Bird Technologies 8860 Series - Manual (Page 13)

3
Chapter 2 Theory of Operation
Load Resistor
Bird 8860 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 transmitter power. The interlock system will not per-
mit reoperation of the transmitter until the load has reached a lower, safe temperature.
Figure 2 Shipping Plug
Figure 3 Vent Plug
O-ring seal
O-ring seal