§ 73.185 Computation of interfering

signal from a directional antenna.

(a) In case of an antenna directional in the horizontal plane, the groundwave interference shall be computed from the calculated horizontal pattern by determining the vectors toward the service area of the station to be protected and applying these values to the groundwave curves set out in § 73.183.

(b) For signals from stations operating on clear channels, skywave interference shall be determined from Figures la and 6a of § 73.190.

(c) For signals from stations operating on regional and local channels, skywave interference is determined from (Certain Figures 2 and 6a of § 73.190. simplifying assumptions may be made in the case of Class IV stations on local channels. See note to § 73.182(a) (4).)

(d) Figure 6a of § 73.190, 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. To provide for variation in the pertinent vertical angle due to variations of ionosphere height and ionosphere scattering, 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 shall be used to determine the multiplying factor to apply to the 10 percent skywave field intensity value read from Figure la or Figure 2 of § 73.190. The multiplying factor is found by dividing the maximum radiation between the pertinent angles by 100 mv/m. (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.)

(e) Example of the use of skywave curves for stations operating on clear channels: Assume a Class II station with which interference may be expected is located at a distance of 450 miles from a proposed Class II station. The critical angles of radiation as determined from Figure 6a of § 73.190 are 9.6° and 16.3°. If the vertical pattern of the antenna of the proposed station, in the direction of the other station, is such that between the angles of 9.6° and 16.3° above the

horizon the maximum radiation is 160 mv/m at 1 mile, the value of the 10 percent field, as read from Figure 1a of § 73.190, is multiplied by 1.6 to determine the interfering field intensity at the location in question.

(f) For stations operating on regional and local channels, interfering skywave field intensities shall be determined in accordance with the procedure specified in (d) of this section and illustrated in (e) of this section, except that Figure 2 of § 73.190 is used in place of Figure la of § 73.190. In using Figure 2 of § 73.190, one additional parameter must be considered, i.e., the variation of received field with the latitude of the path.

(g) Figure 2 of § 73.190, "10 percent Skywave Signal Range Chart," shows the signal as a function of the latitude of the transmission path, which is defined as the geographic latitude of the midpoint between the transmitter and receiver. When using Figure 2 of § 73.190, latitude 35° should be used in case the mid-point of the path lies below 35° North and latitude 50° should be used in case the mid-point of the path lies above 50° North.

(h) 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 of 73.190. 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. In cases where the radiation in the vertical plan, in the pertinent azimuth, contains a large lobe at a higher angle than the pertinent angle for one reflection, the method of calculating interference will not be restricted to that just described, but each such case will be considered on the basis of the best knowledge available.

(i) Example of the use of skywave curves for stations operating on regional and local channels: It is desired to determine the amount of interference to a Class III station at Portland, Oregon, caused by another Class III station at Los Angeles, California. The Los Angeles station is radiating a signal of 560 mv/m at one mile, in the horizontal plane, in the great circle direction of Portland, using a 0.5 wavelength antenna. The distance is 825 miles. From Figure 6a of § 73.190, the upper and lower pertinent angles are 7° and 3.5° and. from Figure 5 of § 73.190, the maximum

radiation within these angles is 99 percent of the horizontal radiation or 554 mv/m at one mile. The mid-point latitude of the transmission path is 39.8° N and, from Figure 2 of § 73.190, the 10 percent skywave field at 825 miles is 0.050 mv/m for 100 mv/m radiated. Multiplying by 554/100 to adjust this value to the actual radiation gives 0.277 mv/m as to the interfering signal intensity. At 20 to 1 ratio, the limitation to the Portland station is to the 5.5 mv/m contour.

(j) When the distance is large, more than one reflection may be involved and due consideration must be given each appropriate vector in the vertical pattern, as well as the constants of the earth where reflection takes place between the transmitting station and the service area to which interference may be caused.

NOTE: In applying the provisions of this section to applications tendered on or before September 29, 1965, for new or changed facilities on the clear channels listed in § 73.25 (b). Figure 1 of § 73.190, entitled "Average Skywave Field Intensity," shall be used instead of Figure 1a, and Figure 6 of § 73.190, entitled "Variation with Distance of Two Important Parameters in the Theory of Skywave Propagation" shall be used instead of Figure 6a. In determining skywave interference from an antenna with a vertical pattern different from that on which Figure 1 of § 73.190 is predicated, it is necessary to compare the appropriate vectors in the vertical plane. The skywave curves shown in Figure 1 of § 73.190 are based on antenna systems having height of 0.311 wavelength (112) and producing a vertical pattern as shown in Figure 5 of § 73.190. A non-directional antenna system, as well as a directional antenna system having vertical patterns other than essentially the same as shown, must be converted to the pattern of a 0.311 wavelength antenna having the same field intensity at the critical angle as does the pattern of the antenna involved.

