this from memeber (reg)
There has been some confusion about using DD coil on The TDI series of detectors. In fact, someone from OZ mentioned to Miner John that Eric Foster indicated that DD coils wouldn't work on the TDI units. To the best of my knowledge, Eric never said that. What he might have said is that DD coils weren't necessary and the TDI would work find in most areas with a mono coil. Many years ago, Eric mentioned DD coils would probably be necessary in OZ, but this was stated well before he designed the GS 5 (the TDI initial design).
DD coils will work fine on any TDI including the SPP units. DD coils may work a little better on the TDI Pro simply because of the filtering modification on the Pro which allows for a little faster sweep speed.
The regular TDI and the SL series have a little slower filtering, thus requiring slower sweep action for best results and maximum depth. This is especially true when searching for small gold. So, regardless of the coil used, the SL series clearly does require a slower sweep speed for the best results.
Since the typical overlap of a DD is quite narrow, the detection range is nowhere near as wide as that found on a mono coil, thus the signal has more time to increase on a mono and this longer time of detection makes it easier to hear. Different designs of a DD could be tried including the rather odd design of the ATX DD coil which appears to have a small round overlap area.
DD coils are popular on VLF units because the reduce the ground signal. This is also true on a PI but also isolate the transmit from the receive on a PI, thus reducing noise further.
DD coils also have a narrow detection range which is quite sensitive, thus making them preferred on some areas when looking for small gold. However, keeping the DD coil low and going slow are required for proper results.
White's developed the Dual Field (DF) as a means of using a mono instead of a DD coil. Dan Geyer was the engineer responsible for the design. Unfortunately, Eric Foster found that the DF had some limitations in OZ and were not recommended for use there. Since the DF coil design was patented by White's, some other idea had to be implemented if the same concept was to be used that would allow a mono coil to have an enhanced small gold response.
Thus, the folded or foldback design was developed by me many years ago. The idea was to enhance signals from small gold without the use of a DF or a DD design. The specifics can be found back around 2009 on the Geotech forum where I posted how to build one. The idea was to build a figure 8 coil but fold the top part of the 8 back into the bottom half and then elongate the inner portion of the winding. The result was an increased field that enhanced small gold but didn't suffer some of the ground problems of the DF. Here is the link to that idea
http://www.geotech1.com/forums/showthre olded+coil
Drop down to my post about a different dual field design.
So, now you know a little more about the history of DF and the folded mono design.
Reg
