Also known as: diplexer
A diplexer is a three-port filter network that splits or combines two frequency bands on a single common port.1 It lets two radios operating in different bands share one antenna or feedline: energy in band A flows between the common port and the band-A port, energy in band B flows between the common port and the band-B port, and the two bands stay isolated from each other.
Overview
The defining feature of a diplexer is that it separates by frequency band, and — unlike a switch — it does so passively and simultaneously, so both bands are live at once. A classic example is a dual-band VHF/UHF station: a diplexer joins a 2 m and a 70 cm radio to one dual-band antenna, each seeing only its own band. It is essentially an RF crossover network, the radio-frequency analogue of the crossover that splits woofer and tweeter in a loudspeaker.
How it works
A diplexer is built from two complementary filters meeting at the common port. In the simplest two-band form, a low-pass filter carries the lower band and a high-pass filter the upper band; their crossover is placed in the guard band between them. Each filter presents a high impedance (an open) to the other’s band, so the two paths do not load one another and energy is steered to the correct port. For bands that are close together, the simple high/low-pass pair is replaced by two band-pass sections — often sharp cavity filters — to get enough separation. A triplexer extends the idea to three bands.
Key specifications are insertion loss in each path (kept low so little signal is wasted) and isolation between the two band ports (how well band A is kept out of the band-B radio).
Diplexer vs duplexer
The names are often confused. A duplexer separates the transmit and receive frequencies of one service — a closely spaced pair on the same band — to share an antenna between a TX and an RX. A diplexer separates two different frequency bands, each of which may itself carry both TX and RX. In short: a duplexer splits a TX/RX pair; a diplexer splits two bands.
Relevance to SDR
Diplexers are common in multi-band installations and are handy for SDR users who want to feed a single wideband antenna and feedline to more than one receiver or to combine antennas optimised for different bands. A diplexer can also serve as a crude preselector, keeping a strong out-of-band signal (say broadcast FM) out of the path feeding a VHF/UHF receiver, reducing overload and intermodulation.
GopherTrunk is software and contains no diplexer; the component is purely part of the antenna and feed system upstream of the SDR. It is relevant to GT users only as a way to share or clean up the RF feeding the receiver that produces GT’s I/Q stream.