This diplexer from Perfect Vision is a high performance device with outstanding specifications. It separates or combines off-air  TV- Antenna and Satellite signals from or onto a single feed coaxial line You will need one of these at each end of the cable. One to combine the signals on one side and the other split the signal at the other end . They are also used with a multi-switch (combines the signals) to split the  signals prior to connecting to the receiver.

Features:

•  40-2250 MHz with 75OHM terminators insert off=air signals into four outputs of Phase III LNBFs
•  Use this diplexer to connect your off air antenna to a 1820DISH
•  Mixes or diplexes 950-2250 MHz and 40-2150 MHz bands
•  Low through loss and high isolation design
•  Use to receive local HDTV digital channels
•  Designed for outdoor use
•  Allows off-air and satellite signals to be fed on a single cable
•  DC power pass provided from the IRD to the LNB

Diagram: