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§ 73.185 Computation of interfering horizon the maximum radiation is 160

signal from a directional antenna. mv/m at 1 mile, the value of the 10 per(a) In case of an antenna directional

cent field, as read from Figure la of in the horizontal plane, the groundwave

$ 73.190, is multiplied by 1.6 to determine interference shall be computed from the

the interfering field intensity at the localculated horizontal pattern by deter

cation in question. mining the vectors toward the service

(f) For stations operating on regional

and local channels, interfering skywave area of the station to be protected and

field intensities shall be determined in applying these values to the groundwave curves set out in $ 73.183.

accordance with the procedure specified (b) For signals from stations operat

in (d) of this section and illustrated in

(e) of this section, except that Figure 2 ing on clear channels, skywave interfer

of $ 73.190 is used in place of Figure la ence shall be determined from Figures la and 6a of $ 73.190.

of $ 73.190. In using Figure 2 of § 73.190, (c) For signals from stations operat

one additional parameter must be con

sidered, i.e., the variation of received ing on regional and local channels, skywave interference is determined from

field with the latitude of the path. Figures 2 and 6a of $ 73.190. (Certain

(g) Figure 2 of $ 73.190, “10 percent simplifying assumptions may be made in

Skywave Signal Range Chart,” shows the the se of Class IV stations on local

signal as a function of the latitude of channels. See note to $ 73.182(a) (4).)

the transmission path, which is defined (d) Figure 6a of $ 73.190, entitled

as the geographic latitude of the mid“Angles of Departure vs. Transmission

point between the transmitter and reRange" is to be used in determining the

ceiver. When using Figure 2 of $ 73.190, angles in the vertical pattern of the

latitude 35° should be used in case the antenna of an interfering station to be

mid-point of the path lies below 359 considered as pertinent to transmission

North and latitude 50° should be used in by one reflection. To provide for varia

case the mid-point of the path lies above tion in the pertinent vertical angle due to

50° North. variations of ionosphere height and ion

(h) In the case of non-directional osphere scattering, the curves 4 and 5 vertical antennas, the vertical distribuindicate the upper and lower angles

tion of relative fields for several heights, within which the radiated field is to be

assuming sinusoidal distribution of curconsidered. The maximum value of field

rent along the antenna, is shown in Fig. intensity occurring between these angles

ure 5 of $ 73.190. In the case of direcshall be used to determine the multiply

tional antennas the vertical pattern in ing factor to apply to the 10 percent sky

the great circle direction toward the wave field intensity value read from

point of reception in question must first Figure la or Figure 2 of $ 73.190. The

be calculated. In cases where the radiamultiplying factor is found by dividing

tion in the vertical plan, in the pertinent the maximum radiation between the

azimuth, contains a large lobe at a higher pertinent angles by 100 mv/m. (Curves

angle than the pertinent angle for one 2 and 3 are considered to represent the

reflection, the method of calculating invariation due to the variation of the ef

terference will not be restricted to that fective height of the E-layer while

just described, but each such case will Curves 4 and 5 extend the range of perti

be considered on the basis of the best nent angles to include a factor which al

knowledge available. lows for scattering. The dotted lines are

(i) Example of the use of skywave included for information only.)

curves for stations operating on regional (e) Example of the use of skywave

and local channels: It is desired to decurves for stations operating on clear

termine the amount of interference to a channels: Assume a Class II station with

Class III station at Portland, Oregon, which interference may be expected is

caused by another Class III station at located at a distance of 450 miles from a

Los Angeles, California. The Los Anproposed Class II station. The critical

geles station is radiating a signal of 560

mv/m at one mile, in the horizontal angles of radiation as determined from

plane, in the great circle direction of Figure 6a of $ 73.190 are 9.6° and 16.3°.

Portland, using a 0.5 wavelength anIf the vertical pattern of the antenna of

tenna. The distance is 825 miles. From the proposed station, in the direction of

Figure 6a of $ 73.190, the upper and lower the other station, is such that between pertinent angles are 7o and 3.5° and, the angles of 9.6° and 16.3° above the 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 la, 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. 130 F.R. 13783, Oct. 29, 1965) $73.186 Field intensity measurements

in allocation; establishment of effec

tive 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 eflective 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 measure. ments 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:

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

(11) 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 (ii) Description of method employed. in $ 73.184, hold to the light if necessary (iii) Tabulation of complete data. and adjust until the curve most closely (iv) Curve showing antenna resistmatching the points is found. This curve ance versus frequency. should then be drawn on the sheet on (7) Antenna current or currents which the points were plotted, together maintained during field intensity measwith the inverse distance curve cor- urements, responding to that curve. The field at 1 (8) Description, accuracy, date, and mile for the radial concerned shall be the by whom each instrument was last caliordinate on the inverse distance curve at brated. 1 mile.

