X10 RF has multiple design issues which result in very poor RF range for things like motion sensors, door/window switches and handheld keychain remotes.
The remotes, motion sensors and door/window switches have LC components for setting the carrier frequency. These tend to be poorly tuned and this can worsen as the LC components age resulting in a carrier frequency that may differ significantly from that of the receiver (e,g. TM751, RR501, CM15A). Second, most have very inefficient PCB trace antennas resulting in very low transmitted power.
For stationary transmitters, adding a passive radiator adjacent to the device, as shown here, can result in a large increase in signal strength.
Unlike the TM751 and CM15A which have RF daughterboards, the RR501 has super-regenerative RF Receiver circuitry built in to its printed circuit board. It uses LC components, which can change value with age, to set its frequency. And, like the TM751, its antenna is isolated from mains voltage by being capacitively coupled (very inefficiently), through the plastic case. When all this is combined with poorly tuned transmitters, range is far from optimal.
Adding an isolated antenna connector, shown below, and a telescoping antenna or a more efficient eggbeater antenna can result in significant improvement. Only the 50-ohm coax bulkhead connector protrudes from the case of the RR501 and it is galvanically isolated from mains voltage by the 50-ohm RF transformer. For added safety, two X2/Y3 safety capacitors block DC and offer significant impedance (2.6M ohms) to 60Hz while presenting minimal impedance (0.5 ohms) at 310MHz. The connector is also secured mechanically on the inside of the case to deter tampering.
CAUTION: The RF transformer has a 300VAC breakdown (or isolation) rating. While this is significantly higher than the ~170V peak mains voltage, it is much less isolation than is offered by the original antenna design.
Install the connector where there's an area free of components. This will vary with different versions of the RR501. It requires about 1"L x 1/2"W x 1/2"H. In some cases the relay may need to be removed and the outlet made a pass-through outlet. Dimensions are shown below for an HC50RX, the RCA version of the RR501.
The connection details will vary with the specific PCB but all that we have seen have a ground trace outboard of the brass blade used for the original antenna. Connect the SIGNAL line to the brass blade (solder at midpoint up the blade) and the GND to the ground trace (which may require drilling a hole in the PCB).
If still more range is needed, a wideband preamp can be added to the eggbeater antenna base.
Schematic of the RR501
Add a Lightning Protector if antenna is outside.