Theory of operation – Bird Technologies 8401 User Manual

Page 15

Chapter 2

Theory of Operation

General

The Models 8401, 8402, and 8404 Termaline Load Resis-

tors consist essentially of a carbon film-on-ceramic resis-

tor immersed in a dielectric coolant. The resistor,

individually selected for its accuracy, is enclosed in a spe-

cial exponentially tapered housing. This provides a linear

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 housing, forming the return conductor of the coaxial

circuit. This produces a uniform and practically reflection-

less line termination over the stated frequencies for the

load resistor.

Cooling

The dielectric coolant is chosen for its desirable dielectric

and thermal characteristics. Cooling of the load is accom-

plished by natural fluid and air convection currents. The

0.7 gallon of dielectric coolant carries the electrically gen-

erated heat from the resistor to the walls of the cylindri-

cal cooling tank. This tank is encased in a set of

radiating fins constructed from heavy gauge aluminum al-

loy and firmly pressed onto the cylinder. The heat from

the dielectric fluid is transferred to the surrounding air

by the fins.

Expansion of the coolant, caused by the rise in its tem-

perature, is allowed by means of a synthetic rubber dia-

phragm (not visible) inside the rear cover dome of the

unit.