Model 425/426/427 FM Isolation Transformers

The ERI 425, 426, and 427 FM Isolation Transformer Series includes models rated to handle FM power levels of 25 kW to 50 kW across an AM tower base insulator operating with up to a peak voltage of 40,000 volts. Each model has three (3) versions that cover one portion of the FM band and they are factory tuned to a single FM frequency. Some multi-channel applications can be addressed with factory modifications.  They are 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 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 cradle fitted with a 3-inch pipe flange on the bottom to accommodate a mounting pipe and two stainless steel straps to secure the tank to the mounting cradle.  A horn gap is installed for lightning protection.  For lightning protection heavy-duty DC short circuits between the inner and outer conductors of both the input and output.

ERI Model 425-426-427 FM Isolation Transformer Outline Drawing

425/426/427 FM Isolation Transformer Specifications

  • FM Operating Frequency:
    88 to 108 MHz, factory tuned to specified operating frequency
  • AM Peak Voltage:
    40,000 volts
  • AM Shunt Capacity:
    Approximately 60 to 70 pf
  • Continuous FM Power Rating:
    Model 425 25 kW
  • Model 426 40 kW
  • Model 427 50 kW
  • FM Insertion Loss:
    0.05 dB, maximum
  • VSWR:
    <1.05:1 at tuned operating frequency ±500 KHz
  • <1.1:1 at tuned operating frequency ±1 MHz
  • 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.
  • Mounting:
    Supplied with cradle fitted with a 3-inch pipe flange on bottom and two stainless steel straps to secure tank to mounting cradle. Mounting pipe is customer supplied.
  • 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 Connector:
    Model 425 and 426 3-1/8-inch EIA, male
  • Model 427 6/18-inch, 50-ohm, EIA, female
  • Output Connector:
    Model 425 and 426 3-1/8-inch EIA, female
  • Model 427 6/18-inch, 50-ohm, EIA, female
  • Weight (transformer and cradle):
    Model 425 256 pounds (116.1 kg)
  • Model 426 300 pounds (136.1 kg)
  • Model 427 325 pounds (147.4 kg)
  • Length of Transformer:
    Model 425 and 426 44 inches (1,118 mm)
  • Model 427 60 inches (1,524 mm)
  • Tank Diameter:
    28 inches (711 mm)

Ordering Information

  • Part No.
    Description
  • IT0002A
    FM Isolation Transformer Type 425 rated at 25 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 88.1 MHz to 92.9 MHz.
  • IT0002B
    FM Isolation Transformer Type 425 rated at 25 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 93.1 MHz to 102.9 MHz.
  • IT0002C
    FM Isolation Transformer Type 425 rated at 25 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 103.1 MHz to 107.9 MHz.
  • IT0031A
    FM Isolation Transformer Type 426 rated at 40 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 88.1 MHz to 92.9 MHz.
  • IT0031B
    FM Isolation Transformer Type 426 rated at 40 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 93.1 MHz to 102.9 MHz.
  • IT0031C
    FM Isolation Transformer Type 426 rated at 40 kW. Input is 3-1/8 inch EIA captive male. Output is 3-1/8 inch female. Input and output impedance is 50 ohms. Any single FM frequency from 103.1 MHz to 107.9 MHz.
  • IT0067A
    FM Isolation Transformer Type 427 isolation transformer, 50 kW 4-1/8 inch input and output with adapters to 6-1/8 inch input and output, both female. Input and output impedance is 50 ohms. Any single FM frequency from 88.1 MHz to 92.9 MHz.
  • IT0067B
    FM Isolation Transformer Type 427 isolation transformer, 50 kW 4-1/8 inch input and output with adapters to 6-1/8 inch input and output, both female. Input and output impedance is 50 ohms. Any single FM frequency from 93.1 MHz to 102.9 MHz.
  • IT0067C
    FM Isolation Transformer Type 427 isolation transformer, 50 kW 4-1/8 inch input and output with adapters to 6-1/8 inch input and output, both female. Input and output impedance is 50 ohms. Any single FM frequency from 103.1 MHz to 107.9 MHz.

Information to be Specified with Order

Model 425/426/427 FM Isolation Transformer Product Brochure