I have a brand new, never installed, INRAD 500 Hz Roofing Filter in the original box. This filter is a direct plug-in to replace the factory-installed 600 Hz filer. Never installed the filter as I no longer have the FTdx5000. It sells for $170 from INRAD. Buy mine for $125, which includes FREE Priority Shipping (CONUS).
Mike, W4UM email@example.com