Therefore, if you trace a pipe or cable under a through concrete or a building, LF is best, assuming if course that the pipe or cable is continuous metal without gaps or breaks, or, you are looking for a break in a cable. For the same reason, LF may be best in a building, provided you can connect to the target, and you can get a proper ground (remember a completed circuit by the signal from the transmitter through the target line and back to ground, is needed in the conductive mode). It is possible to 'light-up" the entire building if everything is running to the same ground. You can also clip onto a terminal of a cable or a pipe at one end, then use a long return wire back to ground. Some people make up their own long ground wire for this purpose.