Can you provide some specifics?, like, how are you hooking this up, are you trying to combine the comcast and sat signal at a multiswitch, then seperting them out at the receiver, if not, can you elaborate? along the lines of what receiver you have, dish type (dual, 3lnb, or at9), is their a multiswitch in place for the sat, if so what configuration is the switch (i.e. 4x8, 5x8 etc).
BTW, if you are trying to put a cable tv splitter on a coax that carries the satellite signal, it won't work.
Your cable modem and directv are two completely different systems. Like damaged said, you cannot split your directv cable and plug your cable modem into one side. Your cable modem needs to be plugged into the coax coming from your local cable system. Now, are you talking about combining your satellite and cable on one line? If so, then you would need a pair of diplexors.