With the NanoVNA in hand, it is worth looking at the passband response of a typical duplexer over the range of, say, 10-900 MHz.
In most cases - and with almost every brand and model used in typical amateur communications - you will note that the off-frequency response (>20 MHz away from the intended frequency) that signals will sail right through most duplexers: The fact that they might be labeled "Bp/Br" (Band-Pass/Band-Reject) should be viewed with suspicion as this typically applies only at frequencies very close to the notch responses.
What this means is that as they are, duplexers used in Amateur Radio service are not usable at shared repeater sites - at least if you want to be a good neighbor. In this area, many repeaters are on mountain tops that are shared with other land-mobile users and FM/TV broadcast stations and it was discovered, when they "upgrade" to a new repeater (replaced an old GE or Motorola with a Yaesu or especially Icom) repeater that they suddenly had a very deaf - or intermittently deaf - receive system since the "new" radios usually do not have the narrow-band helical resonators on the front end since they are typically parts of modified mobile radios that are crammed into a box - and we all know how easily Mobile rigs are to overload!
In all cases, the "fix" has been to add at least one "pass" cavity (Icom D-Star stacks seem to require two on the receive side at busy sites since their receivers are so miserably bad) on both the receive and transmit antenna paths - an preferably include a low-pass filter (especially on 2 meter repeaters) to attenuate the inevitable "3x" odd-order response that all of these cavities have, and to prevent these off-frequency signals from getting back into the isolator (all your repeaters have one on the TX side, right?) which will do very little on frequencies far-removed from the desired transmit frequency - and a true band-pass only cavity - along with proper grounding - provides excellent lightning protection, too.
73,
Clint
KA7OEI
