Bird 8890-315 and 8890-320 Series RF Loads are self-contained high-power 50 ohm coaxial transmission line terminations requiring either 115 VAC or 230 VAC for operating the blower unit which provides forced-air cooling for power between 1250 and 5000 watts. It provides accurate, dependable, and practically non-reflective termination for testing and adjusting transmitters under non-radiating conditions.

Model selection

Frequency range and VSWR covered by these by these models* is as follows:

• DC - 1GHz @ 1.1 max.
• 1 GHz - 2 GHz @ 1.25 max.
• 2 GHz - 2.4 GHz @ 1.3 max.

Only models 8890-315 and 8890-320 can be used with Bird QC Quick Connect connectors.

*8892D320 is designed for digital broadcasting and has different specifications: 470-860 MHz @ 1.065:1 max

Bird 8890 series loads consist of a thin-film-on-ceramic resistor immersed in a dielectric coolant. The resistor, individually selected for its accuracy, is enclosed in a special housing. When surrounded by the coolant, this produces a uniform, practically reflectionless line termination over the specified frequencies.

The load is cooled by natural fluid and air convection currents. The coolant, chosen for its dielectric and thermal characteristics, carries heat from the resistor to the walls of the cooling tank, where radiator fins surrounding the tank transfer the heat to the air.

When the coolant is heated, thermal expansion causes an increase in the internal pressure. The vent plug relieves this pressure while protecting the opening from dirt or other contaminants.

When installed, the blower assembly provides forced airflow with two blowers. Baffles direct the airflow from the blowers over the radiator fins, doubling the heat transfer efficiency.

For example, a 2.5 kW load will safely dissipate 5 kW. A passive, normally open control thermoswitch closes when the coolant reaches 155 °C (311 °F), turning the fans on.

As an option, the load may be provided with a thermoswitch assembly, P/N 8890-008. The thermoswitch is used as an interlock switch to disable transmitter power if the coolant temperature exceeds a preset temperature, thus preventing possible damage from accidental power overloading from the transmitter or equipment malfunction.

The thermoswitch is normally closed and opens at a maximum safe temperature. Since the thermoswitch is connected in series with the transmitter interlock, it cuts off transmitter power if the coolant temperature exceeds this value.

Many models available

Bird offers several models of oil cooled loads. Models are available with power ratings from 150W to 10kW, many connection options, and utilizing convection or blowers for cooling.

For more detailed information on Bird oil cooled loads with power ratings, connector types and more, please see pages 88-89 (PDF pages 9-10) of the document linked here.