iBiquity Corporation has developed a specification for its “in-band on-channel” (IBOC®) broadcasting system that meets the requirements of the Federal Communications Commission (FCC). Transmitting a hybrid (both analog and digital) IBOC®-compatible broadcast requires radiating an analog signal with frequency modulation (FM) technology and a digital signal with orthogonal frequency division multiplexing (OFDM) technology. The OFDM signal occupies the edges of the FM signal's emissions mask and has a total radiated power one hundredth (−20 dB) that of the FM signal. Each hybrid IBOC® signal uses one of the hundred radiotelephone channels for public reception established between television channels 6 and 7 in the very high frequency (VHF) band (88.1 MHz to 107.9 MHz). IBOC® also defines standards for all-digital VHF and for AM-band (535 KHz to 1705 KHz) radio.
A previous IBOC® antenna design disclosed in U.S. Pat. No. 7,084,822 (“the '822 patent”), incorporated herein by reference, includes crossed dipoles for radiation of analog and digital signals. The propagation concept disclosed includes, in at least one embodiment, two pairs of dipoles in each bay, with the dipoles in each pair spaced horizontally by a quarter wavelength, oriented at right angles to each other within parallel planes, and driven with two substantially unrelated signals, where the two signals are fed as traveling waves from opposite ends of a coaxial line and coupled therefrom to drive the dipoles.
A crossed-dipole pair so driven reinforces signal emission at some azimuths and cancels signal emission at other azimuths to produce generally peanut-shaped and overlaid circularly polarized patterns—beams—for the two signals. Each beam has two lobes; the lobes for that beam have the same circular polarization, but are opposite in phase at each instant. The '822 patent discloses a second dipole pair that taps the coaxial line a quarter wavelength from a first dipole pair for impedance cancellation, and that has an azimuthal orientation at right angles to that of the first pair, so that each bay radiates two circularly polarized signals with opposite handedness and oppositely rotating phase. The signals generally fill in at intermediate azimuths to an extent sufficient for the antenna to be termed omnidirectional.
While effective, this embodiment is somewhat constrained by the traveling-wave feed method, and is better suited to tower-top mounting and a small number of bays. A second embodiment in the '822 patent feeds crossed dipole pairs from taps on a traveling wave coaxial line, splitting the tapped signals to drive the pairs. This allows all of the radiating elements to be placed to one side of the coaxial line, but is still further limited in power by halving the number of coupling taps per radiator.
Another previous IBOC® antenna design is disclosed in copending U.S. application Ser. No. 11/698,065, filed Jan. 26, 2007, titled “Antenna System and Method to Transmit Cross-Polarized Signals from a Common Radiator with Low Mutual Coupling,” incorporated herein by reference. This design includes separate corporate feed from analog and digital transmitters to a plurality of hybrid couplers per bay, each hybrid including unbalanced inputs and balanced outputs, so that multiple crossed-dipole radiators with integral cross-coupling cancellation can be provided in a plurality of bays with low mutual coupling. While highly effective, broad banded (>20% BW for VSWR<1.05:1), and high power capable, this design can be complex, preferably using either a tower-top mounting scheme or a plurality of discrete mountings around a tower or other structure to realize omnidirectional coverage.
Multiple-channel broadcast towers are costly to build and occupy significant amounts of real estate in rare locations (high up and near the center of population regions but low in local population, so transmitters can be clustered around them). Many such broadcast towers are relatively full, that is, they are limited in the number of antennas that can be mounted on them with adequate vertical separation, and desirable positions such as tower tops are typically already taken, leaving small or low positions or replacement of existing antennas as enhancement possibilities. Some IBOC®-compatible antenna designs are not readily adapted to tower-side mounting, because they use highly symmetrical structures to achieve omnidirectional patterns and would require robust, extended—and massive—cantilever brackets for tower side mounting.