This is for the Domain 5000 only. The Domain 3000 uses the newer Powered RF Antenna that normally provides excellent reception and comes with its own extension cord.
The RF antenna that is shipped with the Domain Controller is a good quality antenna. However, its location on the back of the Domain Controller often puts it in a poor location to pick up the RF signal from the remote controller. Therefore, you may want move the antenna to a better location. Use a 50 ohm coax cable. Commercially available cables will have female connectors on both ends, so you will also need a male/male adaptor. Connect the coax cable to the RF antenna BNC connector on the back of the Domain Controller. Connect the other end of the coax cable to the male/male adaptor and then to the antenna that came with the Domain Controller. Place the antenna at least 2 feet away from all AV components and metal enclosures. The reception will improve dramatically.