This being such an issue I wonder why no combiner manual I've read so far mentioned it. Also why not sell the units with terminators for each port.
Is this also best practice for open splitter outputs?
All active combiners or splitters as sold for low power transmit and receive applications are based on the
Wilkinson circuit, with circuits cascaded to achieve more inputs or outputs. The key benefit of this circuit is a relatively high isolation between the two individual ports, but with a minimal insertion loss between either individual port and the summed port. Although the Wilkinson circuit can be designed with individual port counts that are not 2^x, the simplest designs are in fact 2, 4, 8, 16 way, etc. Ports are simply cascaded.
So taking the port pairs at the opposite ends of the overall cascaded circuit will provide the greatest isolation.
Regarding the termination of unused ports, it's always a good practice, more so with receive multicouplers (splitters) to ensure proper impedance matching at each output port so as to minimize any reflected energy back into the circuit.
With TX combiners, it's not as crucial because each input goes directly to an amplifier stage which has a directivity spec; basically a reverse gain value (usually in the -20 to -30dB range), so reflected energy is rarely an issue. If there is a partial internal fault however, the termination on the input port can substantially reduce any oscillating reflections getting magnified in the amplifier stage.
"Also why not sell the units with terminators for each port?" Because the manufacturer has no idea how many ports you'll wan to use. Just
buy them for your work kit. (Don't buy cheap no-name ones; their return loss is usually awful.)
