They could use the ISM band around 2.4GHz world-wide, but there's a reason it's called the "microwave oven band" or the "WiFi Band"...
http://www.linxtechnologies.com/I have to wonder if we are going overboard a bit.
We are trying to locate a roving module which will be in plain sight all the time by spending hundreds (possibly thousands) of dollars on electronics.
Unless it is on Mars there must be a more realistic way of doing it.
What about marking a 30 M radius circle (with a piece of string if you like) and get some clear flexible plastic tubing and lay it out around the circumference of the circle?
Use tent pegs to hold it in position.
It would take 189 metres of plastic tubing to do this.
Then you put a plug in one end of the tube and a pressure sensor at the other end and ring a bell if something runs over the pipe.
What is the actual application?
I know of none that are commercially available so you can have the fun of building it yourself. For the transmitter you'll want an oscillator, perhaps tunable, and a buffer amplifier driving a tuned ferrite rod.Are LF transmitters and receivers readily available or as you said is this something I will have to build myself? Size is sort of an issue (smaller the better).
And what else needs to be part of the ferrite rod antenna and first gain stage circuit...?The receiver will take the signal from the ferrite rod and amplify it, rectify it and compare it to your set point. The easy way is to use an IF integrated circuit with an RSSI output such as the MC3372. If you need more sensitivity you can always add a low noise, high frequency opamp at the input.