47 CFR § 90.241 - Radio call box operations.
(a) The frequencies in the 72-76 MHz band listed in § 90.257(a)(1) may be assigned in the Public Safety Pool for operation of radio call boxes to be used by the public to request fire, police, ambulance, road service, and other emergency assistance, subject to the following conditions and limitations:
(1) Maximum transmitter power will be either 2.5 watts plate input to the final stage or 1 watt output.
(2) Antenna gain shall not exceed zero dBd (referred to a half-wave dipole) in any horizontal direction.
(3) Only vertical polarization of antennas shall be permitted.
(4) The antenna and its supporting structure must not exceed 6.1 m (20 feet) in height above the ground.
(5) Only A1D, A2D, F1D, F2D, G1D, or G2D emission shall be authorized.
(6) The transmitter frequency tolerance shall be 0.005 percent.
(7) Except for test purposes, each transmission must be limited to a maximum of two seconds and shall not be automatically repeated more than two times at spaced intervals within the following 30 seconds. Thereafter, the authorized cycle may not be reactivated for one minute.
(8) All transmitters installed after December 10, 1970, shall be furnished with an automatic means to deactivate the transmitter in the event the carrier remains on for a period in excess of three minutes. The automatic cutoff system must be designed so the transmitter can be only manually reactivated.
(9) Frequency selection must be made with regard to reception of television stations on channels 4 (66-72 MHz) and 5 (76-82 MHz) and should maintain the greatest possible frequency separation from either or both of these channels, if they are assigned in the area.
(c) Frequencies in the 450-470 MHz band which are designated as available for assignment to central control stations and radio call box installations in § 90.20(c) or § 90.20(d)(58) may be assigned in the Public Safety Pool for highway call box systems subject to the following requirements:
(1) Call box transmitters shall be installed only on limited access highways and may communicate only with central control stations of the licensee.
(2) Maximum transmitter power for call boxes will be either 2.5 watts input to the final amplifier stage or one watt output. The central control station shall not exceed 25 watts effective radiated power (ERP).
(3) The height of a call box antenna may not exceed 6.1 meters (20 feet) above the ground, the natural formation, or the existing man-made structure (other than an antenna supporting structure) on which it is mounted. A central station transmitting antenna, together with its supporting structure shall not exceed 15 m. (50 ft.) above the ground surface.
(4) Only F1D, F2D, F3E, G1D, G2D, or G3E, emission may be authorized for nonvoice signaling, radiotelephony, and multiplexed voice and nonvoice use. The provisions in this part applicable to the use of F3E or G3E emission are also applicable to the use of F1D, F2D, G1D or G2D emission for call box transmitters.
(5) The station identification required by § 90.425 shall be by voice and may be transmitted for the system from the central control station. Means shall be provided at each central control station location to automatically indicate the call box unit identifier when a call box unit is activated.
(6) Call box installations must be so designed that their unit identifier is automatically transmitted when the handset is lifted.
(7) Each application for a call box system must contain information on the nonvoice transmitting equipment, including the character structure, bit rate, modulating tone frequencies, identification codes, and the method of modulation (i.e., frequency shift, tone shift, or tone phase shift).
(8) Call box installations may be used secondarily for the transmission of information from roadside sensors. Central control station transmitters may be used secondarily to interrogate call box roadside sensors and for the transmission of signals to activate roadside signs.
(9) Each call box transmitter must be provided with a timer which will automatically deactivate the transmitter after 2 minutes unless the central control station operator reactivates the timer cycle.
(10) The central control station must include facilities that permit direct control of any call box in the system.
(11) Call box transmitter frequency tolerance shall be 0.001 percent.
(12) Transmitters certificated under this part for use of F3E or G3E emission may be used for F1D, F2B, G2B or G2D emission provided that the audio tones or digital data bits are passed through the low pass audio filter required to be provided in the transmitter for F3E or G3E emission. The transmitter must be adjusted and operated so that the instantaneous frequency deviation does not exceed the maximum value allowed for F3E or G3E emission.
(d) In addition to the frequencies available pursuant to § 90.20(c) the frequencies set forth in § 90.20(d)(58) may be used for central control station and call box installations in areas where such frequencies are available for fixed system use subject to the requirements and limitations of that section and subject to the provisions of paragraphs (c) (1), (4), (5), (6), (7), (8), (9), (10), and (12) of this section.