Bird Technologies 8329-300 - Manual (Page 15)

Chapter 2
Theory of Operation
The Model 8329-300 Attenuator is a symmetrical “T” pad,
with the power distribution on the legs being different.
Therefore, the value of the resistance on each leg is different
according to the power it is to absorb. On the input resistor
element, a proportionately larger resistor is of course re-
quired for its much greater power dissipation. A “T” configu-
ration is used to provide equal input and output
impedance’s for the 50 ohm transmission line attenuation.
The input resistor is joined by the “T” leg joint in an expo-
nentially tapered housing to provide a linear reduction in
surge impedance of 50 ohms. This arrangement produces a
uniform and practically reflectionless attenuation character-
istic over the stated frequencies of the attenuator.
This system of carbon-film-on-ceramic cylindrical resistors
immersed in a dielectric coolant constitutes the RF section
assembly. The cooling liquid and the tapered input resistor
housing provide the proper electrical characteristics of the
coaxial line termination.
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. Expansion of the coolant, when power is ap-
plied to the attenuator, is accomplished by allowing the air,
which is compressed by the expanding coolant, to escape
through the vent plug located near the top and front face of
the unit.
By convection, the coolant carries the heat generated in the
various resistor elements to the walls of the coolant hous-
ing. This housing is encased in a set of radiating fins which
are attached to its outer surface. These radiating fin sur-
faces dissipate the heat of the coolant into the surrounding
air.
The Model 8329-300 may be used for isolation of power
sources up to 2000 W and for low level monitoring. The low
power value obtained at the output of the attenuator can
easily be read on an oscilloscope or terminated in a small
RF load resistor.
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