Oil Cooled RF Termination (150W)
The Bird Model 8135 Termaline Load Resistor is a portable, general-purpose 50-ohm coaxial transmission line termination. It is a self-contained unit requiring no outside power source or additional equipment. This coaxial load resistor provides an accurate, dependable, and practically non-reflective termination for adjustment, standby, and testing of transmitters under non-radiating conditions.
The Model 8135 can absorb up to 150 watts continuously and dissipate it harmlessly as heat over a frequency range of DC to 4000 MHz. It is designed to show low reflectivity throughout this range.
VSWR is as follows:
- 1.1:1.0 maximum DC-1000 MHz
- 1.2:1.0 maximum 1000-2500 MHz
- 1.3:1.0 maximum 2500-4000 MHz
The 8135 is compatible with Bird Quick Change “QC” connectors.
The only other item required for operation is a matching connector on the coaxial transmission line.
Theory of operation
The Model 8135 Termaline Load consists essentially of a cylindrical film-type resistor immersed in a dielectric coolant. The resistor, individually selected for its accuracy, is enclosed in a special tapered housing which provides a linear reduction in surge impedance directly proportional to the distance along the load resistor. This produces the uniform, practically reflection-less line termination over the stated frequencies of the load.
The coolant oil is chosen for its desirable dielectric properties and thermal characteristics. Cooling of the load is accomplished by natural fluid and air convection. The dielectric coolant carries the electrically generated heat from the resistor to the walls of the cylindrical cooling tank.
The tank is encased in a set of heavy gauge metal radiating fins, which are firmly pressed on the cylinder. The heat from the oil is transferred to the surrounding air by the radiating fins.
A synthetic rubber diaphragm located in the rear dome of the load allows the coolant to expand as the temperature rises.
Many models available
Bird offers several models of oil-cooled loads. Models are available with power ratings from 150W to 10kW, many connection options, and utilizing convection or blowers for cooling.
For more detailed information on Bird oil-cooled loads with power ratings, connector types, and more, please see pages 88-89 (PDF pages 9-10) of the document linked here.