A triplexer is used by dish net its a diplexer and seperator built into one . a diplexer is a signal combiner one side is power passing sat it runs 950 to 2150 mghz the uhf side runs the 2-950 mghz you can diplex a cable modem and a reciever in a swm system however you will loose connected home frequencey they run from 2-75 mghz which would end up on the uhf side of diplexer. If you want to use a single line for a bbd and cable modem you can do a back door splitter sat and cable modem on the output side line to modem on the input then reverse that at the modem and bbd a splitter rated for 1000mghz will work.
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