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Critical Hours Computations, Section 73.187
For AM broadcast stations, the term critical hours refers to the
time periods of sunrise to two hours after sunrise, and two hours before sunset to sunset.
During these periods, the ionosphere has commenced its transition from daytime to
nighttime conditions (or vice versa), resulting in greater coverage than would be
expected from a daytime-only analysis. But
because the transmitting station operates with its daytime power between sunrise and sunset,
the extended skywave signal can be strong enough to interfere with other stations.
This daytime skywave phenomenon was the focus of a protracted
rulemaking proceeding that commenced in 1947 and finally culminated in 1959 with the adoption of
Section 73.187, Limitation on daytime radiation (also called the
"critical hours" rule), and Figures 9, 10, and 11 in Section 73.190. See the Report
and Order in Docket 8333, adopted September 18, 1959 [ PDF ], petitions for reconsideration denied, adopted December 9, 1959 [ PDF ]. The rule provides for operation at a lower power during critical hours time
periods to limit interference from new or changed Class B or D stations (where the changes
were made after 1959) operating on frequencies specified
in Section 73.25,
to Class A AM stations on the same frequency.
Figures 9, 10, and 11 in Section 73.190 have been digitized and converted to metric units:
In addition, the digitized curves have been distilled into a simple function, intended
for use primarily by broadcast engineers. Select the frequency, then enter the
distance and azimuth from the AM station being considered to a point on the
0.1 mV/m contour of a Class A station on the same frequency. A
field strength value will be returned. The critical hours AM station may not exceed
that computed field strength toward that point during the critical hours periods.
Several computations should be made for various points on the Class A station's
0.1 mV/m contour to ensure compliance with the rule.
Results will be displayed immediately below the input form.
Please note that a browser capable of handling the IFRAME tag is required to properly
display the results.
