(Reprinted with permission of BE, and converted from the 1988 copyrighted printed copy by Rich Rarey. Every effort has been made to make this HTML version faithful to the original. While the content is the same, small editorial changes have been made to make the material more readable in its HTML form, and some layout changes have been made to take advantage of a Web browser's display capabilities.)
INTRODUCTION
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On July 16, 1987, the FCC adopted Docket MM
87-13 which authorized FM stations to increase the power of their on frequency booster
facilities from 10 watts maximum to "20 percent of the maximum permissible ERP for
the class of primary station they rebroadcast" [FCC Docket MM 87-13, p4, July 15,
1987]. The station may not, however, transmit beyond the predicted 1 mv/m contour of
the main transmitter for Class A and Class C stations, the 0.5 mv/m contour for Class B,
or the 0.7 mv/m contour for Class B1 stations. This decision has created a surge of interest from broadcasters as to the feasibility of a booster for their particular station, and specifically what means are available to accomplish this. The increase in power brings with it new problems to be dealt with, including widened interference zones, adequate signal ratios, and the need to synchronize carrier frequencies. |
This paper will deal with basic properties of "real world" effects caused by the presence of two or more radio signals of identical frequency, together with their effects on an FM receiver. Capture ratio, carrier ratios, and residual signal strength caused by destructive interference are discussed. The reasons behind frequency locked carriers are reviewed, along with the consequences of non-synchronous signals.
Time delay effects on the modulation information are examined, especially as they relate to the composite FM stereo signal and subcarriers. Conclusions are drawn as to the desirability of delay equalized modulation.
Various configurations to transmit both the program material and the frequency locking information are presented. A working system is described giving actual field test results, including the effectiveness of the frequency locking scheme, signal improvement in the primary area of interest, and signal reception in areas of relatively equal carrier ratios.
A recommendation for minimum acceptable carrier ratios is presented along with some basic guidelines for the selection of a suitable location for a synchronous FM booster.
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