How can intermodulation interference between two repeater transmitters in close proximity often be reduced or eliminated?

To prevent intermodulation interference at a shared transmitter site, you must stop the RF energy from one transmitter from flowing backward into the final amplifier of another. A terminated circulator or ferrite isolator is the standard commercial solution for this problem. A ferrite isolator acts like a one-way valve for RF energy. It allows the transmitter's own signal to pass forward to the antenna with minimal loss, but it diverts and absorbs any external signals coming back down the feedline into a terminated dummy load. Standard filters like low-pass or high-pass filters are generally ineffective here because the interfering transmitters are usually operating on frequencies very close to the victim transmitter's frequency, passing right through standard LC filters.