By design, the dual field (DF) coil should not go quite as deep on larger gold. However, on smaller gold, less than 1/8 oz the DF should begin to do better. Your 4 gram nugget is about the crossover size. Keep in mind that a 4 gram nugget is about 62 grains and a lot if not most of the nuggets found are much smaller than this size.
Keep in mind the basic design was to provide the depth of a mono coil but enhance the signal strength on smaller more common gold, thus the DF design worked for large gold while making it easier to detect the smaller gold also.
BTW, one can hunt for smaller gold with a DD coil but you have to sweep slower for the best results. Tests performed by AZO indicated the DD coil did a little better on smaller gold in the few grain size, but the coil didn't go quite as deep on larger gold when compared to a similar size mono. A DD coil can be a little quieter because the transmit is isolated from the receive plus the receive coil is a little smaller than a similar size diameter wise mono coil.
As mentioned, the DF coil was designed by a White's engineer named Dan Geyer. I was truly saddened to hear that he passed away. Dan was a great guy and a brilliant engineer who loved nugget hunting. I was fortunate enough to have met him a few years ago and I can honestly say, I enjoyed my talks with him.
On a different note, the SL does operate on AA batteries, but can run on LI Ion rechargeable types also. RNB makes a Li Ion battery pack that works very well.
My recommendation is to change the AA batteries well before the warning light comes on. As they diminish in voltage, the detector will begin to lose depth and can become a little noisier. Fortunately, or unfortunately, the noise reducing design makes it difficult to hear the noise increase. In any case, as the noise increases, the depth is lost. This depth loss isn't much but it is there and any depth loss when hunting gold can be detrimental.
My recommendation is to use the high/low conductivity switch and hunt initially in the low conductor mode. Most gold found will be found in this mode. Also, depending upon the gold purity, even a lot of the larger gold will be detected in this mode. Very pure gold will cross over from the low conductor to the high conductor mode at somewhere around a 1/4 oz size. Less pure gold may never cross over even in the oz + size range.
On another different note, I have to thank two people, Steve Hershback and George Baker for giving me the idea which caused me to develop the conductivity switch idea. I installed it on a GS 5 prior to the TDI's existence. Fortunately, White's elected to add that feature to the TDI. I also have to thank Steve for his information about the gold conductivity in Alaska. From what he mentioned, I conducted several tests plus searched for technical information on what happens to gold and how its conductivity changes as the purity changes.
There used to be a website that displayed how the conductivity gold changes from a very high conductor to a very low conductor simply be changing the purity of the gold by adding something as common as silver and/or copper to the gold. Since this commonly happens in nature, a gold nugget can respond as a high conductor, much like aluminum to a very low conductor similar to lead. This is why it is important to know more about the gold in the area one is hunting. In simple terms, knowing more about the gold nature can mean in some areas one only needs to hunt in the low conductor mode and not miss any gold, but be able to ignore most iron junk. In areas where the gold is pure, then one will have to use the all mode or detect an area using the low conductor mode and then go over the area in the high conductor or all mode.
Why use the low or high conductor mode and not the all mode? Simple, to reduce the noise level on the TDI and the SL for that matter. Also, a person can search an area again in the all mode or the high conductor mode if they are worried about missing anything.
As for the coil question as to which is best, the answer lies in what a person expects to find in terms of size of gold. A DF does better on the smaller gold down in the few grain size. A foldback design does well also if the coil is built properly. Basically, the foldback design gives you the depth of a similar size mono plus enhanced response to small gold when under the fold back area.
BTW, for those interested, the foldback design is something I developed shortly after the DF coil was introduced as an alternative to the DF. Reference to this foldback design can be found on the Geotech 1 forum for those interested. Since the DF was patented, any duplication by anyone selling coils would be infringing on the patent. The foldback is a totally different design, but does have a similar ability to detect smaller gold.
Here is the link to my DF idea. Just go down to my post and open the pic to see the basic design.
http://www.geotech1.com/forums/showthread.php?15585-Coax-for-duel-field-coil&highlight=dual+field
Again for those interested, Jason elected to build the foldback design and he referenced where he got the idea way back when he mentioned on his forum he was going to build coils.
One last note, I have a couple of new ideas are in the works including a different coil idea. Actually, the coil is only different to a PI. If wired a little differently, special low noise parts can be used, thus reducing the noise even on the TDI. Add this feature to a simple mod or two to add the noise reducing design and the TDI becomes a different detector. All of this takes time so don't look for it right away.
Reg
