The 7 1/2" dual field coin is my go to coil on the SL. I purchased the coil on one of the forum sites. The coil was advertised as a mono coil but it turned out to be the dual field. The reason I was wanting the dual field coil is because of the areas I was to hunt. I knew right from the start there was something different about the coil. I ask Carl Morland how to test and sure enough the coil was a dual field. I contacted the seller and he basically shut me off (he knew what he had but I was asking for a mono and he thought he could fool me). But I decided to give the coil a try and found the heaviest gold ring ever in a completely hunted out site (with VLF machines).
The following is a success story and you will understand my need for a true mono coil.
More than 10 years ago I started inquiring about some old locations I hunt. Most of the locations are old shut down schools where coal was used as a heating source for decades. Before the EPA was birthed most people did whatever pleased them when it came to disposing of waste, regardless of what it was and these old school sites are no exception. The burnt coal waste was spread over many acres of school property which created some extremely harsh ground conditions. Grass and weeds find it hard to get a start and most areas are void of any vegetation. Most of the school grounds look like Martian landscapes with small BB size or smaller pieces of coal waste everywhere. This material attracts to a magnet with little effort and can reduce depth of all VLF detectors by well over half. In fact until recently maximum detection depth was actually 2-3”, any target deeper would give a solid iron audio report if any sound at all. After many years of hunting these areas all but completely unsuccessfully I finally purchase a White’s TDI SL.
It turned out the SL opened up these old sites and many nice coins and relics were unearthed, but not without many trials and numerous adjustments. Case in point: one particular area had been, in my opinion hunted out with many different VLF machines over a 10 year period and I was certain there were no good targets left. I had been hunting about 15 minutes and all the SL was giving were very short audio reports, which sounded more like chatter or EMI and not targets. This prompted me to increase the time delay to about 15 “which increases the time before a transmitted signal is analyzed” thinking the small pieces of coal waste were the short reports I was hearing. Continued hunting another 5 minutes and noticed the short audio chatter continued but not to the same magnitude. Stopping and increased the delay to around 17 and off I went hunting again. Suddenly I noticed the machine was running very quiet, to quiet. A minute or two later and a very loud low tone, which on the SL means a high conductor, I stopped and reduced the delay to 10 and found my definite answer. The coal waste was causing all the ground chatter and false reports. Increased the delay to 17 and recovered a wheat penny around 4” deep. Now to be honest I had to stop for a moment and think about what just happened. Decided to start over I returned to where I began hunting and discovered I had passed right over many good targets. After digging a few more wheat’s I decided to start checking these targets before digging and discovered if I decreased the delay most of these targets became the short sounding audio reports I had heard earlier. The PI was just the trick to discovering some nice coins deeper than 4” in these barren areas. I must add the SL is not the best choice to make if there is an over abundance of nails because of the very limited discrimination capabilities (using the ground balance adjustment).
Just one of many successful hunts.