Bird Technologies 8320 - Manual (Page 17)

Chapter 3
Theory of Operation
General The Series 8320 Attenuators are symmetrical “T” pads, with
the power distribution on the legs being different. The value
of the resistance on each leg varies; therefore, according to
the power it is to absorb. On the input resistance element, a
proportionately larger resistor is of course required for its
much greater power dissipation. A “T” configuration is used
to provide equal input and output impedances for the 50
ohm transmission line attenuation.
The input resistor is joined to the “T” leg joint in an expo-
nentially tapered housing to provide a linear reduction in
surge impedance directly proportional to the distance along
the resistor. The output resistor is enclosed in a housing de-
signed to return the attenuator to the characteristic imped-
ance of 50 ohms. This arrangement produces a uniform and
practically reflectionless attenuation characteristic over the
stated frequencies of the attenuator.
This system of film-on-ceramic cylindrical resistors im-
mersed in a dielectric coolant constitutes the RF section as-
sembly. The dielectric constant of the cooling fluid and the
tapered input and output resistor housings provide the
proper electrical characteristics for the coaxial line attenua-
tion throughout the internal circuitry.
The dielectric coolant is carefully chosen for its desirable di-
electric properties, to which the diameters of the resistors
and housings are matched, and for its high thermal stability
characteristics. The coolant used is 10C Transformer Oil,
P/N 5-030. Model 8321 requires approximately 1 pint (0.47
liter); Model 8322, 0.7 gallons (2.65 liter); Model 8323, 0.35
gallons (1.3 liter) and Model 8325, 0.9 gallons (3.4 liter). For
Model 8325 expansion of the coolant when power is applied
to the attenuator, is accomplished by allowing the air, which
is compressed by the expanding coolant, to escape through
the vent plug located on the top and near the rear face of
the unit. Models 8321, 8322 and 8323 allow for expansion of
the coolant by using underfilled radiator tanks and a derat-
ing of their load carrying capacities.
By convection, the cooling fluid carries the heat generated
in the various resistor elements to the walls of the coolant
housing. This housing is encased in a set of radiating fins
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