Sec.  21.906   Antennas.

   

   (a) Main and booster station transmitting antennas shall be
   omnidirectional, except that a directional antenna with a main beam
   sufficiently broad to provide adequate service may be used either to
   avoid possible interference with other users in the frequency band, or
   to provide coverage more consistent with distribution of potential
   receiving points. In lieu of an omnidirectional antenna, a station may
   employ an array of directional antennas in order to reuse spectrum
   efficiently. When an applicant proposes to employ a directional
   antenna, or a licensee notifies the Commission pursuant to Sec. 21.42 of
   the installation of a sectorized antenna system, the applicant shall
   provide the Commission with information regarding the orientation of
   the directional antenna(s), expressed in degree of azimuth, with
   respect to true north, and the make and model of such antenna(s).

   (b) The use of horizontal or vertical plane wave polarization, or
   right hand or left hand rotating elliptical polarization may be used
   to minimize the hazard of harmful interference between systems.

   (c) Transmitting antennas located within 56.3 kilometers (35 miles) of
   the Canadian border should be directed so as to minimize, to the
   extent that is practical, emissions toward the border.

   (d) Directive receiving antennas shall be used at all points other
   than response station hubs and response stations operating with an
   EIRP no greater than -6 dBW per 6 MHz channel and shall be elevated no
   higher than necessary to assure adequate service. Receiving antenna
   height shall not exceed the height criteria of Part 17 of this
   chapter, unless authorization for use of a specific maximum height
   (above ground and mean sea level) for each location has been obtained
   from the Commission prior to the erection of the antenna. (See part 17
   of this chapter concerning construction, marking and lighting of
   antenna structures.) A response station operating with an EIRP no
   greater than -6 dBW per 6 MHz channel may use an omnidirectional
   receiving antenna. However, for the purpose of interference
   protection, such response stations will be treated as if utilizing a
   receive antenna meeting the requirements of the reference receiving
   antenna of Figure 1 of Sec. 21.902(f)(3).

   [ 44 FR 60534 , Oct. 19, 1979, as amended at  52 FR 37786 , Oct. 9, 1987;
    58 FR 44896 , Aug. 25, 1993;  63 FR 65104 , Nov. 25, 1998;  64 FR 4054 ,
   Jan. 27, 1999;  64 FR 63733 , Nov. 22, 1999;  65 FR 46617 , July 31, 2000]