(9) Name, address, and qualifications (4) When all radials have been ana- of the engineer making the measurelyzed in accordance with subparagraph ments. (3) of this paragraph, a curve shall be (10) Any other pertinent information. plotted on polar coordinate paper from the fields obtained, which gives the in- § 73.187 Limitation on daytime radiaverse distance field pattern at 1 mile.

tion. The radius of a circle, the area of which (a) (1) Except as otherwise provided is equal to the area bounded by this in subparagraphs (2) and (3) of this pattern, is the effective field. (See

paragraph, no authorization will be $ 73.14.)

granted for Class II facilities if the pro(5) While making the field intensity

posed facilities would radiate, during the survey, the output power of the station

two hours after local sunrise and the shall be maintained at the licensed power two hours before local sunset, toward as determined by the direct method. To

any point on the 0.1 mv/m contour of do this, it is necessary to determine ac- a co-channel U.S. Class I station, at or curately the total antenna resistance

below the pertinent vertical angle deter(the resistance variation method, the

mined from Curve 4 of Figure 68 of substitution method or bridge method

§ 73.190, values in excess of those obis acceptable) and to measure the an

tained as provided in paragraph (b) of tenna current by means of an ammeter

this section. of acceptable accuracy. (See $$ 73.39

(2) The limitation set forth in suband 73.54.)

paragraph (1) of this paragraph shall (b) Complete data taken in conjunc

not apply in the following cases: tion with the field intensity measure

(1) Any Class II facilities authorized ments shall be submitted to the Commis

before November 30, 1959; or sion in affidavit form including the

(ii) For Class II stations authorized befollowing:

fore November 30, 1959, subsequent (1) Tabulation by number of each

changes of facilities which do not inpoint of measurement to agree with the

volve a change in frequency, an increase map required in (2) below and the field

in radiation toward any point on the intensity meter reading, the attenuation

0.1 mv/m contour of a co-channel U.S. constant, the field intensity (E), the dis

Class I station, or the move of transmite tance from the antenna (D) and the

ter site materially closer to the 0.1 mv/m product of the field intensity and dis

contour of such Class I stations. tance (ED) (if data for each radial are

(3) If a Class II station authorized beplotted on semi-logarithmic paper, see

fore November 30, 1959, is authorized above) for each point of measurement.

to increase its daytime radiation in any (2) Map showing each point of meas

direction toward the 0.1 mv/m contour urement numbered to agree with tabu

of a co-channel U.S. Class I station lation required above.

(without a change in frequency or a (3) Description of method used to

move of transmitter site materially take field intensity measurements. closer to such contour), it may not,

(4) The family of theoretical curves during the two hours after local sunrise used in determining the curve for each or the two hours before local sunset, radial properly identified by conductivity radiate in such directions a value exand dielectric constants.

ceeding the higher of: (5) The curves drawn for each radial (i) The value radiated in such direcand the field intensity pattern.

tions with facilities last authorized be(6) Antenna resistance measurement: fore November 30, 1959, or

(i) Antenna resistance at operating (ii) The limitation specified in subfrequency.

paragraph (1) of this paragraph.

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(b) To obtain the maximum permis- $ 73.188 Location of transmitters. sible radiation for a Class II station on

(a) The four primary objectives to be a given frequency (fxc/s) from 640 kc/s

obtained in the selection of a site for a through 990 kc/s, multiply the radiation

transmitter of a broadcast station are as value obtained for the given distance

and azimuth from the 500 kc/s chart
(Figure 9 of $ 73.190) by the appropriate

(1) To serve adequately the center of interpolation factor shown in the Soo

population in which the studio is located column of paragraph (c) of this section;

and to give maximum coverage to adand multiply the radiation value ob

jacent areas. tained for the given distance and azi

(2) To cause and experience minimum muth from the 1000 kc/s chart (Figure 10

interference to and from other stations. of $ 73.190) by the appropriate interpola

(3) To present a minimum hazard to tion factor shown in the Klane column of

air navigation consistent with objectives paragraph (c) of this section. Add the (1) and (2). two products thus obtained; the result (4) To fulfill certain other requireis the maximum radiation value appli- ments given in the following paragraphs cable to the Class II station in the per- of this section. tinent directions. For frequencies from

(b) The site selected should meet the 1010 kc/s to 1580 kc/s, obtain in a similar following conditions: manner the proper radiation values (1) A minimum field intensity of 25 from the 1000 kc/s and 1600 kc/s charts

to 50 mv/m will be obtained over the (Figures 10 and 11 of $ 73.190), multiply

business or factory areas of the city. each of these values by the appropriate

(2) A minimum field intensity of 5 to interpolation factors in the K'1000 and

10 mv/m will be obtained over the most K 1800 columns in paragraph (c) of this

distant residential section. section, and add the products.

(3) The absorption of the signal is the (c) Interpolation factors. (1) Fre

minimum for any obtainable sites in the quencies below 1000 kc/s.

area. As a guide in this respect the absorption of the signals from other sta

tions in that area should be followed, as Ekolo K 300 K1000 fk clo K 500 K 1000 well as the results of tests on other sites.

(4) The population within the blanket 640. 0.720 0.280 780

0.440 0.560 contour does not exceed that specified
0.700 0.300 800

0.400 0.600
0.680 0.320 810

0. 380 0.620

by § 73.24(g).
0.660 0.340 820.

0.360 0.640

(c) In selecting a site in the center of 0.640 0.360 830

0.340 0.660
0. 620 0.380 840.

