§  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 § 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 § 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
   § 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 § 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 § 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 § 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 § 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]