G55 manual recommends at least 6 feet to avoid "near/far" issues...
Not sure how that applies to earlier ones.
From the manual:
minimizinG neaR / faR tRansmitteR effects
Line 6 digital wireless systems are designed so that a receiver only passes audio from a transmitter that is set to the same channel. While other nearby transmitters and RF sources will not create audio in a receiver not on their channel, under certain conditions they can have an effect on range. When you are using several channels of wireless, following some simple procedures will minimize any near / far effects.
The Relay G55 receiver constantly monitors the signal from its transmitter, and increases gain (sensitivity) as the transmitter moves farther away to maintain a good RF signal level. The near / far effect can happen when the transmitter is at a distance from the receiver’s antennas, and transmitters on different channels are being used near the antennas. The strong signal from the nearby transmitters, especially if they are close in frequency to the channel the receiver is set on, can mask the signal from the distant transmitter – and sometimes cause the audio from that transmitter to drop out.
For example, if the transmitter on the same channel as the receiver is 50 feet away, and another transmitter is 3 feet from the receiver’s antenna, the range of that distant transmitter might be affected. Avoid this potential problem by positioning the receivers and their antennas at a more equal distance from the transmitters that are in use.
• Making sure that any transmitter is at least 6 feet away from the receivers, and that other RF sources (such as WiFi routers) are also at a distance from them.
• Placing the antennas higher, which can lessen the difference in distance as well as increase line-of-sight with the distant transmitter.
• Using remote antennas (with the XD-AD8 antenna distribution system) and placing them approximately equidistant from each group of transmitters (for example, positioning a remote antenna connected to Antenna A nearer to the closer transmitters, and one connected to Antenna B nearer to the distant transmitters).
• Moving the receiver associated with the distant transmitter closer to it.