2
Chapter 2
Theory of Operation
General
Th load resistor is unique in that it utilizes an external water supply for the direct cooling of the resistor element. By
using this technique, the need for an intermediate dielectric fluid to transfer the heat generated in the resistor
element has been eliminated, reducing the physical size of the load to a virtual minimum. This simplified system
allows the use of the loads in more varied environments, and the attachment in any orientation.
Heat Transfer
The 50 ohm resistor consists of a high temperature substrate tube with a deposited resistive film. The heat
generated by absorption of RF power is transferred from the heated film to the water flowing over it, through a
restricted chamber surrounding the resistor body. This water, first diverted to the front of the load resistor by a
special inside tube passes over the entire length of the resistor and is discharged through the sealed water chamber
at the rear. The dielectric characteristics and distinctive design of these enclosures provide a very accurate 50 ohm
termination over the specified frequency range of this load - 1000 Hz to 800 MHz.
The absence of intermediate cooling fluids considerably simplifies the construction and sealing of this unit. It can be
readily disassembled in the field for resistor element replacement (see
Because there is practically no heat transfer to the outer housing of the load, the housing remains at a cool ambient
temperature even under full power conditions. Virtually all of the power input to the load is transformed into heat
which is carried away by the cooling water. Therefore, the differential in output and input temperatures of the
water multiplied by the amount of flow constitutes an accurate measurement of the power consumed by the load.
The amount of this power dissipation may be calculated from the following formula:
For Celsius Temperatures:
P = 0.263(T1-T2)GPM
For Fahrenheit Temperatures:
P = 0.146(T1-T2)GPM
Where:
P = Power in Kilowatts
T1 = Outlet Water Temperature
T2 = Inlet Water Temperature
GPM = Water Flow in Gallons Per Minute
