The Frequency Transmitter are items that are used to link Teleporters. When right clicked on a teleporter with the Frequency Transmitter, it sets itself to the current frequency of that Teleporter. To link it to another Teleporter, right click on the destination Teleporter. Linked Teleporters effectively allow you to cover long distances instantly by using some EU power. After linking two Teleporters, it needs a reset of frequency or else it's going to link the previous starting Teleporter to whichever Teleporter you right click on. To reset right click anywhere.