 1945 Supp., pages 4350-4355) was deleted, and the text

2 Two stations are considered to be operated synchronously when the carriers are maintained within one-fifth of a cycle per second of each other and they transmit identical programs.

set forth above was inserted in lieu thereof, by Order, Federal Communications Commission, Jan. 31, 1947, effective Feb. 10, 1947, 12 F.R. 925, except for the two paragraphs immediately following the note under TableInterference Ratios, which were added by Order, Federal Communications Commission, June 9, 1947, effective July 17, 1947, 12 FR. 3893.

ANNEX I

Interference From Ground Wave Signals

*

CODIFICATION: The sixth paragraph of Annex I (47 CFR, 1945 Supp., page 4356) was amended in the following respects, by Order, Federal Communications Commission, Jan. 31, 1947, effective Feb. 10, 1947, 12 F.R. 926:

1. The tenth sentence was amended to read as follows:

"Since the stations are separated by 10 kc, the undesired signal at that point can have a value up to 500 uv/m without interference, but if the interference signal had been found to be greater than this value, then interference would have been determined."

2. In the twelfth sentence, the words "or twenty-five times when the frequency separation is 20 kc" were deleted.

3. The last sentence was amended to read as follows:

"In this case the interference must be estimated on the basis of the sky wave interference and the propagation curve in Figure 1, or Figure 1-A, used to determine the interfering signals rather than Figure 4 for the ground wave signals."

ANNEX II

Computation on Interfering Signal From a Directional Antenna

For signals from stations on regional and local channels, in computing the 10% skywave (interference) field intensity values of Class III and Class IV stations,' 18a Fig. 1-A is to be used in place of Figure 1. Since Figure 1-A is predicated upon a radiated field of 100 mv/m at one mile in the pertinent direction, no comparison with the vertical pattern of a 0.311 wavelength antenna is to be made. Instead the appropriate radiated field in the vertical plane corresponding to the distance to the receiving station, divided by 100, is multiplied into the value of 10% skywave field intensity determined from Figure 1-A. There are two new factors to be considered, however, namely the variation of received

18 Certain simplifying assumptions may be made in the case of Class IV stations on local channels: See footnote 3a.

field with latitude of the path and the variation of pertinent vertical angle due to variations of ionosphere height and ionosphere scattering.

Figure 1-A, "10% Skywave Signal Range Chart," shows the 10% skywave signal as a function of the latitude of the transmission path and the distance from a transmitting antenna with a radiated field of 100 mv/m at the pertinent angle for the distance. The latitude of the transmission path is defined as the geographic latitude of the midpoint between the transmitter and the receiver. Latitude 35° should be used in case the midpoint of the path lies below 35° North and latitude 50° should be used in case the midpoint of the path lies above 50° North.

Figure 6-A, entitled "Angles of Departure vs. Transmission Range," is to be used in determining the angles in the vertical pattern of the antenna of an interfering station to be considered as pertinent to transmission by one reflection. Corresponding to any given distance, the curves 4 and 5 indicate the upper and lower angles within which the radiated field is to be considered. The maximum value of field intensity occurring between these angles will be used to determine the multiplying factor for the 10% skywave field intensity determined from Figure 1-A. (Curves 2 and 3 are considered to represent the variation due to the variation of the effective height of the E-layer while Curves 4 and 5 extend the range of pertinent angles to include a factor which allows for scattering. The dotted lines are included for information only.)

In the case of non-directional vertical antennas, the vertical distribution of relative fields for several heights, assuming sinusoidal distribution of current along the antenna, is shown in Figure 5. In the case of directional antennas the vertical pattern in the great circle direction toward the point of reception in question must first be calculated. Then for the distance to the points, the upper and lower pertinent angles are determined from Figure 6-A. The ratio of the largest value of radiated field occurring between these angles, to 100 mv/m (for which Figure 1-A is drawn) is then used as the multiplying factor for the value of the field read from the curves of Figure 1-A. Note that while the accuracy of the curves is not as well established by measurements for distances less than 250 miles as for distances in excess of 250