Oil Cooled RF Termination (500W)
The Bird 8201 RF load is a portable, self-contained general purpose 50-ohm coaxial transmission line termination requiring no outside power source or additional equipment. It provides accurate, dependable, and practically non-reflective termination for testing and adjusting transmitters under non-radiating conditions. The 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 resistor. This produces the uniform, practically reflectionless line termination.
The 8201 Termaline Load Resistor can absorb and dissipate up to 500 W of RF power continuously over a frequency range of DC to 2500 MHz.
VSWR is as follows:
- DC to 1000 MHz is 1.1:1 max
- 1000 to 2500 MHz is 1.25:1 max
The 8201 is compatible with Bird Quick Change “QC” connectors.
Only a coaxial cable with the appropriate connector plug is needed for connecting these load resistors to the source of RF power.
Theory of operation
The Birds 8201 consists essentially of a carbon film-on-ceramic resistor immersed in a dielectric coolant. The resistor, individually selected for its accuracy, is enclosed in a special exponentially tapered housing. This provides a linear reduction in surge impedance directly proportional to the distance along the resistor. When surrounded by the dielectric coolant, the characteristic impedance is therefore 50 ohms at the front (connector end), 25 ohms at the mid-point to compensate for the resistance already passed over, and zero ohms at the rear where the resistor joins the housing, forming the return conductor of the coaxial circuit. This produces a uniform and practically reflection-less line termination over the stated frequencies for the load resistor.
The dielectric coolant is chosen for its desirable dielectric and thermal characteristics. Cooling is accomplished by natural fluid and air convection currents. The dielectric coolant carries the electrically generated heat from the resistor to the walls of the cylindrical cooling tank. This tank is encased in a set of radiating fins constructed from heavy gauge aluminum alloy and firmly pressed onto the cylinder. The heat from the dielectric fluid is transferred to the surrounding air by the fins. Expansion of the coolant, caused by the rise in its temperature, is allowed by means of a synthetic rubber diaphragm (not visible) inside the rear cover dome of the unit.
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.