Copyright ©1998, RF Specialties Group.
Here are
a few combiner ground rules, starting with type. Branched combiners are effective
for combining two to four stations. Each station's signal is passed through
a multi-stage bandpass cavity filter, and the outputs of the filters are summed
together to feed the antenna. (In some cases, notch or band-reject cavities
are also used.) Balanced (constant impedance) combiners are effective for combining
more than four stations. A balanced combiner comprises individual modules for
each station, usually constructed of six or eight bandpass cavity filters and
a pair of hybrid splitters. The modules are connected in series, with each station
adding its power to the chain. It's not uncommon to combine 10 to 15 stations
in this manner. Not only is the balanced combiner more cost effective as the
number of stations goes up, but, unlike a branched system, frequencies can be
added or changed easily.
In any combiner, the number of cavities used for each station determines the amount of isolation achieved between stations. Adding cavities will improve the impedance bandwidth, and as more cavities are added, the more frequency response starts to approach ideal — especially important if what's being combined are FMs close in frequency.
There also is group delay to consider. Ideally, all sidebands of the signal should pass through a cavity at the same time. But, in reality, those at the edges of the bandpass tend to move through more slowly than those at the center, resulting in a time or phase distortion referred to as group delay. To the listener, the resulting distortion sounds a lot like multipath, the bane of FM stations. Group delay is minimized through proper tuning of the cavities and by using the right number of cavities. Shively Labs has found three branched cavities or four balanced cavities to be the optimum for lowest group delay. Group delay equalizers are sometimes built into bandpass combiner modules. Many stations also have successfully eliminated group delay by using low-level pre-distortion networks at the output of the FM exciter. (This is the most common fix for group delay problems in TV transmitters.)
There are other considerations that help determine which combiner type to use. Many of today's transmitters are broader in bandwidth, and any energy reflected back into the transmitter from the combiner results in spurious emissions. This is not a problem with balanced combiners because these have hybrids with reject loads built in that prevent out-of-band energy from reflecting back into the transmitter. Not so the typical branched system, which requires careful adjustment to the transmission line length as part of the design process. "You've got a tuned circuit in between the transmitters and the master antenna, so there's potential for spurious emissions," said Chief Engineer Chris Murray with McKenzie River Broadcasting in Eugene, Ore., a long-time RF Specialties customer whose branched Shively combiner system for his group's two FMs was cable-adjusted by Shively to minimize spurious emissions from cropping up on the output of the transmitters.
Line size between the combiner and antenna becomes an issue in larger systems, where six- or nine-inch rigid line is often required. Redun-dancy here becomes a priority, because the failure of one bullet can knock a great many stations off the air. Often, such systems use two smaller runs of transmission line, each carrying half the power to separate halves of the antenna. In this way, the failure of one run of transmission line or one part of the antenna will only result in a reduction in power.
Overall, when planning a combiner system, the design goals should be near VSWR unity, minimal insertion loss (transmitter power is lost as heat at the resonant frequency), a good frequency response within the bandpass, and a good balance between high isolation and low group delay. In the 15 years RF Specialties has been providing combiner solutions, there have been many cases in which all of these goals have been achieved with a little planning.
PHOTO: This 11-station Shively combiner system at the John Hancock building in Chicago, Ill., was installed over ten years ago. The number of stations on the system now stands at ten. The system is digital-ready.