783 Series Constant Impedance FM Combiner Module

The ERI Model 783 Series Constant Impedance FM Combiner Modules provide stable high performance combining system for high power multi-station FM combining systems.


ERI FM filters and combiners have served the broadcast industry for over 40 years. The basic building block of ERI’s combiner and multiplexers system is a unique filter cavity. Only ERI offers a cylindrical cavity and all internal components in the RF path are silver plated. The result is an extremely efficient (low loss) and stable filter. ERI filters include a unique bellows temperature compensation assembly which maintains filter performance from a cold start to normal operating temperature without causing high transmitter VSWR. Only ERI filters incorporate a loop coupled design that allows more control over the filter pass band and offers performance superior to what can be achieved with the iris coupled cavities used by other manufacturers. The use of cylindrical tanks provides the benefit of a mechanically rigid tank design. ERI’s 783-6 constant impedance combiner module is modular in construction and is designed so that additional stations may be added in the future. The frequency of the additional station may be located anywhere within the FM broadcast band 2.0 MHz or more from any other system frequency. For closer frequency spacing use the Model 783-8 Constant Impedance FM Combiner Module.

To further enhance system reliability ERI FM filter systems do not include air filters that require replacement. In filter system designed to handle higher FM power levels the forced air cooling option uses a high reliability motorized fans that are mounted on each cavity and requires no additional floor space and does not require filtering.

The filter system offered is configured in a floor mounted configuration to maximize heat transfer, power handling capability, and simplify system modification in the future. The individual modules can be optionally forced air cooled, for higher power handling capability. There are also optional configurations that include ceiling hung frames and floor stands to accommodate a wide variety of physical space requirements. The combiner module can also be equipped with circulators for the dump load ports of the system to enable the capability or “reverse feeding” the combiner with the digital IBOC FM signals.

Features and Benefits

  • Cylindrical construction provides better mechanical and electrical stability than square or rectangular cavities
  • Factory tuned to customer’s specified channel, yet can be easily field converted to any FM channel
  • Minor adjustments of cavity resonant frequency can be made during full power operation of filters
  • High stability temperature compensated cavities
  • ERI filters are loop coupled and fixed with bolted connections to couple cavities
  • No Teflon or other insulating material used in the coupling between cavities, eliminating a primary point of failure, present in competitor’s designs
  • Loop coupling provides an adjustment (without disassembly) at the input and output of each cavity
  • Marman clamp connection allows close cavity placement
  • Ability to change frequency quickly and with a minimum of disassembly
  • Filter can be easily broken down for shipment and placement
  • Temperature indicator provided on all filter sets
  • Optional non-adjacent coupling and group delay compensation available
  • Easily assembled floor standing units (hardware for ceiling suspension available on request


  • Band:
  • Frequency Range:
    88 to 108 MHz
  • Number of Cavities:
    Three (3) or Four (4)
  • Power Handling:
    40 kW convection cooled
  • 70 kW with forced air cooling
  • Mounting Configuration:
    Floor or Optional Vertical Rack Stands
  • Insertion Loss:
    < 0.35 dB at ±200 kHz
  • VSWR:
    < 1.1:1 at ±200 kHz
  • Group Delay:
    < 75 nsec overall variation ±150 kHz
  • < 25 nsec with optional group delay correction

Models Available

  • 783-6
    ERI Model 783-6 Constant Impedance FM Combiner Module, convection cooled
  • 783-6A
    ERI Model 783-6 Constant Impedance FM Combiner Module, with optional forced air cooling
  • 783-8
    ERI Model 783-8 Constant Impedance FM Combiner Module, convection cooled
  • 783-8-NA
    ERI Model 783-8 Constant Impedance FM Combiner Module with optional non-adjacent coupling, convection cooled
  • 783-8A
    ERI Model 783-8 Constant Impedance FM Combiner Module, with optional forced air cooling
  • 783-8A-NA
    ERI Model 783-8 Constant Impedance FM Combiner Module with optional non-adjacent coupling and forced air cooling

Product Brochures

783 Series Constant Impedance FM Combiners

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