Bird Technologies 8201 - Manual (Page 15)

Chapter 2
Theory of Operation
General
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 imped-
ance directly proportional to the distance along the resistor.
When surrounded by the dielectric coolant, the characteris-
tic impedance is therefore 50 ohms at the front (connector
end), 25 ohms at the mid-point to compensate for the resis-
tance 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 practi-
cally reflectionless line termination over the stated frequen-
cies 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 elec-
trically generated heat from the resistor to the walls of the
cylindrical cooling tank. This tank is encased in a set of ra-
diating fins constructed from heavy gauge aluminum alloy
and firmly pressed onto the cylinder. The heat from the di-
electric fluid is transferred to the surrounding air by the
fins.
Expansion of the coolant, caused by the rise in its tempera-
ture, is allowed by means of a synthetic rubber diaphragm
(not visible) inside the rear cover dome of the unit.
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