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


2 Answers 2


No. Do not even think of doing this. There are huge safety issues. How is the LNB going to get power?

The LNB gets its power from the receiver or set-top box inside the house. This phantom power is sent "up" the same coaxial cable that carries the received signals "down" to the receiver, eliminating the need for a separate power cable. -- Wikipedia.org

  • I knew the receiver powered the LNB, but I didn't believe the power levels were that high (I of course have no idea where/why I made this assumption). I am curious then, why does anyone sell these diplexers then? Jan 14, 2014 at 13:19
  • Furthermore I see the model diplexer I purchased has a DC block on the CATV side. Jan 14, 2014 at 17:47
  • @FredThomsen That might solve some of the issue. Does it pass the DC to the satellite side? Jan 14, 2014 at 22:38
  • Yes, there is DC passthrough on the satellite side. Jan 15, 2014 at 13:22

I did end up going this route, and I can confirm it works just fine, as the frequency range doesn't overlap and the diplexer only passes the power through one side. I also added a dc block for the cable modem feed on each end due to extra paranoia. The only issue I can foresee with this setup is that DOSCIS 3.0 frequencies can be greater than 1GHz and then there would be issues with the frequency ranges overlapping if my ISP decided to use those channels on the downlink.

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