The transmitters have a soldering spot/hole for an antenna. I discovered this via a blogpost I read this morning. It will extend the range significantly with the given voltage/amps, so no need to get a repeater or mess with the voltage.
In that example, they used a "12 inch piece of wire from the inside of a cat 5 ethernet cable", but there are specific lengths that are optimal for RF antennas at certain frequencies. The length needs to be a harmonic of the wavelength in order to optimize harmonic resonance. To get the best signal, the length of the antenna should be a multiple or 1/4 or 1/2 of the wavelength of the frequency. There are antenna length calculators online to determine these lengths. Make sure you use one that calculates monopole lengths (not dipole). However, here's a quick cheat-sheet:
- Full wavelength antenna length: 27.2"
- Half wavelength antenna length: 13.6"
- Quarter wavelength antenna length: 6.8"
I went with the 27.2" length (I rounded it to 27 & 1/8") and it covered my whole house.
I have not yet soldered the antenna. I ran it though the hole in the transmitter board and twisted the end back and around in a loop just to test it out (like how you twist a twist-tie).
Note, on my transmitter, the antenna hole was not precisely marked. It wasn't the closest hole to the "ANT" label. It was a hole on the corner of the transmitter board, similar to this:
The wire I used was an old copper speaker wire I had lying around. Note any wire should do. Diameter is unimportant.