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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 discrete LC components (inductors and capacitors) for setting the carrier frequency. These tend to be poorly tuned from the factory 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). Plus, most have very inefficient PCB trace antennas resulting in very low transmitted power.
RF reception problems are made worse by the design of the TM751. Older units use a super-regenerative receiver with a fairly wide bandwidth which tends to compensate somewhat for poor tuning of the transmitters although they also exhibit the same tuning issues as the transmitters. Newer units use a super-heterodyne receiver which not only has a narrow bandwidth but also uses LC components to set the frequency, sometimes resulting in very poor frequency alignment between it and the transmitters. But the biggest limitation is the TM751 antenna which, because the electronics are not isolated from mains voltage, is capacitively coupled (very inefficiently), using the plastic case as the capacitor dielectric.
For stationary transmitters, adding a passive radiator adjacent to the device, as shown here, can result in a significant increase in signal strength and range but this is not practical for handheld remotes.
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 bulkhead BNC coax connector protrudes from the case of the TM751 and it is galvanically isolated from mains voltage by a 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. However, to make room (requires about 1"L x 1/2"W x 1/2"H.) for the small PCB attached to the bulkhead BNC connector, the relay must be removed. The formerly switched outlet can be made into a pass-through outlet as X10 has done with their most recent version (c2012), the TM751A.
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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.
The three TM751 variants are shown below. Older versions used a superregenerative RF receiver (left, below) while newer versions use a superheterodyne receiver (center and right). Some of the newer ones come without a relay. (TM751A).
 TM751, SR receiver |
 TM751, SH receiver |
 TM751A, SH receiver, pass-through outlet |
Desoldering of the relay (refer to leftmost picture, below) will vary with different PCBs, especially for the two small wires (red circles, lower right) from the coil. You may have to trace these wires to be certain. The main connection (red circle, lower left) also serves to mount the relay and the other connection (red circle, upper left) is the relay output to the switched outlet. After removing the relay, adding a wire from the lower left point (may need to drill out rectanglular opening) to the upper left point creates a pass-through outlet as shown in the center picture below (black wire). Desoldering the relay is not for neophytes. For those without extensive soldering experience, it is recommended to buy a TM751A which has no relay.
For the newer models with super-heterodyne receivers, install the connector on the side where the relay was mounted. The older version with the super-regenerative receiver requires mounting the connector on the top of the case in fairly tight quarters. Center the connector horizontally and vertically in this small space. Dimensions for both types are shown in the drawings above.
 Relay desolder points |
 SR Receiver Connections |
 SH Receiver Connections |
Slide the original antenna out of its pocket prior to reassembly.
 TM751, SH receiver |
 TM751, SR-WB receiver |
In some cases, adding a wideband preamp to the antenna, may be required for even longer range. However, you may then discover there are many other, previously unknown, X10 users in the neighborhood.
Add a Lightning Protector if antenna is outside.