§  73.316   FM antenna systems.

   (a) It shall be standard to employ horizontal polarization; however,
   circular or elliptical polarization may be employed if desired.
   Clockwise or counterclockwise rotation may be used. The supplemental
   vertically polarized effective radiated power required for circular or
   elliptical polarization shall in no event exceed the effective radiated
   power authorized.

   (b) Directional antennas. A directional antenna is an antenna that is
   designed or altered for the purpose of obtaining a non-circular
   radiation pattern.

   (1) Applications for the use of directional antennas that propose a
   ratio of maximum to minimum radiation in the horizontal plane of more
   than 15 dB will not be accepted.

   (2) Directional antennas used to protect short-spaced stations pursuant
   to § 73.213 or § 73.215 of the rules, that have a radiation pattern which
   varies more than 2 dB per 10 degrees of azimuth will not be authorized.

   (c) Applications for directional antennas. (1) Applications for
   construction permit proposing the use of directional antenna systems
   must include a tabulation of the composite antenna pattern for the
   proposed directional antenna. A value of 1.0 must be used to correspond
   to the direction of maximum radiation. The pattern must be tabulated
   such that 0° corresponds to the direction of maximum radiation or
   alternatively, in the case of an asymmetrical antenna pattern, the
   pattern must be tabulated such that 0° corresponds to the actual
   azimuth with respect to true North. In the case of a composite antenna
   composed of two or more individual antennas, the pattern required is
   that for the composite antenna, not the patterns for each of the
   individual antennas. Applications must include valuations tabulated at
   intervals of not greater than ten (10) degrees. In addition, tabulated
   values of all maximas and minimas, with their corresponding azimuths,
   must be submitted.

   (2) Applications for license upon completion of antenna construction
   must include the following:

   (i) A complete description of the antenna system, including the
   manufacturer and model number of the directional antenna. It is not
   sufficient to label the antenna with only a generic term such as
   "dipole." In the case of individually designed antennas with no model
   number, or in the case of a composite antenna composed of two or more
   individual antennas, the antenna must be described as a "custom" or
   "composite" antenna, as appropriate. A full description of the design
   of the antenna must also be submitted.

   (ii) A plot of the composite pattern of the directional antenna. A
   value of 1.0 must be used to correspond to the direction of maximum
   radiation. The plot of the pattern must be oriented such that 0°
   corresponds to the direction of maximum radiation or alternatively, in
   the case of an asymmetrical antenna pattern, the plot must be oriented
   such that 0° corresponds to the actual azimuth with respect to true
   North. The horizontal plane pattern must be plotted to the largest
   scale possible on unglazed letter-size polar coordinate paper (main
   engraving approximately 18 cm × 25 cm (7 inches × 10 inches)) using
   only scale divisions and subdivisions of 1, 2, 2.5, or 5 times 10-nth.
   Values of field strength less than 10% of the maximum field strength
   plotted on that pattern must be shown on an enlarged scale. In the case
   of a composite antenna composed of two or more individual antennas, the
   composite antenna pattern should be provided, and not the pattern for
   each of the individual antennas.

   (iii) A tabulation of the measured relative field pattern required in
   paragraph (c)(1) of this section. The tabulation must use the same zero
   degree reference as the plotted pattern, and must contain values for at
   least every 10 degrees. Sufficient vertical patterns to indicate
   clearly the radiation characteristics of the antenna above and below
   the horizontal plane. Complete information and patterns must be
   provided for angles of -10 deg. from the horizontal plane and
   sufficient additional information must be included on that portion of
   the pattern lying between + 10 deg. and the zenith and -10 deg. and the
   nadir, to conclusively demonstrate the absence of undesirable lobes in
   these areas. The vertical plane pattern must be plotted on rectangular
   coordinate paper with reference to the horizontal plane. In the case of
   a composite antenna composed of two or more individual antennas, the
   composite antenna pattern should be used, and not the pattern for each
   of the individual antennas.

   (iv) A statement that the antenna is mounted on the top of an antenna
   tower recommended by the antenna manufacturer, or is side-mounted on a
   particular type of antenna tower in accordance with specific
   instructions provided by the antenna manufacturer.

   (v) A statement that the directional antenna is not mounted on the top
   of an antenna tower which includes a top-mounted platform larger than
   the nominal cross-sectional area of the tower in the horizontal plane.

   (vi) A statement that no other antenna of any type is mounted on the
   same tower level as a directional antenna, and that no antenna of any
   type is mounted within any horizontal or vertical distance specified by
   the antenna manufacturer as being necessary for proper directional
   operation.

   (vii) A statement from an engineer listing such individual engineer's
   qualifications and certifying that the antenna has been installed
   pursuant to the manufacturer's instructions.

   (viii) A statement from a licensed surveyor that the installed antenna
   is properly oriented.

   (ix)(A) For a station authorized pursuant to § 73.215 or Sec. § 73.509, a
   showing that the root mean square (RMS) of the measured composite
   antenna pattern (encompassing both the horizontally and vertically
   polarized radiation components (in relative field)) is at least 85
   percent of the RMS of the authorized composite directional antenna
   pattern (in relative field). The RMS value, for a composite antenna
   pattern specified in relative field values, may be determined from the
   following formula:

   RMS = the square root of:

   [(relative field value 1)2 + (relative field value 2)2 + .... + (last
   relative field value)2]

   total number of relative field values

   (B) where the relative field values are taken from at least 36 evenly
   spaced radials for the entire 360 degrees of azimuth. The application
   for license must also demonstrate that coverage of the community of
   license by the 70 dBu contour is maintained for stations authorized
   pursuant to § 73.215 on Channels 221 through 300, as required by
   § 73.315(a), while noncommercial educational stations operating on
   Channels 201 through 220 must show that the 60 dBu contour covers at
   least a portion of the community of license.

   (d) Applications proposing the use of FM transmitting antennas in the
   immediate vicinity (i.e. 60 meters or less) of other FM or TV broadcast
   antennas must include a showing as to the expected effect, if any, of
   such approximate operation.

   (e) Where an FM licensee or permittee proposes to mount its antenna on
   or near an AM tower, as defined in § 1.30002, the FM licensee or
   permittee must comply with § 1.30003 or § 1.30002, depending on whether
   the antenna is proposed to be mounted on an AM tower (§ 1.30003) or near
   an AM tower (§ 1.30002).

   [ 28 FR 13623 , Dec. 14, 1963, as amended at  34 FR 14222 , Sept. 10, 1969;
    37 FR 25841 , Dec. 5, 1972;  43 FR 53738 , Nov. 17, 1978;  48 FR 29508 ,
   June 27, 1983;  51 FR 17028 , May 8, 1986;  54 FR 9804 , Mar. 8, 1989;  56 FR 57294 , Nov. 8, 1991;  62 FR 51058 , Sept. 30, 1997;  63 FR 70047 , Dec.
   18, 1998;  78 FR 66298 , Nov. 5, 2013]