Bird Technologies 8578B150 Series - Manual (Page 17)

5
Chapter 2 Theory of Operation
General
The 8578A and 8578B series consists of high powered, air cooled, RF loads
used for termination of coaxial transmission lines. The RF energy, when con-
verted into heat, is transmitted directly to the surrounding area by the forced
air system.
RF Section Description
The RF section, of the 8578A and 8578B series, is composed of a parallel com-
bination of tubular resistors. These resistors are carefully positioned to pro-
vide a reduction in surge impedance 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 reflec-
tionless line termination over the stated frequencies of the load resistor.
Heat Transfer
The resistors used in the 8578 series are of a tubular type, situated in a vertical
position within their housing. When the unit is in operation, a fan located at
the bottom of the unit draws air into the top grille openings and directs it 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 lower grille opening in the unit.
Interlock Control Circuit
The interlock control circuit provides fail-safe protection of the transmitter
and load resistor in the event of restricted air flow or excassive ambient tem-
perture to the load. This protection is necessary because dissipation of the
heat generated by the RF power is critically dependent upon a required mini-
mum flow of cooling air at all times. If the air flow 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 actu-
ate the interlock relay to reverse the process 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.