said by james1979:What is the the purpose of using a grounding rod if the ground is going to be connected to the house ground?
The purpose of tying the two grounds together is to keep ground current off the coaxial cable and to (hopefully) lower the overal resistance to ground.
As Greg said, there are two parts to the grounding process. Grounding of the antenna, and installation of a ground block on the coaxial cable prior to building entry.
The length of these ground connections must not exceed 20 ft. If that is not possible, the code provides this text: "Exception: In one- and two-family dwellings where it is not practicable to achieve an overall maximum bonding conductor or grounding electrode conductor length of 6.0 m (20 ft), a separate grounding electrode as specified in 250.52(A)(5), (A)(6), or (A)(7) shall be used, the grounding electrode conductor shall be connected to the separate grounding electrode in accordance with 250.70, and the separate grounding electrode shall be connected to the power grounding electrode system in accordance with 820.100(D)."
The referenced sections talk about bonding the supplemental grounding electrode to the building structure's ground system.
The modem itself is not separately grounded. It gets its ground over the coaxial cable and through the connection from the DC power supply, which in turn is grounded through the internal house wiring.