Question for Mr. D (or anyone else who understands coils)...

While in my test garden today, trying to sort out my F70/10" stock coil VDI issues, I tried some coil swaps. I put the Gold Bug 11" DD on the F70, and the F70 5" DD on the Gold Bug. Got some strange results that I wondered if you could help me understand...

GB Pro 11" DD on the F70 -- no loss in depth compared to the stock 10", at least not enough to "lose" my 12" coins. It still saw them fine. Better VDI results on 6" coins too -- I could get fairly consistent audio/VDI when careful to center the coil hot-spot over the coin. Marginally better audio/VDI on the 8" quarter; no better on deeper quarters (all iron audio/VDI on the 10"/12" quarter). I expect the odds to be fairly high that the 11" F70 DD may show even more improvement.

More strangely though, and this is the part I would like to understand...

F70 5" DD on the GB Pro -- SUBSTANTIAL decrease in performance, compared to the GB 11" DD, when running the GB in disc. mode; HOWEVER, equal or POSSIBLY EVEN IMPROVED performance, compared to the GB 11" DD, when running in all-metals mode.

With the GB in disc mode, the F70 5" DD lost ALL COINS in my garden, except for the 6" nickel and 6" quarter (my shallowest coins) -- whereas, I can easily hear ALL but the 12" quarter (which is VERY iffy at best) when in disc mode with the GB 11" DD. I was shocked with this, but then not nearly as shocked as I was when I switched to all-metals mode. Not only did the F70 5" DD in all-metals mode out-do the GB 11" DD coil in disc mode (depth-wise), but I think it even out-did the GB 11" DD's all-metals mode performance. The 12" quarter hit LOUDER, I'm almost sure, on the F70 5" DD, than it did when using the GB 11" DD. It MAY have been because I was able to run slightly higher threshold, but still -- I would have expected a depth loss with the 5" DD coil (though not as substantial as I experienced when in disc mode).

Is there any obvious way to explain this strange set of findings when running the GB Pro with an F70 DD, and could this have implications regarding the results I got when using the GB 11" DD on the F70 (i.e. is that another reason to expect the F70 11" DD to perform better on the F70 than the GB 11" DD did?) Obviously, testing is needed, just wanted some speculation/information...

Steve

Now get the right coil.....for the right detector!!!!! ((( F70 coil for F70....and GB coil for GB ))). I do not believe they were meant to be interchanged....due to different operating freqs.

Clearly, that is the right suggestion! I was not expecting much in the way of performance, with my "coil swapping." However, I was just wondering why the 5" DD from the F70 might have been SO POOR on the GB in disc mode, and SO GOOD on it in all-metals mode. Just weird!

Steve

Resonant frequency and inductive capacitance of the coil. This also plays a role as to why the Grnd Bal is critical in the all-metal mode.......and not quite as important in the ID mode (yet, still important).

Interesting; thanks much for the answer. So there IS a logical way to explain what I experienced. I figured there was; I will now google those terms. Interesting.

Steve