Bird Technologies 8255 - Manual (Page 17)

Chapter 2
Theory of Operation
These loads consist essentially of a carbon film resistor
on a ceramic substrate immersed in a dielectric coolant.
The resistor, individually selected for its accuracy, is en-
closed in a special exponentially tapered housing. This
provides a linear reduction in surge impedance directly
proportional to the distance along the resistor. When sur-
rounded by the dielectric coolant, the characteristic im-
pedance is therefore: 50 ohms at the front (connector
end), 25 ohms at the mid-point (to compensate for the re-
sistance 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,
practically reflectionless line termination over the stated
frequencies of the load resistor.
Cooling
The dielectric coolant was chosen for its desirable dielec-
tric and thermal characteristics. Cooling of the load is ac-
complished by natural fluid and air convection currents.
The 0.8 gallons (3.03 liters) 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 into the cylinder. the
heat from the dielectric oil is transferred to the surround-
ing air by the fins.
Expansion of the coolant, caused by the rise in its tem-
perature, is allowed for by the use of an expansion tank
located on the upper rear of the load. The expansion tank
is equipped with a spring loaded vent for relief of the in-
ternal pressure.
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