cleaning out the CPARC shack last month, we happened upon a helical duplexer set, in additional to various other pieces of equipment, which seem to indicate that someone in the club in the 1990s was collecting parts for a homebrew 2m repeater.
Duplexers are an interesting part of a repeater. Repeaters are a common radio installation where many low-power mobile radios transmit on a single frequency, which is received by a strategically placed receiver. This received signal is then re-transmitted at higher power, and usually a fixed frequency offset, so that every other mobile unit can hear each other, as long as they're within the coverage area of the repeater.
The problem with repeaters that you need duplexers to solve is the fact that you are trying to receive and transmit on two unusually close frequencies at the same time from the same place. For example, a 2 meter (146MHz) repeater will only have a 600kHz offset between the input and output. Even using two separate antennas, when you're trying to transmit 15 watts and still receive a 2μV signal only 600kHz away from it, things can prove problematic. Duplexers solve this puzzle by being a pair of ridiculously good notch filters; one to filter out the input frequency from the transmitted signal, and a second to filter out the transmitted frequency from the receiver.
Notch filters are traditionally built from inductors and capacitors, but each pole of a passive filter only gets you 20dB of attenuation per decade of frequency change. A good rule of thumb for the needed isolation between the two sides of a duplexer is 90dB, and instead of being a decade apart (i.e. 14MHz vs 140MHz), a typical VHF repeater only has 0.00178 decades of separation between the input and output frequencies (146.160MHz vs 146.760MHz). Using traditional passive filters, you would need to build a 2,500 pole filter to get the needed 90dB, which is comically large. Duplexers instead operate on the principle of using a resonant chamber as the notch filter, at which point getting a Q in the thousands with only a few stages becomes possible.
VK6UU's article about traditional duplexers.
Now what to do with a really nice set of helical filters?