[30 F.R. 13783, Oct. 29, 1965]

§ 73.186 Field intensity measurements in allocation; establishment of effective field at one mile.

(a) Section 73.45 provides that certain minimum field intensities are acceptable in lieu of the required minimum physical vertical heights of the antennas proper. Also, in other allocation problems, it is necessary to determine the effective field at 1 mile. The following requirements shall govern the taking and submission of data on the field intensity produced:

(1) Beginning as near to the antenna as possible without including the induc

tion field and to provide for the fact that a broadcast antenna not being a point source of radiation (not less than one wave length or 5 times the vertical height in the case of a single element, i. e., nondirectional antenna or 10 times the spacing between the elements of a directional antenna), measurements shall be made on eight or more radials, at intervals of approximately one-tenth mile up to 2 miles from the antenna, at intervals of approximately one-half mile from 2 miles to 6 miles from the antenna, at intervals of approximately 2 miles from 6 miles to 15 or 20 miles from the antenna, and a few additional measurements if needed at greater distances from the antenna. Where the antenna is rurally located and unobstructed measurements can be made, there shall be as many as 18 or 20 measurements on each radial. However, where the antenna is located in a city where unobstructed measurements are difficult to make, measurements shall be made on each radial at as many unobstructed locations as possible, even though the intervals are considerably less than stated above, particularly within 2 miles of the antenna. In cases where it is not possible to obtain accurate measurements at the closer distances (even out to 5 or 6 miles due to the character of the intervening terrain), the measurements at greater distances should be made at closer intervals. (It is suggested that "wave tilt" measurements may be made to determine and compare locations for taking field intensity measurements, particularly to determine that there are no abrupt changes in ground conductivity or that reflected waves are not causing abnormal intensities.)

(2) The data required by subparagraph (1) of this paragraph should be plotted for each radial in accordance with either of the two methods set forth below:

(i) Using log-log coordinate paper, plot field intensities as ordinate and distance as abscissa.

(ii) Using semi-log coordinate paper, plot field intensity times distance as ordinate on the log scale and distance as abscissa on the linear scale.

(3) However, regardless of which of the methods in subparagraph (2) of this paragraph is employed, the proper curve to be drawn through the points plotted shall be determined by comparison with the curves in § 73.184 as follows: Place the sheet on which the actual points have

been plotted over the appropriate Graph in § 73.184, hold to the light if necessary and adjust until the curve most closely matching the points is found. This curve should then be drawn on the sheet on which the points were plotted, together with the inverse distance curve corresponding to that curve. The field at 1 mile for the radial concerned shall be the ordinate on the inverse distance curve at 1 mile.

(4) When all radials have been analyzed in accordance with subparagraph (3) of this paragraph, a curve shall be plotted on polar coordinate paper from the fields obtained, which gives the inverse distance field pattern at 1 mile. The radius of a circle, the area of which is equal to the area bounded by this pattern, is the effective field. (See § 73.14.)

To

(5) While making the field intensity survey, the output power of the station shall be maintained at the licensed power as determined by the direct method. do this, it is necessary to determine accurately the total antenna resistance (the resistance variation method, the substitution method or bridge method is acceptable) and to measure the antenna current by means of an ammeter of acceptable accuracy. (See §§ 73.39 and 73.54.)

(b) Complete data taken in conjunction with the field intensity measurements shall be submitted to the Commission in affidavit form including the following:

(1) Tabulation by number of each point of measurement to agree with the map required in (2) below and the field intensity meter reading, the attenuation constant, the field intensity (E), the distance from the antenna (D) and the product of the field intensity and distance (ED) (if data for each radial are plotted on semi-logarithmic paper, see above) for each point of measurement.

(2) Map showing each point of measurement numbered to agree with tabulation required above.

(3) Description of method used to take field intensity measurements.

(4) The family of theoretical curves used in determining the curve for each radial properly identified by conductivity and dielectric constants.

