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 deg. 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

[[Page 61]]

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]