Bird 8930 series RF loads are general purpose, 50 ohm, coaxial RF transmission line terminations. They are useful as standby reject loads for single or double sideband transmitters and provide accurate, dependable, and low reflection line terminations.

Up to 10,000 watts can be dissipated.

The loads have a coolant chamber surrounded by radiator fins. The front and rear fins form mounting flanges which can be used as supports for freestanding use or as brackets for fixed mounting. A single vent plug at the top of the unit relieve internal pressure from coolant expansion and a blower assembly with three axial fans that line the bottom of the load.

Only model 8931 can be used with Bird QC Quick Change connectors.

Theory of operation

Bird 8930 Loads consist of a thin-film-on-ceramic resistor immersed in a dielectric coolant. The resistor, individually selected for its accuracy, is enclosed in a special housing. When surrounded by the coolant, this produces a uniform, practically reflection-less line termination over the specified frequencies.

The load is cooled by forced air and natural fluid convection currents. The coolant, chosen for its desirable dielectric and thermal characteristics, carries heat from the resistor to the walls of the cooling tank, where radiator fins surrounding the tank transfer the heat to the forced air flow.

When the coolant is heated, thermal expansion causes an increase in internal pressure. A vent plug relieves this pressure while protecting the opening from dirt or other contaminants.

Forced airflow is provided by three axial fans beneath the heat exchanger. Baffles direct the air over the radiator fins for more efficient cooling. A passive, normally open control thermoswitch closes when the coolant reaches 60 °C (140 °F), turning the fans on.

Thermal interlock (optional - part number 8890-017)

When installed, a passive, normally closed over-temperature thermoswitch opens above the maximum safe load temperature of 226 °C (439 °F), turning off transmitter power. The interlock will not permit use of the transmitter until the load has reached a safe temperature.

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.