Bird Technologies 8327-300 - Manual (Page 13)

Chapter 2
Theory Of Operation
The Model 8327-300 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
propotionately 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 reflectionless 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.
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. 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 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 housing. This 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.
The Model 8327-300 may be used for isolation of power sources up to 1000 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.
3