Rich, thanks for sharing your experiences. In my case, I've tried 9.5" coils and 15" coils and found no better depth detections in the soils I've hunted! I've primarily hunted the S.F. Bay Area and the Portland Oregon areas, with moderate mineralization. Oh, and one time, the OTTS went for a club hunt in Washington Park in Portland. I saw a guy go over some ground with a "WOT" coil, which I think is 18" diameter. He found nothing. I went over the same ground with a 4X6" "shooter" coil with my MXT and found a Wheatie at ~4"!

I made a slight error in my response to kickindirt. I should have asked where (more specifically, the level of mineralization) he hunts in. This makes all the difference in the world.

Mineralization conducts the transmit field of the detector (in a typical "VLF" or induction balance detector). It "channels" the field into a smaller volume beneath the coil. The more mineralization, the more detection depth is lost, because the depth of the transmit field is reduced. This is the main reason for the value of the pulse induction detector, which is effected by the soil to a much lesser degree.

I found this coil selection chart on the web at an Aussie site:

• Small jewelry (e.g., necklaces and thin gold rings) at 7 – 15 cm
• Big jewelry (e.g., class ring) at 15 to 30 cm
• Small coins (e.g., Ticky, 1 cent) at 10 to 15 cm
• Large coins: (e.g., Kruger pond, 5 shilling) at 15 to 30 cm
• Jam jar lid, bicycle licence disc at 22 to 40 cm
• Coffee can size at 30 to 60 cm
• Cannon shell, etc. at 60 to 90 cm

Note: 7cM = 2.8", 10cM = 4", 15cM = 6", 30cM = 12"

Medium Searchcoils
For general-purpose hunting, which typically includes coins and coin-sized targets, a medium sized searchcoil (8 - 9”) is the best choice. Because a medium sized searchcoil provides the best combination of magnetic field concentration, detection depth and capability to detect the greatest range of target sizes within the detection area, it is standard with most detectors. In addition, it's lightweight and easy to maneuver.

Speaking of George Payne, this from a paper he wrote on detector design:


"As most of you have observed a larger coil will pick-up more ground mineralization. Because of the coil size it is effectively closer to the ground than a smaller coil. Therefore, it may make more sense to use a smaller coil in high mineral. Keep in mind that a coil picks up the ground in a very non- linear way. Pushing the coil down against a high mineral ground may get you say 1/2" closer to the target. However, the increase in ground signal may be several times greater than a deep target's signal. In this case you would wind up with less sensitivity. For better results raise the coil 1 or even 2 inches and use a smaller coil in highly mineralized soils.

Here are some general design parameters for a coil:

Transmit Coil -- 25 to 30 turns of 22 gauge wire. Diameter 7 to 8 inches.
Receive Coil -- 200 to 300 turns of 31 gauge wire. Diameter 3 to 4 inches
Feedback Coil -- 6 to 10 turns of 22 gauge wire. Diameter same as receive.
Tank Capacitor -- 0.47uF"


The bottom line: The largest coil size you can use to detect a penny/dime, depends upon your soil. In moderate mineralization that I'm familiar with, 8" is about it. In air, or soil without minerals, a larger coil can get you more depth - until the size of the target becomes the limiting factor.