Search Coils for Metal Detectors, page 6
Search Coil Discipline in Details:
Proper Coil Angle Adjustment, Using 8-Position Loop Support
1) Proper Coil Angle
Make sure that the angle of the search coil is properly adjusted: the coil is parallel to the ground when touching the surface.
If the coil is tilted upwards or backwards, a considerable amount of operational depth will be lost, and a few targets will be missed during every sweep as shown in illustration below.
Tilted Detection Patterns of 2D (blue) and Concentric (pink) Coils with Wrong Angularity
To make a necessary coil angle adjustment, simply loosen the bolt that connects the lower shaft to the coil, while holding the detector as if detecting, lightly press the coil to the ground until it sits flat/parallel with the ground. The coil should remain parallel with the ground when you stand with the detector at your side, and the coil is lifted to the sweep height, approximately 25mm (1") above the ground. And then tighten the bolt, but do not over-tighten it.
A shaft length should be adjusted correctly. If the coil is too far from your body, it will be difficult to balance and maneuver, and cause a lot of uncomfortable stretching. If the coil is too close to your body, it may detect your digging tools or any other metallic objects such as steel plates in your boots (always leave them at home!) which you are carrying, causing destructive sounds. Plus, you will experience a lot of stooping and may accidentally step on the coil (I did it a few times).
Some metal detectors have inadequate isolator/washer/fastener designs used for attaching search coils to the detector's lower shaft. If the shaft attachment is not located at the coil's center (open-center coils), the inadequate isolator/washer/fastener system does not hold the coil's angularity stable. To correct this problem, an after-market metal detecting accessory - an attachment called 8-Position Loop Support, is manufactured.
It can be attached to your search coil to keep it constantly parallel with the ground to ensure the coil's detection pattern envelops the coins that may happen to be close to the sweep edge. Search coils of the conventional solid housing style usually do not cause this problem.