We all want to use the biggest coil we can for the condition. The ground ant type of terrain will determine what you can use. In big open areas with medium ground you can swing a good size coil and cover more ground, however, big coils tend to give up sensitivity to smaller gold targets. If hunting relics and coins, this will not hurt much. This can also be a bonus, allowing you to not detect the very tiny shards of metal.
Areas that are very rocky and have lots of brush will present a challenger for big coils. Ground mineralization can also be a huge factor with big coils. Noisy ground from high mineral content will need a smaller coil to get a smooth threshold. Choose the largest coil for the conditions. If the big coil is not smooth, you have ground balanced well, and have lowered the gain by several numbers, this would be the time to drop coil sizes… Small coils have their place, and if one only has a choice of one or two coils due to the backpacking in or funds, then go for small to medium coils. They will work everywhere no matter what. You just will not get the coverage. It’s easier to pinpoint a target with smaller coils, and they’re more
sensitive to smaller items and gold. They ground balance much easier and are also much lighter.
DD or Mono?
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Mono coils, under most conditions, are deeper coils than DDs. They give a bowl shaped field and “see” a lot of ground under the coil. Your first choice should be a Mono if you can. However, if the mineral content of the ground is very high, or there is a huge amount of trash, then a DD is going to serve you better.
DD coils have a narrow blade field running the center of the coil. These coils have both a transmit (TX) and receive (RX) coil inside, which overlap in the middle. These coils also do well in areas where you have lots of electromagnetic interference (EMI). The blade shape of the DD also has a greater target separation. This means that, if a quarter were next to a pull tab, the detector could “see” both targets as separate items, where a mono field would cover both targets at once and you’re going to have a harder time determining that there are two separate items.
If a larger piece of iron is next to gold, the detector will “see” the iron and pass gold with a mono, but you have a chance of separating the gold from the iron with a DD coil, DD coils do not hit as deep as monos, but under severe hot ground, they are deeper as they “see” less ground under the coil than a mono of equal size. If you are finding that you just cannot seem to get a smooth threshold in a spot, or the trash is very high, then you should switch to a DD for better results.
Ask Bob 