Inductive pick-up

Need help on an inductive pick-up coil. Building a robot that basically will follow a buried wire. Need a coil pick-up sensor similar to what is used for an invisible dog fence, or some of the robotic lawn mowers that stay with-in a given area, or some of the automated transporters used in warehouses. I have looked at several parts catalogs and just haven’t been able to find what I want. --- Suggestions for a low cost DIY coil idea that I can build, or buy and tune, and be able to work with? --- Thank You

You need to decide how the system will work before you can build a sensing coil. For example, some dog fences use audio frequency radio signals. What will you use for the transmitter?

I am looking for what is being used. If I can locate sources for the pick-ups at a good price, (I need 12 of them) I will just order them. If someone can point me in the right direction to make my own, great. All I am looking for here is some information and possible sources. As to the transmitter and receiver/control system, I will take care of. (I don’t want to reinvent the wheel if I don’t have to, as far as the sensors are concerned.) TNX

FYI update: After finally being able to get back on this project, and giving a little thought to it, a possible idea hit me. I got some small chokes (unshielded), very small, cheap and rugged. I am getting very nice sensitivity, up to a few feet with no problems. By using various size caps, I can tune them where there are different frequencies for various commands. All looking very good. (KISS came through again.)