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required. Bandwidths in excess of 5 Mc/s will not be authorized.

(f) The frequencies listed in this paragraph are available for assignment to control and repeater stations functioning in conjunction with the Domestic Public Land Mobile Radio Service, on a shared basis with certain other radio services. A repeater station normally will not be authorized unless the land mobile radio system with which it is associated is continuously open for public correspondence and the emission of the repeater station are under the operational surveillance of the land mobile system's operating personnel.

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1 Assignments made to stations on frequencies in this band are subject to the condition that no harmful interference will be caused to operational fixed stations or reception of television stations on channels 4 or 5. (See § 21.103.) Existing stations authorized in the 73 to 74.6 Mc/s band as of December 1, 1961, may continue to operate and shall not be required to afford protection to the radio astronomy service.

(g) New control and repeater stations will not be authorized in the 890-940 Mc's band. However, stations which were authorized to operate on such frequencies on April 16, 1958, may be granted renewed licenses subject to the following conditions:

(1) Operations shall not be protected against any interference received from the emission of industrial, scientific, and medical equipment operating on 915 Mc s or from the emission of radiolocation stations in the 890-942 Mc/s band.

(2) No harmful interference shall be caused to stations operating in the radiolocation service in the 890-942 Mc/s band.

(h) Stations now authorized in the band 890-940 Mc/s may be authorized to operate in the band 942-952 Mc/s on the following conditions:

(1) That such stations can show that harmful interference is being caused by Government radiopositioning stations in

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the 890-942 Mc/s band or by ISM equipment operating on 915 Mc/s.

(2) That an engineering study by the Commission indicates that the proposed frequency assignment in the band 942952 Mc/s is likely to eliminate the interference.

(3) That the bandwidth of emission does not exceed 1100 kc/s.

(4) That the proposed frequency assignment will not cause interference to existing operations in the band 942-952 Mc/s.

(i) In lieu of a wireline circuit for control of a specific base station transmitter from its required control point or in lieu of wirelines for an audio circuit to a base station control point from a remotely located fixed receiver used for reception of mobile station transmissions, and upon an affirmative showing that the conditions set forth in subparagraphs (1) through (5) of this paragraph are satisfied, a single control and repeater station may be authorized to miscellaneous common carriers upon the frequencies indicated below:

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(1) The control station, and the base station controlled thereby, are located over 50 airline miles from the nearest geographical boundary of the nearest urbanized area having a population over 300,000 (as determined and defined in the most recent census reports of the U.S. Bureau of the Census).

(2) The repeater station, and the point to which its transmissions are directed, are located over 50 airline miles from the nearest geographical boundary of the nearest urbanized area having a population over 300,000 (as determined and defined in the most recent census reports of the U.S. Bureau of the Census).

(3) The effective radiated power of the control or repeater station does not exceed 150 watts.

(4) The use of the frequencies by a control or repeater station will not cause harmful interference to any other station authorized to use such frequencies and shall be on a secondary basis to the

provision of mobile and rural radio service by other classes of stations.

(5) Series operation of more than one control or repeater station is not involved.

NOTE: The provisions of subparagraphs (1) and (2) of this paragraph may be waived by the Commission upon a factual showing, supported by such engineering proof as may be necessary, that all of the currently assignable pairs of 152-162 Mc/s band frequencies listed in paragraph (c) of this section are not assigned or applied for within interference range of existing or possible station assignments within the urbanized area having a population of over 300,000 and, upon a satisfactory showing, that in such area over a substantial period of years the growth of the public land mobile radio service has not been hampered and is not likely to be hampered by a shortage of frequencies allocated to such service in the 152 to 162 Mc/s band. Facilities authorized under the provisions of such waivers shall be on a secondary basis and subject to the condition that, in the event the frequencies are required for assignment to base and mobile stations in the area, operation thereon shall be terminated within 60 days after notice is received from the Commission.

[28 F.R. 13002, Dec. 5, 1963, as amended at 29 F.R. 2700, Feb. 26, 1964]

§ 21.502 Classification of base stations.

Base stations of miscellaneous common carriers in the Domestic Public Land Mobile Radio Service shall be classified, as set forth below, according to their transmitting antenna height above average terrain, and according to their effective radiated power in the horizontal plane of the antenna.

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In any particular case, where it appears that unusual radio wave propagation conditions are involved, the Commission may require greater separation than indicated in the above table, or make assignments at lesser station spacing. Reference may be made to § 73.611(d) of this chapter, Radio Broadcast Services, for methods of computing milage separation between station locations.

§ 21.504 Service area of base station.

(a) The limits of reliable service area of a base station are considered to be described by a field strength contour of 37 decibels above one microvolt per meter for stations engaged in two-way communication service with mobile stations and 43 decibels above one microvolt per meter for stations engaged in one-way signaling service. Service within that area is generally expected to have an average reliability of not less than 90 percent.

(b) The field strength contours described in paragraph (a) of this section shall be regarded as determining the limits of the reliable service area of the related base stations for the purpose of providing protection to such stations from co-channel electrical harmful interference and defining the area within which consideration will be accorded claims of economic competitive injury. § 21.505 Antenna height-power limit

for base stations.

In view of the fact that the predominant need for mobile communication service can usually be met by base stations within the classification set forth in § 21.502, and because widespread coverage is undesirable in areas where no substantial need exists for mobile communication service through a distant

base station, base stations will not be authorized to employ transmitting antennas in excess of 500 feet above average terrain unless the effective radiated power of the base station is reduced below 500 watts by not less than the 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.)

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REQUIRED REDUCTION IN EFFECTIVE RADIATED POWER FOR ANTENNAI HEIGHTS IN EXCESS OF 500 FEET ABOVE AVERAGE TERRAIN

600

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3000

4000

ANTENNA HEIGHT IN FEET ABOVE AVERAGE TERRAIN (See 21.505)

5000

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

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(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 emission 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.

§ 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 de

viation 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 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 audio frequencies between 3 kc/s and 15 kc/s, the filter shall have an attenuation greater than the attenuation at 1 kc/s by a least:

40 logo (f/3) decibels

where "f" is the audio frequency in kilocycles. At audio frequencies above 15 kc/s, the attenuation shall be at least 28 decibels greater than the attenuation at 1 kc/s.

§ 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

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