Fi 30 is a diplex filter used to combine the signals from an AM and an FM-TV antenna, eliminating one coaxial downlead. To prevent interference when operating short-wave transmitters, it is possible to mute the AM-signals by activating a 24V relay. Unwanted AM-signals from the FM-TV antenna is attenuated by the filter. The AM-input is protected against high voltages by a surge arrester. This is particularly beneficial to prevent damage to underlying equipment when using a passive antenna. DC-voltage is fed to active AM-antennas, eg. VPA 30, via a jumper (R2), if a passive antenna is used, the jumper should be cut.