Bird Technologies 8630B Series - Manual (Page 21)

5
Chapter 2 Theory of Operation
The 8730 Series style Econoload Coaxial load resistor installed in this
unit is unique in having its water supply primarily directed over the
outer, wet film type, resistive coating of the substrate. This
eliminates the need for an intermediate heat transfer system,
reducing the load size to a minimum. 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 (refer to the Chapter 5 - Maintenance).
Heat Transfer The 50 ohm resistor consists of a substrate which has a permanently
deposited resistive film, covered by a thin layer of electrically
nonconductive material. The heat generated by the absorped RF
power is transferred from the film to the water which flows over it
through a restricted chamber surrounding the resistor body. This
water, first carried to the front of the load resistor, passes over the
entire length of the resistor and discharges through the sealed water
chamber at the rear. The composite dielectric characteristics and the
distinctive design of these enclosures provide an accurate 50 ohm
termination over the specified frequency range of this load - 1 kHz to
1000 MHz.
Flow Interlock
Control Circuit
The interlock control circuit provides instantaneous fail-safe
protection of the transmitter and Moduload in the event of even
momentary interruption of the coolant supply. This protection is
necessary because dissipation of the heat generated by the RF power
is critically dependent on a required minimum water flow of 2.5
gallons (9.5 liters) per minute at all times, regardless of the coolant
temperature.
The water flow switch, installed directly on the coolant input line to
the load resistor, is a “normally open” type; i.e. its electrical contacts
open when deactivated. Closed during equipment operation, the
switch is adjusted to open when the coolant flow drops below the
minimum. When the water flow switch contacts open, the time delay
switch is deactivated, which at once opens the interlock switch,
causing immediate transmitter shutdown. Also, the centrifugal
impeller of the pump is carefully selected for the necessary flow rate.
CAUTION
Do not attempt to alter the flow switch setting or disturb the
coolant pump.