0.320 0.680 a city it is usually necessary to place the
0.600 0. 400



radiating system on the top of a build0.420 860.

0.280 0.720
0.560 0.440 870.


0.740 ing. This building should be large
0.540 0. 460 880

0. 240 0.760
0.520 0.480 890

0.220 0.780 enough to permit the installation of a
0.500 0.500 900

0. 200 0.800

satisfactory ground and/or counterpoise
0.480 0.520 940.

0. 120 0. 880
0.460 0.540 990

0.020 0.980 system. Great care must be taken to

avoid selecting a building surrounded by

taller buildings or where any nearby (2) Frequencies above 1000 kc/s.

building higher than the antenna is

located in the direction which it is def'kala f'keto K 1000 K 1600

sired to serve. Such a building will tend

to cast "radio shadows" which may ma1010. 0.983 0.017 1170. 0.717 0.283

terially reduce the coverage of the sta1020. 0.967 0.033 1180.

0.700 0. 300

tion in that direction. Irrespective of
0.950 0.050 1190

0.683 0. 317
0.933 0.067 1200.

0.667 0.333

the height of surrounding buildings, the 1050. 0.917 0.083 1210.

0.650 0.350 1060.

building on which the antenna is located 0.900 0.100 1220.

0.633 0. 367
0.883 0.117 1500.

0. 167

should not have height of approximately
0.867 0.133 1510.

0. 150 0.850 1090.

one-quarter wavelength. A study of 0.850 0.150


0. 133 0.867
0.833 0.167 1530.

0. 117 0.883

antenna systems located on buildings
0.817 0.183 1540.

0. 100 0.900
0.800 0. 200 1550.


tends to indicate that where the build

0.783 0.217 1560.

0.067 0.933

ing is approximately a quarter wave-
0.767 0.233 1570.

0.050 0.950

length in height, the efficiency of radia0. 267 1580.

0.033 0.967

tion may be materially reduced.

91 51-096-66


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K 1000

K 1000

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(d) Particular attention must be given to avoiding cross-modulation. In this connection, attention is invited to the fact that it has been found very unsatisfactory to locate broadcast stations so that high signal intensities occur in areas with overhead electric power or telephone distribution systems and sections where the wiring and plumbing are old or improperly installed. These areas are usually found in the older sections of a city. These conditions give rise to cross-modulation interference due to the nonlinear conductivity characteristics of contacts between wiring, plumbing, or other conductors. This type of interference is independent of the selectivity characteristics of the receiver and normally can be eliminated only by correction of the condition causing the interference, Cross-modulation tends to increase with frequency and in some areas it has been found impossible to eliminate all sources of cross-modulation, resulting in an unsatisfactory condition for both licensee and listeners. The Commission will not authorize (1) new stations, (2) a major change of facilities of existing stations, (3) a change in transmitter location of an existing station, or (4) auxiliary transmitters, for use with other than the authorized antenna system of the main transmitter, if such new stations, physical facilities of existing stations after a major change, transmitters or auxiliary transmitters would be located in such areas or would utilize a roof-top antenna and the operating power would be in excess of 1000 watts.

(e) If it is determined that a site should be selected removed from the city, there are several general conditions to be followed in determining the exact site. Three maps should be given consideration if available:

(1) Map of the density of population and number of people by sections in the area. (See Bureau of Census series P-D and H-E available from Superintendent of Documents, Government Printing Office, Washington, D.C., 20402.)

(2) Geographical contour map with contour intervals of 20 to 50 feet.

(3) Map showing the type, nature and depth of the soil in the area with special

reference to the condition of the moisture throughout the year. From these maps a site should be selected with a minimum number of intervening hills between it and the center of the city. In general, because of ground conditions, it is better to select a site in a low area rather than on top of a hill, and the only condition under which a site on top of a hill should be selected is that it is only possible by this means to avoid a substantial number of hills, between the site and the center of a city with the resulting radio shadows. If a site is to be selected to serve a city which is on a general sloping area, it is generally better to select a site below the city than above the city.

(f) If a compromise must be made between probable radio shadows from in. tervening hills and locating the transmitter on top of a hill, it is generally better to compromise in favor of the low area, where an efficient radiating system may be installed which will more than compensate for losses due to shadows being caused by the hills, if not too numerous or too high. Several transmitters have been located on tops of hills, but so far as data has been supplied not a single installation has given superior efficiency of propagation and coverage.

(g) The ideal location of a broadcast transmitter is in a low area of marshy or "crawfishy" soil or area which is damp the maximum percentage of time and from which a clear view over the entire center of population may be had and the tall buildings in the business section of the city would cast a shadow across the minimum residential area.

(h) The type and condition of the soil or earth immediately around a site is very important. Important, to an equal extent, is the soil or earth between the site and the principal area to be served. Sandy soil is considered the worst type, with glacial deposits and mineral-ore areas next. Alluvial, marshy areas and salt-water bogs have been found to have the least absorption of the signal. One is fortunate to have available such an area and, if not available, the next best condition must be selected.

(i) Figure M3 (See Note to $ 73.183 (c)) and Figure R3 of $ 73.190 indicate

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