2
Chapter 2
Theory of Operation
Load Resistor
Bird 8860 series 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.
Coolant
The load is cooled by natural fluid and air convection currents. The coolant, chosen for its 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 air.
When the coolant is heated, thermal expansion causes an increase in the internal pressure. The vent plug relieves
this pressure while protecting the opening from dirt or other contaminants.
Thermal Interlock (Optional)
Loads can be supplied with an optional passive over-temperature thermoswitch. Normally closed, it opens at the
maximum safe load temperature of 236 °C (457 °F), turning off power to the RF generator. The interlock system will
not permit reoperation of the generator until the load has reached a lower, safe temperature.
