ANTENNA HEIGHT IN FEET ABOVE AVERAGE TERRAIN 1500 (See § 21.505)

amount shown in the following chart entitled "Required Reduction in Effective Radiated Power for Antenna Heights in Excess of 500 Feet Above Average Terrain".

§ 21.506 Power limitations.

Stations in this service shall not be permitted to exceed 500 watts effective radiated power and shall not be authorized to use transmitters having a rated power output in excess of the limits set forth in § 21.107(b): Provided, however, That the effective radiated power of dispatch stations shall not be permitted to exceed 100 watts. A base station standby transmitter having a rated power output in excess of that of the main transmitter of the base station with which it is associated will not be authorized.

§ 21.507 Bandwidth and emission limitations.

(a) Stations in the Domestic Public Land Mobile Radio Service shall normally be authorized to use only type A3 or F3 emission for radiotelephony, and type A1, A2, F1, or F2 emission for selective signaling. The authorization to

use type A3 or F3 emission for radiotelephony shall be construed to include the use of tone signals or signaling devices the sole function of which is to establish and maintain communication. Stations providing one-way signaling service by means of selective signaling which is not associated with a voice message are required to be authorized to employ telegraph type emission appropriate to the type of transmission employed. The use of types A0 or FO emission in the 72-76 Mc/s band will not be authorized, except for temporary or short periods necessary for testing incident to the construction or maintenance of a radio station. (Further reference should be made to §§ 21.103 to 21.105, inclusive.)

(b) The maximum authorized bandwidth of emission and, for the cases of frequency or phase modulated emissions, the maximum authorized frequency deviation shall be as follows:

(c) On frequencies above 500 Mc/s, the bandwidth authorized shall not exceed 400 kc/s for each derived communication channel and may be restricted to lesser bandwidth when appropriate to the type of operation involved in any particular case.

(d) Other types of emission or bandwidth in excess of those specified in paragraph (b) of this section may be authorized upon an adequate showing of need therefor. An application requesting such authorization shall fully describe the modulation characteristics (for FM include maximum modulating frequency and

maximum frequency deviation) emission and bandwidth desired, shall specify the bandwidth to be occupied, and shall state the reasons why such emission or bandwidth is required.

(e) The authorization to employ any of the various types of modulated emis

sion in this service shall be construed to include authority to employ unmodulated emission only for temporary or short periods necessary for equipment testing incident to the construction and maintenance of a radio station.

[28 F.R. 13002, Dec. 5, 1963, as amended at 33 F.R. 4578, Mar. 15, 1968]

§ 21.508 Modulation requirements.

(a) The use of modulating frequencies higher than 3000 cycles per second for radiotelephony or tone signaling is not authorized on frequencies below 500 Mc/s.

(b) [Reserved]

(c) When amplitude modulation is used, the modulation percentage shall be sufficient to provide efficient communication and shall normally be maintained above 70 percent on peaks, but

shall not exceed 100 percent on negative peaks.

(d) When phase or frequency modulation is used for single channel operation on frequencies below 500 Mc/s, the deviation arising from modulation shall not exceed the limits specified in § 21.507(b).

(e) Each transmitter, which has more than 3 watts plate power input to the final radio frequency stage and was initially authorized or installed at the station in this service after July 1, 1950, employing type A3 or F3 emission shall be equipped with a device which will automatically prevent greater than normal audio level from modulating in excess of the limits specified in paragraphs (c) and (d) of this section.

(f) Each transmitter, which operates on frequencies below 450 Mc/s and employs type A3 or F3 emission, shall be equipped with a modulation limiter in accordance with the provisions of paragraph (e) of this section and also shall be equipped with a low-pass audio filter installed between the modulation limiter and the modulated stage. At audiofrequencies between 3 kc/s and 15 kc/s, the filter shall have an attenuation greater than the attenuation of 1 kc/s by at least:

40 log10 (1/3) decibels

where "f" is the audiofrequency in kilocycles per second. At audiofrequencies above 15 kc/s, the attenuation shall be at least 28 decibels greater than the attenuation at 1 kc/s.

(g) Each transmitter which operates on frequencies between 450 Mc/s and 470 Mc/s and employs type A3 or F3 emission, shall be equipped with a modulation limiter in accordance with the provisions of paragraph (e) of this section and also shall be equipped with a low-pass audio filter installed between the modulation limiter and the modulated stage. At audiofrequencies between 3 kc/s and 20 kc/s, the filter shall have an attenuation greater than the attenuation at 1 kc/s by at least:

60 log10 (1/8) decibels

where "f" is the audio frequency in kilocycles per second. At audio frequencies above 20 kc/s, the attenuation shall be

at least 50 decibels greater than the attenuation at 1 kc/s: Provided, however, That in lieu of such filter, transmitters authorized to operate between 450 Mc/s and 470 Mc/s prior to June 1, 1968, may continue to operate until November 1, 1971, with a filter meeting the requirements prescribed in paragraph (f) of this section.

[28 F.R. 13002, Dec. 5, 1963, as amended at 33 F.R. 4578, Mar. 15, 1968]

§ 21.509

Permissible communications.

(a) Mobile stations in this service are authorized to communicate with and through base stations only. Such communication between base and mobile stations shall be upon frequencies which are paired in the manner set forth in § 21.501 (a), (b), and (c).

(b) Base stations in this service are normally authorized to communicate with associated land mobile stations in the same service: Provided, however, That base stations may also render service to properly licensed transient land mobile stations normally associated with another common carrier base station, including mobile units of Canadian registry. Under specific authorization from the Commission, service may be rendered to stations on board vessels. Authority to provide service to vessels may be granted upon a showing that the rendition of service to vessels will not degrade, by kind or extent of usage, the service which would be available, in the absence of service to vessels, to land vehicles receiving or subsequently requesting such services in the area: Provided, That any authorization to render service to vessels, other than itinerant vessels requiring service in other areas where VHF public coast service is not available, shall automatically terminate at the expiration of 60 days after inauguration of VHF public coast service in the area involved. Applications for authority to communicate with vessels shall be accompanied by a showing of the following:

(1) The total number of land mobile units being served through the base station.

(2) The total number of land mobile units to which it is reasonably expected