Model 404 FM Isolation Transformer

The Model 404 is broadband, 88 to 108 MHz, FM Isolation Transformer rated to handle up to 10 kW of FM power across an AM tower base insulator operating with up to a peak voltage of 7500 volts. It is designed to be pressurized with nitrogen or dry air with 3 to 5 PSI (21 to 35 kPa), not to exceed ten PSI (69 kPa).

Isolation transformers couple FM power across the base insulator of an AM broadcast station transmitting tower, with little or no effect on the AM base impedance and no mismatch into the FM antenna feed line. An isolation transformer is especially desirable for feeding high impedance AM radiators or towers that are part of a directional array where “bazooka” isolation systems would have adverse effects.

Each isolation transformer is supplied with a weather shield fitted with a 2-inch pipe flange on the bottom to accommodate a mounting pipe.  Each Type 404 isolation transformer is factory tested to operate anywhere in the 88 to 108 MHz FM band with no adjustment necessary A horn gap is installed for lightning protection. Both the input and output have a DC short circuit between the inner and outer conductors of the transformer.

ERI Model 404 FM Isolation Transformer Outline Drawing

404 FM Isolation Transformer Specifications

  • FM Operating Frequency:
    Any frequency from 88 to 108 MHz
  • AM Peak Voltage:
    7500 volts
  • AM Shunt Capacity:
    Approximately 100 pf
  • FM Insertion Loss:
    0.05 dB, maximum
  • Continuous FM Power Rating:
    10 kW
  • VSWR:
    <1.1:1 over the 88 to 108 MHz band
  • Lightning Protection:
    DC short circuit across inner and outer conductors on both input and output.
  • FM Leakage Isolation:
    Signal level measured between the outside of the coaxial output line and the transformer grounded case will be at least 30 dB down from the FM carrier level.
  • Pressurization:
    Designed for gas pass operation with a recommended pressure of 3 to 5 PSI (21 to 35 kPa) not to exceed 10 PSI (69 kPa).
  • Input and Output Connectors:
    Female 1-5/8-inch EIA, 50 ohm (order flange hardware and inner connectors separately)
  • Weight:
    112 pounds (49.98 kg)
  • Length of Transformer:
    72-inches (1830 mm)
  • Tank Diameter:
    6-1/8-inch (155 mm)
  • Rain Shield Measurements:
    Approximately 12-inches (305 mm) square
  • Mounting:
    2-inch pipe flange, mounting pipe is customer supplied

Ordering Information

  • Part No.
  • IT0038
    FM Isolation Transformer Type 404 rated at 10 kW. Input and output is 1-5/8-inch EIA. Broadband, 88.1 MHz to 107.9 MHz. 1-5/8-inch EIA female, 50 ohm input and output. Does not include inner connectors or flange hardware kits. These should be ordered separately if required. HWK1102 Hardware Kit contains screws and O Rings for both the input and output flange. Inner connector is part number ACX150-20.

Accessory Items (Order Separately)

  • Part No.
  • HWK1102
    3 inch Block Bolt Kit - 2 Sides – with two O Rings. Sufficient hardware for input and output flange. Also can be used as a flange hardware kit for the Model 404 FM Isolation Transfomer.
  • ACX150-20
    1-5/8-inch inner connector.

Information to be Specified with Order

Model 404 FM Isolation Transformer Brochure