Bird 8251 – Oil Cooled RF Termination (1000W)

Bird 8251 RF loads are portable, self-contained general purpose 50-ohm coaxial transmission line RF terminations requiring no outside power source or additional equipment. These loads provide oil-assisted convection cooling for safe, continuous operation up to 1000W full-rated power. They provide accurate, dependable, and practically non-reflective termination for testing and adjusting transmitters under non-radiating conditions.

Frequency ranges and VSWR for standard models:

• Frequency range: DC – 2.4 GHz
• DC – 1 GHz @ 1.1 max
• 1 GHz – 2 GHz @ 1.25 max
• 2 GHz to 2.4 @ 1.3 max

Frequency range and VSWR for D models:

• Frequency range: 470-860 MHz
• VSWR: 1.065 max

Bird offers its 8251 with multiple connector options, as can be seen below. The 8251D and 8251D7-16 are designed for digital broadcasting (VSWR = 1.065 max, 470-860 MHz).

• 8251: LC connector installed, and an additional QC-N connector for lower power testing
• 8251N: QC-N connector for lower power testing
• 8251D: 1-5/8” EIA, flanged
• 8251D7-16: 7/16-DIN (f)

Bird 8251 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. An expansion tank on the upper rear of the load allows for coolant expansion due to the rise in temperature and is equipped with a vent plug to relieve this pressure while protecting the opening from dirt or other contaminants.

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.