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.
Reception problems are made worse by the design of the CM15A which uses a super-heterodyne receiver which not only has a narrow bandwidth but also uses LC components to set the frequency resulting in very poor frequency alignment between it and the transmitters. However, unlike the TM751 and RR501, the CM15A antenna is isolated from mains voltage by T1, T2 & U4 (see CM15A schematic).
Some users have reported they added a more efficient antenna without seeing significant improvement while others have reported significant improvement by replacing the narrow-band super-heterodyne RF Receiver daughterboard with a wideband super-regenerative one. These mixed results are a strong indication that the poor RF reception is due to misalignment of the receiver and various transmitter frequencies.
In a few cases, adding this eggbeater antenna may be necessary to achieve maximum range even after changing the RF Receiver. In extreme cases it may be necessary to add a wideband preamp to the antenna but the stronger signal may overwhelm the receiver, resulting in decreased performance. Or you might begin receiving X-10 RF from other users in your neighborhood.
Since the CM15A electronics circuits are isolated from the mains, adding a bulkhead 50 ohm BNC connector and connecting it to the receiver is all that is necessary to use either a telescoping antenna or the eggbeater. Remove the flange on the inside of the slot for the original antenna and widen the slot just enough to accept the BNC connector. Connect the original antenna lead to the center of the BNC connector and add a ground wire from receiver (just outboard of the antenna lead) to the BNC ground lug. The ground lug is hard to discern in the photo below due to the reflectivity of the connector so it is outlined in red.
Add a Lightning Protector if antenna is outside.
Adding 100nF decoupling capacitors in parallel with C11 & C12 and to U1 & U2 as shown in this CM15A schematic and in the photo below will reduce the noise that the chips have to deal with. I have scraped away the soldermask and installed 1206 size SMD capacitors. For the Cypress processor, I used an axial lead through-hole capacitor. See red lines for locations.