Off topic or on topic well this is on topic.

We have heard of the double D coils what about a triple D coil. Well that outside my knowledge but let look at a quad D coil the oldest ground balancing coil. Have a look at these links.

....LINK....
https://www.phys.k-state.edu/reu2011/nnorvell/Metal_Detector_Research.html

and more detail.

....LINK....
http://41.67.20.41/bitstream/handle/123456789/18621/Metal%20Detector.pdf?sequence=1&isAllowed=y

Are we on topic.

G'day geof

Both Interesting & On Topic. Although there are discrepancies at the very low end frequency scale, detracting & discrediting frequency peaks from that that were expected, in layman's terms, can this set up of coil winding, be successfully used for the purpose of enhancing what we currently have in use today?
I myself do believe, that all coil windings (regardless of the type of winding configuration) are but a form of antenna, able to both transmit & receive signals, according to the output power & or signal filtering system of the detector.
Surely there must be some way, of doubling up on that transmit coil wave, attaining far greater depth & or a far better form of target Identification, or would it be more advantageous to somehow further enhance the receive winding instead or maybe even both at the same time?
One way or the other, I know there's a lot more to coil technology than currently made available. Mind you fellow detectorists, I might also be lost in thinking/talking, a whole lot of mumbo jumbo?    
BTW, don't ask me if I had found any gold during the couple hrs out there detecting this arvo (over thrashed ground), for apart from the odd bit of junk targets, I came back skunked of colour.    
Cheers Kon.

Detector companies should work on leaving the detectors maximum output frequency as a standard operating function & work around making coils to suit that maximum detector output frequency. Knowing PI's have an extra say 30% output power in reserve & not being able to use that extra power, without the use of choke modes or filters being incorporated (causing a dramatic loss in depth) is the biggest nonsense I've seen under the sun.  This is why to this day the older SD's are able to match or even surpass that of the much newer/later PI's on or over large gold buried deep in areas of high mineralization.
It is quite evident that detector manufacturers are placing more emphasis on smaller & smaller gold, with each new model of detector brought forth, than ever seen before.
We've gone from fly sh*t, to the fly's feet & now were a step away from chasing the fly's toes in gold size.  
What's next, a $15,000 detector specifically made for chasing the 0.001gm?

Cheers Kon.