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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 reflectionless 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 Loads can be supplied with an optional passive overtemperature
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.
