Sec.  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
    Sec. 73.213 or  Sec. 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  Sec. 73.215 or Sec.  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  Sec. 73.215 on
   Channels 221 through 300, as required by  Sec. 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 an AM
   antenna tower, or locate within 3.2 km of an AM antenna tower, the FM
   licensee or permittee must comply with  Sec. 73.1692.

   [ 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]