This is a device with three ports (or inputs) and a single output. One of the ports is internally terminated with a matched load to isolate it from the input. In other words, it reflects any energy sent into the one or two open ports away from the input, which is grounded on one side while being left open on the other.
All external mating surfaces are machined to extreme flatness to provide connection to a standard waveguide flange for minimum discontinuity.
Junction isolators are useful in test setup and operational systems. These devices provide a high degree of isolation between signal sources and system loads by sharply attenuating reflected signals with very low loss in the forward direction.