(5) The curves drawn for each radial and the field intensity pattern.

(6) Antenna resistance measurement: (i) Antenna resistance at operating frequency.

(ii) Description of method employed. (iii) Tabulation of complete data. (iv) Curve showing antenna resistance versus frequency.

(7) Antenna current or currents maintained during field intensity measurements.

(8) Description, accuracy, date, and by whom each instrument was last calibrated.

(9) Name, address, and qualifications of the engineer making the measurements.

(10) Any other pertinent information. § 73.187 Limitation on daytime radiation.

(a) (1) Except as otherwise provided in subparagraphs (2) and (3) of this paragraph, no authorization will be granted for Class II facilities if the proposed facilities would radiate, during the two hours after local sunrise and the two hours before local sunset, toward any point on the 0.1 mv/m contour of a co-channel U.S. Class I station, at or below the pertinent vertical angle determined from Curve 4 of Figure 6a of § 73.190, values in excess of those obtained as provided in paragraph (b) of this section.

(2) The limitation set forth in subparagraph (1) of this paragraph shall not apply in the following cases:

(1) Any Class II facilities authorized before November 30, 1959; or

(ii) For Class II stations authorized before November 30, 1959, subsequent changes of facilities which do not involve a change in frequency, an increase in radiation toward any point on the 0.1 mv/m contour of a co-channel U.S. Class I station, or the move of transmitter site materially closer to the 0.1 mv/m contour of such Class I stations.

(3) If a Class II station authorized before November 30, 1959, is authorized to increase its daytime radiation in any direction toward the 0.1 mv/m contour of a co-channel U.S. Class I station (without a change in frequency or a move of transmitter site materially closer to such contour), it may not, during the two hours after local sunrise or the two hours before local sunset, radiate in such directions a value exceeding the higher of:

(i) The value radiated in such directions with facilities last authorized before November 30, 1959, or

(ii) The limitation specified in subparagraph (1) of this paragraph.

(b) To obtain the maximum permissible radiation for a Class II station on a given frequency (fkc/s) from 640 kc/s through 990 kc/s, multiply the radiation value obtained for the given distance and azimuth from the 500 kc/s chart (Figure 9 of § 73.190) by the appropriate interpolation factor shown in the K500 column of paragraph (c) of this section; and multiply the radiation value obtained for the given distance and azimuth from the 1000 kc/s chart (Figure 10 of § 73.190) by the appropriate interpolation factor shown in the K1000 column of paragraph (c) of this section. Add the two products thus obtained; the result is the maximum radiation value applicable to the Class II station in the pertinent directions. For frequencies from 1010 kc/s to 1580 kc/s, obtain in a similar manner the proper radiation values from the 1000 kc/s and 1600 kc/s charts (Figures 10 and 11 of § 73.190), multiply each of these values by the appropriate interpolation factors in the K'1000 and K'100 columns in paragraph (c) of this section, and add the products.

(c) Interpolation factors. (1) Frequencies below 1000 kc/s.

(2) To cause and experience minimum interference to and from other stations.

(3) To present a minimum hazard to air navigation consistent with objectives (1) and (2).

(4) To fulfill certain other requirements given in the following paragraphs of this section.

(b) The site selected should meet the following conditions:

(1) A minimum field intensity of 25 to 50 mv/m will be obtained over the business or factory areas of the city.

(2) A minimum field intensity of 5 to 10 mv/m will be obtained over the most distant residential section.

(3) The absorption of the signal is the minimum for any obtainable sites in the area. As a guide in this respect the absorption of the signals from other stations in that area should be followed, as well as the results of tests on other sites. (4) The population within the blanket contour does not exceed that specified by $73.24(g).

(c) In selecting a site in the center of a city it is usually necessary to place the radiating system on the top of a building. This building should be large enough to permit the installation of a satisfactory ground and/or counterpoise system. Great care must be taken to avoid selecting a building surrounded by taller buildings or where any nearby building higher than the antenna is located in the direction which it is desired to serve. Such a building will tend to cast "radio shadows" which may materially reduce the coverage of the station in that direction. Irrespective of the height of surrounding buildings, the building on which the antenna is located should not have height of approximately onc-quarter wavelength. A study of antenna systems located on buildings tends to indicate that where the building is approximately a quarter wavelength in height, the efficiency of radiation may be materially reduced.