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Chapter 2
Theory of Operation
Attenuator Construction
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 required for its much greater power dissipation. A “T”
configuration 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 exponentially tapered housing to provide a linear reduction in
surge impedance of 50 ohms. This arrangement produces a uniform and practically reflection-less attenuation
characteristic 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.
Cooling
The dielectric coolant is carefully chosen for its desirable dielectric properties, to which the diameters of the
resistors and housings are matched, and for its high thermal stability characteristics.
Coolant Expansion
When power is applied to the attenuator, the coolant expands, as the air in the coolant housing is compressed it is
allowed to escape through the vent plug located near the top and front face of the unit.
Heat Dissipation
Heat Dissipation is accomplished by convection, the coolant carries the heat generated in the various resistor
elements to the walls of the coolant housing. The housing is encased in a set of radiating fins which are attached to
its outer surface. These radiating fin surfaces dissipate the heat of the coolant into the surrounding air.
