It is a NOTCH FILTER and is used to attenuate unwanted frequencies that may be presented to the receiver.
It's a notch filter to prevent the harmonics from your comm radio from interfering with your GPS.
Interesting. I will reconnect it. But it raises a question about the new GPS 20A position source that I am installing. The existing antenna is mounted on the glare shield, which will feed the GPS in the GDU 460, and will serve as the backup GPS source. The new GPS antenna for the GPS 20A necessary for ADS-B integrity is mounted behind the canopy slide rail. Will it require a notch filter as well?
C.