The UC-584 universal transponder antenna coupler provides reliable Effective Radiation Power (ERP) and transponder Minimum Trigger Level (MTL) testing in high multi-path ramp and hanger environments. This antenna coupler provides 20 dB of transponder antenna isolation, ensuring airborne TCAS systems and SSR ground stations do not see UUT transponder altitude replies under test conditions where air-data system is pressurized to flight levels. This innovative design includes an integral positioning slot that improves the ability to replicate measurements and produce repeatable ERP tests. The UC-584 can be connected to the IFR 6000/6015 direct connect port.