Definition
A radio-frequency device that allows two transmitters or receivers operating on different frequencies to share a single antenna without interfering with each other. It uses frequency-selective filters to keep each signal on its own path while routing both through the common antenna line.
Plain English
A small electronic part that lets two radios use the same antenna at the same time without one drowning out the other.
Context Anchor
Seen in avionics installation, antenna wiring, and radio troubleshooting discussions.
Derivation
From 'di-' meaning two, combined with 'multiplexer' (a device that combines multiple signals onto one channel). A diplexer is the two-channel version, built specifically to merge or split two frequency bands.
Why Pilots Care
Allows multiple radios to operate through a single antenna while preventing signal loss or interference that could affect communication clarity.
Analogy
Think of it like a road divider at a junction: two streams of traffic can use the same area, but each is guided into its correct lane instead of running into the other.
Intuition Check
A diplexer is not the same as a duplexer. A diplexer separates two different frequencies sharing one antenna; a duplexer lets a single radio transmit and receive on the same antenna without the transmitter damaging the receiver.
Example Sentence 1
The avionics shop installed a diplexer so the VHF comm and the glideslope receiver could share the same blade antenna.
Example Sentence 2
A faulty diplexer caused weak signals on both comm radios until it was replaced during the annual inspection.