Sec. 21.906  Antennas.

    (a) 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. When an 
applicant proposes to employ a directional antenna, the applicant shall 
provide the Commission with information regarding the orientation of the 
directional antenna, expressed in degree of azimuth, with respect to 
true north.
    (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 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 
antenna height (above ground and above mean sea level) for each location 
has been obtained from the Commission prior to the erection of the 
antenna. Requests for such authorization shall show the inclusive dates 
of the proposed operation. (See part 17 of this chapter concerning the 
construction, marking and lighting of antenna structures.)
[ 44 FR 60534 , Oct. 19, 1979, as amended at  52 FR 37786 , Oct. 9, 1987;  58 FR 44896 , Aug. 25, 1993]