This is the cavity-tuned LRL that I tested some years ago. It was tested under double-blind protocol specifically to determine how the tuning mechanism would influnce the response of this very sensitive swivel-handle type of LRL. The manufacturer was present during the bulk of the testing and agreed to the test environment and the test methodology.
The protocol involved "full-view" testing, followed by double-blind testing, followed by "full-view" testing. Each phase of the test was monitored and data recording was done by a disinterested third party, chosen by the manufacturer and agreed to by myself.
Neither the manufacturer or myself knew the results of any of the tests until after all the tests were completed. A single test consisted of several "trials" wherein the LRL was either operated by the manufacturer or under his direction. Tests were repeated over a span of several hours, in daylight and artificial light, and over a period of three days; thus eliminating as many environmental factors as was thought to be possible.
One claim was; a cavity-tuned LRL established a "signal line" (also called an "energy line") between the chamber of resonance within the LRL and the sought-after target the LRL was tuned to. It was also claimed; the LRL would lock in to the closest viable target, thus eliminating signal lines which may have been present from more distant targets. The manufacturer had a "special" proprietary method for cancelling a signal line once a target was recovered. (I am not at liberty to reveal how that was accomplished)
The results of the tests were pretty much as I expected. In all cases, both the manufacturer and myself were looking for "statistically significant" data to positively support the claims as I iterated above.
The full-view testing was 100% accurate with only one single exception. (In my opinion, the operator merely forgot where he saw the target had been secreted)
The DB testing was statistically the same as if the operator had merely "guessed" at a target location. In other words, using the LRL produced nearly the same correct location idendifications as would be expected from pure guessing. In about 4 of the tests the results were worse than guessing and in 2 of the tests the results were slightly better than guessing. In essence the claims were not validated by the test results.
At least in this one particular instance, and device, there was no observable evidence the device created a "signal line", as was claimed.
It was my understanding the manufacturer was ready to place this device on the market, but wanted some hard data and test results before doing so. To this date, I've never seen an LRL device of this particular design introduced to the consumer market, nor have I seen any devices offered by this particular manufacturer. (Though he may have under some other name I'm not aware of)
You didn't say, Graham.... how do you like your crow? ...well-done, with a touch of hot sauce.... or medium rare with all the feathers! 