Well you would need to know the frequency of the hd being taken out and the frequency of the modem. Yes if they are the same they will clash and one or both won't work. I am hoping I got this right.........are you trying to combine cable signal and satellite signal? If so then .........you will need to run the other line. And the diplexer is for the return which should be way out of the band for HD if the internet is on cable. But I might be missing something.
Re: Diplexer
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