The Wireless Transmitter is used to send signals to wireless network items in Refined Storage.
Keep in mind that you have to place the Wireless Transmitter on a Cable.
The default range is 16 blocks, but can be upgraded with range upgrades.
Placing transmitters efficiently
Placing 2 Wireless Transmitters just next to each other will not do that much.
The wireless signal starts from the Wireless Transmitter's position, so, if you place 2 transmitters next to each other you'll only have a net gain of 1 block.
Instead, you have to spread your wireless transmitters around.