Bird Technologies 8251D208 - Manual (Page 17)

Chapter 2
Theory of Operation
This load consists essentially of a carbon film resistor on a
ceramic substrate immersed in a dielectric coolant. The re-
sistor, individually selected for its accuracy, is enclosed in a
special exponentially tapered housing. This provides a lin-
ear reduction in surge impedance directly proportional to
the distance along the resistor. When surrounded by the di-
electric 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 hous-
ing, forming the return conductor of the coaxial circuit. This
produces a uniform, practically reflectionless line termina-
tion over the stated frequencies of the load resistor.
Cooling
The dielectric coolant was chosen for its desirable dielectric
and thermal characteristics. Cooling of the load is accom-
plished by natural fluid and air convection currents. The 1.1
gallons (4.2 liters) of dielectric coolant carries the electri-
cally generated heat from the resistor to the walls of the cy-
lindrical cooling tank. This tank is encased in a set of
radiating fins, constructed from heavy gauge aluminum al-
loy, and firmly pressed into the cylinder. The heat from the
dielectric oil is transferred to the surrounding air by the
fins.
Expansion of the coolant, caused by the rise in its tempera-
ture, 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 internal
pressure.
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