Bird Technologies 8578A100GIG - Manual (Page 15)

Chapter 2
Theory of Operation
General The 8578A100GIG is a high powered, air-cooled, RF load
used for termination of coaxial transmission lines. The RF
energy, when converted into heat, is transmitted directly to
the external area by a forced air system.
RF Section
Description
The RF section of the 8578A100GIG is composed of a paral-
lel combination of tubular resistors. These resistors are
carefully positioned to provide a reduction in surge imped-
ance proportional to the distance along the resistive system,
which finally terminates to the housing forming the return
path for the coaxial circuit. This produces a very uniform
and almost reflectionless line termination over the stated
frequencies of the load resistor.
Heat
Transfer
The resistors used in the 8578A100GIG are of a tubular ce-
ramic type, situated in an angled position within their hous-
ing. When the unit is in operation, externally forced air is
blown into the side grille opening and directed over the RF
resistor network. The heat, developed in the resistors from
dissipation of the RF energy, is carried off by the flow of air
over the resistors surface. The hot air is then exhausted
through the opposite side grille openings in the unit.
Interlock
Control
Circuit
The interlock control circuit provides fail-safe protection of
the transmitter and load resistor in the event of an overload
situation. This protection is necessary because dissipation of
the heat generated by the RF power is critically dependent
upon a required minimum flow of external cooling air at all
times. If the airflow over the resistor array should stop or be
restricted and the temperature in the RF chamber should
rise beyond a safe limit, the heat sensor unit will sense the
change and actuate the interlock relay to reverse the pro-
cess and turn off the transmitter. The interlock system will
not permit re-operation of the transmitter until the air flow
is restored and a safe low temperature in the RF housing is
once again attained.
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