ABSTRACT
Transmitting several frequencies from a single broadband antenna system requires the use of a combining system, or combiner, composed of radiofrequency (RF) filters and interconnecting transmission line. In general, a combiner can be categorized as one of two types: branched (star point) or balanced (constant impedance). Any of these types may employ bandreject (notch) or bandpass filters. This chapter discusses the use of filters, other components in FM combiners, and the hardware used to combine an in-bandon-channel (IBOC) digital signal into an analog signal.
For years, both the FM spectrum and the FM channel were straightforward and uncomplicated. Until the early 1980s, the number of stations on the air in all but the largest metropolitan areas was low by today’s standards. In most areas, the frequency spacing between stations exceeded the 0.8 MHz minimum that is common in all parts of the country today. These wider frequency spacings, the relative ease of developing new tower sites, and the limited station ownership in any market worked against the economics of combining stations into common antennas; therefore, most stations operated on single-frequency antennas, and large, multi-station antennas were generally only found in a few of the largest markets.
