Sec. 73.151  Field strength measurements to establish performance of 
          directional antennas.

    (a) In addition to the information required by the license 
application form, the following showing must be submitted to establish 
for each mode of directional operation, that the effective measured 
field strength (RMS) at 1 mile is not less than 85 percent of the 
effective field strength specified for the standard radiation pattern 
for that mode of directional operation, or less than that specified in 
Sec. 73.189(b) for the class of station involved, whichever is the 
higher value, and that the measured field strength at 1 mile in any 
direction does not exceed the field shown in that direction on the 
standard radiation pattern for that mode of directional operation:
    (1) A tabulation of inverse field strengths in the horizontal plane 
at 1 mile, as determined from field strength measurements taken and 
analyzed in accordance with Sec. 73.186, and a statement of the 
effective field strength (RMS), based on these measurements. 
Measurements shall be made in at least the following directions:
    (i) Those specified in the instrument of authorization.
    (ii) In major lobes. Generally at least three radials are necessary 
to establish a major lobe; however, additional radials may be required.
    (iii) Along sufficient number of other radials to establish the 
effective field. In the case of a relatively simple directional antenna 
pattern, approximately five radials in addition to those in paragraphs 
(a)(1) (i) and (ii) of this section are sufficient. However, when more 
complicated patterns are involved, that is, patterns having several or 
sharp lobes or nulls, measurements shall be taken along as many radials 
as may be necessary, to definitely establish the pattern(s).

[[Page 38]]

    (2) A tabulation of:
    (i) The phase difference of the current in each other element with 
respect to the reference element, and whether the current leads (+) or 
lags (-) the current in the reference element, as indicated by the 
station's antenna monitor.
    (ii) The ratio of the amplitude of the radio frequency current in 
each other element to the current in the reference element, as indicated 
on the station's antenna monitor.
    (iii) The value of the radio frequency current at the base of each 
element, and the ratio of the current in each other element to the base 
current in the reference element. If there are substantial differences 
between the ratios established in paragraph (a)(2)(ii) of this section 
and the ratios computed in this paragraphs (a)(2)(iii) and/or if there 
are substantial differences between the parameters established in 
paragraphs (a)(2) (i) and (ii) of this section and this paragraph 
(a)(2)(iii), and those used in the design of the standard radiation 
pattern, a full explanation of the reasons for these differences shall 
be given.
    (3) The actual field strength measured at each monitoring point 
established in the various directions for which a limiting field was 
specified in the instrument of authorization together with accurate and 
detailed description of each monitoring point together with ordinary 
snapshots, clear and sharp, taken with the field strength meter in its 
measuring position and with the camera so located that its field of view 
takes in as many pertinent landmarks as possible. In addition, the 
directions for proceeding to each monitoring point together with a rough 
sketch or map upon which has been indicated the most accessible 
approaches to the monitoring points should be submitted.
    (b) For stations authorized to operate with simple directional 
antenna systems (e.g., two towers) in the 1605-1705 kHz band, the 
measurements to support pattern RMS compliance referred to in paragraphs 
(a)(1)(ii) and (a)(1)(iii) of this section are not required. In such 
cases, measured radials are required only in the direction of short-
spaced allotments, or in directions specifically identified by the 
Commission.

[ 36 FR 919 , Jan. 20, 1971, as amended at  42 FR 36828 , July 18, 1977;  49 FR 23348 , June 6, 1984;  50 FR 32416 , Aug. 12, 1985;  56 FR 64862 , Dec. 
12, 1991]