Commonly used to guide automatic lawn mowers, a cable carrying an AC frequency could also possibly guide the WEEDINATOR. This would have a few advantages over using a camera as there would be no problem in very bright sunlight and it would not matter if the cable became slightly covered with soil.

To get started, I bought an incredibly cheap $9 kit from Robotshop  and put together a test rig with an oscilloscope from RS supplies:

For the price of the kit, the results were pretty good, but I soon realised that I'd need something a bit better for guiding this machine. 

On the frequency generator board there is a variable resistor that changes the output frequency to match the receiver board and the 'ideal' calculated frequency is 33.932 kHz., but my receiver worked best at 36 kHz:

I wondered why I was getting much better sensitivity on the red channel than the blue and at first suspected the paralleled inductor / capacitor combinations. To test this, I moved the inductors relative to the transmitting wire to balance out the response and adjusted the frequency on the generator board backwards and forwards from a centre of 36 kHz as shown below:

The difference between the two channels did not change that much, so the LC bandpass filters actually seem to be quite well matched. Since the oscilloscope probes are on the inductors, the problem must be resistance in the inductors or the cables / soldered joints connecting them? Maybe a faulty inductor?