I have searched for this question online and I've seen a variety of answers, most of which are non-technical and simply this is a bad idea and I would never do this without any rational to back it up. I am looking for a concrete technical reason why this should not be done. I want to run the coax from the LNB (Dish Network) and the coax for the Internet (Comcast) over the same coax cable that enters my house.
I do have a DOCSIS 3.0 modem (SB6120) and I know the frequencies range from 5MHz to 1000MHz while the signal coming out of the Dish Network LNB is on the L-Band around 950MHz to 2300MHz, so yes there potential for overlap; however, it seems that my downstream channels are all in the 600MHz range and my upstream channels are in the 20MHz to 40MHz range. Assuming my cable modem keeps the same channels so there is no interference, is it possible to these two services to share a coax entering my home if I use a diplxer to combine them going into the house, and another to split them when the are in the house? Are there other considerations such as power levels that I am not thinking about?
Link to the diplexer I purchased 2 of: http://www.amazon.com/dp/B0018BS728/ref=pe_385040_30332200_pe_309540_26725410_item