2712 Bandpass Filter
Small footprint, up tp 15kW with blowers
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Shively Labs balanced combiner modules allow any number of stations to be combined into a common antenna. Systems have been designed to handle up to 21 analog stations, along with their digital counterparts. Frequency spacings as close as 0.8 MHz are routine, even on large systems. Shively’s unique design allows stations to be added in any frequency order without the need to reconfigure the existing combiner chain – making them ideal for sites where future expansion is planned.
All Shively Labs combiners use bandpass filtering, ensuring higher spectral purity, flat in-band frequency response, and typical isolation values of 50 dB or higher – even for frequencies spaced 0.8 MHz apart. All Shively Labs bandpass filters are iris coupled, eliminating the need to adjust the pass band to compensate for the increased distortion and asymmetry inherent to older, loop coupled designs.
Balanced combiners are generally used for combiner systems of four or more stations, for systems designed to feed two outputs, or for systems where an auxiliary wideband input is desired. Balanced combiner systems generally require more floor space than a branched or starpoint system, and are generally more expensive. Balanced combiner modules are also used to expand branched combiner systems from their original configuration.
A balanced combiner system consists of station-specific modules along with shared “system components” used in the broadband signal line. Typical system components are: interconnecting coax between station modules, output dual directional couplers to facilitate test measurements and monitoring, power splitters and patching networks to split combined output signals between antenna halves, and dummy loads for wideband inputs when not in use as antenna feeds. Lockout/tagout switches may be either station specific or a shared system component.
A station-specific balanced combiner module (right) consists of input hybrid, two four cavity or resonator bandpass filters, and an output hybrid. In addition, a low level group delay equalizer is used when a station’s signal is within 1.0 MHz of any other signal in the combiner system.
Balanced combiner modules and system components are configured based on two factors – the average analog transmitter power of the station, and the combined average and peak transmitter powers of all the stations in the system.
A station’s average transmitter power determines the size of the input hybrid and the bandpass filters for that station’s module. This will also determine the size of station specific input patching networks if used. This should be the maximum power the transmitter will ever run at, including any power increases that may be used to compensate for a lower gain aux antenna. Stations with different sized input hybrids and filters can be combined in the same system.
The output hybrid is designed to handle the combined average and peak power levels of all the stations that may eventually occupy the combiner system. In system configurations that accommodate split and back feeding, hybrids must accommodate any changes in combined power resulting from the feed direction being swapped in an emergency situation. Stations with different sized output hybrids can be combined in the same system, as long as power levels and feed directions are properly accommodated.
In most cases, average power levels will determine the size of the combiner modules and system components. Peak power may be a factor, however, in combiner systems that are designed to handle a large number of relatively low power stations, especially if their digital signals are operating simultaneously. In these cases, broadband line components may need to be larger than would customarily be used to handle the combined average powers. Please consult the factory for assistance in determining these values.
Shively Labs manufactures balanced combiner modules based on three styles of bandpass filters – resonant cavity, interdigital, and comb-line (see below). All are available for low power modules. Medium and high power modules are only available with resonant cavity filters. As a general rule, modules using interdigital filters are less expensive and take up considerably less floor space than their resonant cavity counterparts. Modules using resonant cavity filters have lower insertion losses in the filter portion of the module (the losses for the output hybrid/broadband line remain the same).
Since 1984, all Shively Labs balanced combiners have been manufactured using temperature compensating materials to ensure consistent electrical performance from cold start to full power. No bellows or moving parts are used. All module and system components are designed to operate at temperatures exceeding 160° F (70° C) with no degradation of electrical performance even though at rated power and the maximum ambient temperature of 104° F (40° C) these temperatures will not be reached.
Shively Labs balanced combiners should be maintained and operated at 60% or less relative humidity.
All Shively Labs combiners are designed to pass IBOC signals in a high level combined configuration. In addition, Shively Labs has pioneered a number of techniques that allow the balanced combiner to be used for low level combining. These include back-feeding the IBOC signal through the input hybrid, cross-feeding to combine the analog and digital signals in a way that balances transmission line usage, and input patching arrangements that allow the analog and digital signals of a station to be swapped in the event of transmission line failure. For more information on these and other techniques, please contact the factory.