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When using a 2M/70cm diplexer for low-pass filtering (or with only one antenna on one of the two "outputs"), should the unused connection be left open, or terminated? (using a suitable impedance, such as a 50 Ohm resistor or dummy load?)

hotpaw2
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1 Answers1

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Terminated! An open end would have a reflection coefficient of |R| = 1 (because, where would the energy go).

Terminate

Marcus Müller
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  • It would reflect, yes, but is that unwanted? A typical two-port low-pass filter must also reflect as it has no resistive elements. Is the absorption more necessary in this case than the other for some reason? – Kevin Reid AG6YO Feb 19 '17 at 19:54
  • Well, true, but there's certainly some amplifiers where you wouldn't want that to happen, energy coming back. Next aspect would of course also be that he'd explicitly get rid of the high-pass component – and leaving the port open would at least make it an (inefficient, since horribly mismatched to $120\pi\,\Omega$) antenna. – Marcus Müller Feb 19 '17 at 20:00