Limitations of physics, plain and simple. Metal detectors work on the principle of induction which follows very strict physical laws. Adding a dowsing rod will not get around that limitation.

Almost all of the claims I've read regarding LRLs (MFD or otherwise) are absolutely false. "Signal lines", "rays", "lines of sympathy", "molecular resonance"... all are made-up terms attempting to explain a phenomena that is far better explained using psychology and statistics. If buried metals emit frequencies or gases or electron fields then those effects should be detectable using traditional instruments. They are not.

One reason that THing technology seems to progress slowly compared to other markets is lack of R&D, mostly due to lack of economic return. It's a mighty small pie.

- Carl