47 CFR §74.536

1. (a)Aural broadcast STL and ICR stations are required to use a directional antenna with the minimum beamwidth necessary, consistent with good engineering practice, to establish the link.
2. (b)An aural broadcast STL or intercity relay station operating in the 17.7-19.7 GHz band shall employ an antenna that meets the performance standards for Category A, except that in areas not subject to frequency congestion, antennas meeting standards for Category B may be employed. However, the Commission may require the replacement, at the licensee's expense, of any antenna or periscope antenna system of a permanent fixed station that does not meet performance Standard A, which is specified in the table in paragraph (c) of this section, upon a showing that said antenna causes or is likely to cause interference to (or receive interference from) any other authorized or proposed station; provided that an antenna meeting performance Standard A is unlikely to involve such interference.
3. (c)Licensees shall comply with the antenna standards table shown in this paragraph in the following manner:
   1. (1)With either the maximum beamwidth to 3 dB points requirement or with the minimum antenna gain requirement; and
   2. (2)With the minimum radiation suppression to angle requirement.