Sec. 73.54  Antenna resistance and reactance measurements.

    (a) The resistance of an omnidirectional series fed antenna is 
measured at either the base of the antenna without intervening coupling 
or tuning networks, or at the point the transmission line connects to 
the output terminals of the transmitter. The resistance of a shunt 
excited antenna may be measured at the point the radio frequency energy 
is transferred to the feed wire circuit or at the output terminals of 
the transmitter.
    (b) The resistance and reactance of a directional antenna shall be 
measured at the point of common radiofrequency input to the directional 
antenna system. The following conditions shall obtain:
    (1) The antenna shall be finally adjusted for the required radiation 
pattern.
    (2) The reactance at the operating frequency and at the point of 
measurement shall be adjusted to zero, or as near thereto as 
practicable.
    (c)(1) The resistance of an antenna shall be determined by the 
following procedure: A series of discrete measurements shall be made 
over a band of frequencies extending from approximately 25 kHz below the 
operating frequency to approximately 25 kHz above that frequency, at 
intervals of approximately 5 kHz. The measured values shall be plotted 
on a linear graph, with frequency as the abscissa and resistance as the 
ordinate. A smooth curve shall be drawn through the plotted values. The 
resistance value corresponding to the point of intersection of the curve 
and the ordinate representing the operating frequency of the station 
shall be the resistance of the antenna.
    (2) For a directional antenna, the reactance of the antenna shall be 
determined by a procedure similar to that described in paragraph (c)(1) 
of this section.
    (d) Notification must be filed with the FCC in Washington, DC when 
determining power by the direct method pursuant to Sec. 73.51 and must 
specify the antenna or common point resistance at the operating 
frequency. The following information must also be kept on file at the 
station:
    (1) A full description of the method used to make measurements.
    (2) A schematic diagram showing clearly all components of coupling 
circuits, the point of resistance measurement, the location of the 
antenna ammeter, connections to and characteristics of all tower 
lighting isolation circuits, static drains, and any other fixtures 
connected to and supported by the antenna, including other antennas and 
associated networks. Any network or circuit component used to dissipate 
radio frequency power shall be specifically identified, and the 
impedances of all components which control the level of power 
dissipation, and the effective input resistance of the network must be 
indicated.
    (e) AM stations using direct reading power meters in accordance with 
Sec. 73.51, can either submit the information required by paragraph (d) 
of this section or submit a statement indicating that such a meter is 
being used. Subsequent station licenses will indicate the use of a 
direct reading power meter in lieu of the antenna resistance value in 
such a situation.

[ 34 FR 18305 , Nov. 15, 1969, as amended at  37 FR 7517 , Apr. 15, 1972;  45 FR 26062 , Apr. 17, 1980;  49 FR 49850 , Dec. 24, 1984;  50 FR 32416 , Aug. 
12, 1985;  51 FR 2707 , Jan. 21, 1986;  51 FR 26250 , July 22, 1986]