Sec.  73.189   Minimum antenna heights or field strength requirements.

   (a) Section 73.45 requires that all applicants for new, additional, or
   different broadcast facilities and all licensees requesting authority to
   move 0the transmitter of an existing station, shall specify a radiating
   system, the efficiency of which complies with the requirements of good
   engineering practice for the class and power of the station.

   (b) The specifications deemed necessary to meet the requirements of good
   engineering practice at the present state of the art are set out in detail
   below.

   (1) The licensee of a AM broadcast station requesting a change in power,
   time of operation, frequency, or transmitter location must also request
   authority to install a new antenna system or to make changes in the existing
   antenna system which will meet the minimum height requirements, or submit
   evidence that the present antenna system meets the minimum requirements with
   respect to field strength, before favorable consideration will be given
   thereto. (See  Sec. 73.186.) In the event it is proposed to make substantial
   changes in an existing antenna system, the changes shall be such as to meet
   the minimum height requirements or will be permitted subject to the
   submission of field strength measurements showing that it meets the minimum
   requirements with respect to effective field strength.

   (2) These minimum actual physical vertical heights of antennas permitted to
   be installed are shown by curves A, B, and C of Figure 7 of  Sec. 73.190 as
   follows:

   (i) Class C stations, and stations in Alaska, Hawaii, Puerto Rico and the
   U.S. Virgin Islands on 1230, 1240, 1340, 1400, 1450 and 1490 kHz that were
   formerly Class C and were redesignated as Class B pursuant to  Sec. 73.26(b), 45
   meters or a minimum effective field strength of 241 mV/m for 1 kW (121 mV/m
   for 0.25 kW). (This height applies to a Class C station on a local channel
   only. Curve A shall apply to any Class C stations in the 48 conterminous
   States that are assigned to Regional channels.)

   (ii) Class A (Alaska), Class B and Class D stations other than those covered
   in  Sec. 73.189(b)(2)(i), a minimum effective field strength of 282 mV/m for 1
   kW.

   (iii) Class A stations, a minimum effective field strength of 362 mV/m for 1
   kW.

   (3) The heights given on the graph for the antenna apply regardless of
   whether the antenna is located on the ground or on a building. Except for
   the reduction of shadows, locating the antenna on a building does not
   necessarily increase the efficiency and where the height of the building is
   in the order of a quarter wave the efficiency may be materially reduced.

   (4) At the present development of the art, it is considered that where a
   vertical radiator is employed with its base on the ground, the ground system
   should consist of buried radial wires at least one-fourth wave length long.
   There should be as many of these radials evenly spaced as practicable and in
   no event less than 90. (120 radials of 0.35 to 0.4 of a wave length in
   length and spaced 3 ° is considered an excellent ground system and in case of
   high base voltage, a base screen of suitable dimensions should be employed.)

   (5) In case it is contended that the required antenna efficiency can be
   obtained with an antenna of height or ground system less than the minimum
   specified, a complete field strength survey must be supplied to the
   Commission showing that the field strength at a mile without absorption
   fulfills the minimum requirements. (See  Sec. 73.186.) This field survey must be
   made by a qualified engineer using equipment of acceptable accuracy.

   (6) The main element or elements of a directional antenna system shall meet
   the above minimum requirements with respect to height or effective field
   strength. No directional antenna system will be approved which is so
   designed that the effective field of the array is less than the minimum
   prescribed for the class of station concerned, or in case of a Class A
   station less than 90 percent of the ground wave field which would be
   obtained from a perfect antenna of the height specified by Figure 7 of
    Sec. 73.190 for operation on frequencies below 1000 kHz, and in the case of a
   Class B or Class D station less than 90 percent of the ground wave field
   which would be obtained from a perfect antenna of the height specified by
   Figure 7 of  Sec. 73.190 for operation on frequencies below 750 kHz.

   [ 28 FR 13574 , Dec. 14, 1963, as amended at  31 FR 8069 , June 8, 1966;  33 FR 15420 , Oct. 17, 1968;  44 FR 36038 , June 20, 1979;  50 FR 18844 , May 2, 1985;
    51 FR 2707 , Jan. 21, 1986;  51 FR 4753 , Feb. 7, 1986;  52 FR 10570 , Apr. 2,
   1987;  56 FR 64868 , Dec. 12, 1991]