4
Chapter 2
Theory of Operation
The Models 8201 and 8203 Termaline Load Resistor consist 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.
Cooling
The dielectric coolant is chosen for its desirable dielectric and thermal
characteristics. Cooling of the Models 8201 and 8203 is accomplished by natural
fluid and air convection currents. The 0.7 gallon of 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.
