Bird Technologies DA40 UHF Series-Manual - Manual (Page 19)

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Chapter 2 Theory of Operation
The Digital Air Series Loads are high powered, air-cooled, RF loads
used for termination of coaxial transmission lines. The RF energy,
when converted into heat, is transmitted to the surrounding area by a
forced air system.
RF Section Description
The RF section of a Digital Air load is a broadband tuned device com-
posed of tubular ceramic resistors. These are carefully positioned to
produce a very uniform and almost reflectionless line termination over
the stated frequencies of the load resistor.
Heat Removal
When in operation, air is forced into all four sides of the unit and
directed over the RF resistor network. The heat, developed in the resis-
tors from dissipation of RF energy, is carried off by the flow of air over
the resistors surface. The hot air is then exhausted through the grille in
the back of the unit.
Fan Activation
The fans on the Digital Air loads are triggered with a pair of normally
open snap disk thermoswitches. The top switch is located in the RF
chamber, while the bottom switch is located in the air exit duct. In
steady state operation, a fan cycle happens as follows:
1. Rising air temperature in the RF chamber closes the top thermo-
switch, turning the fans on.
2. The hot air is forced into the exit duct by the fans, heating the bot-
tom thermoswitch until it closes.
3. As the cool air is blown into the RF chamber, the top switch
reopens. Since the bottom switch is closed, the fans stay on.
4. When RF power is removed, cool air reaches the exit duct, the bottom
switch opens and the fans turn off.
For power levels substantially below the unit’s power rating, the air in
the exit duct may not heat up sufficiently to close the bottom switch.
As a result, the fans cycle on and off more often. For these power lev-
els, it is recommended that the unit be operated in manual mode.