Search Coils for Metal Detectors, page 12
Search Coil Discipline in Details:
Slow and Methodical Coil Swinging, Careful Usage and Treatment of Search Coil
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6) Slow and Methodical Coil Swinging
Sweep the search coil slowly and methodically. An average sweep speed is 4 seconds from left to right to left. Do not try to cover an acre in 10 minutes!
The worst possible thing you can do to yourself is to impose a time limit in which to cover an entire hunt site. Unless you are participating in a competition hunt where the detecting speed is the desired objective, you should not swing the search coil 100 mph to cover 50 yards in 15 minutes!
NOTE: Some metal detectors require fast movements of a search coil for achieving maximum detecting depth range. Please consult the instruction manual for your metal detector.
7) Careful Usage and Treatment of Search Coil
Do not bang the search coil against solid objects such as tree roots, large rocks, walls, etc. Although the search coil is constructed from durable materials and designed to withstand shocks and bangs, it could be prone to serious damage if not treated with due care. The construction of transmit and receive windings inside the coil is very delicate, and if it is not solid epoxy-filled, sudden jolts will skew the windings; thus, altering the electronic balance of the windings.
Also, banging the search coil against solid objects can cause random false signals and inaccurate target IDs. Careful sweeping will keep the coil's performance at an optimum level at all times. Excessive brushing of the coil against the ground should be avoided as well.
When attaching the search coil to the lower shaft of a metal detector, ensure the coil cable is not subject to undue stress. But do not allow the search coil cable to become loose and prone to creating false signals.
To solve this problem, use Velcro straps (they usually are provided with some metal detectors) to secure the cable being tightly wrapped around the shaft. Make sure you leave enough cable slack between the shaft and search coil to allow for the coil's movements occurring, for example, during packing your metal detector into the bag or rucksack.
Do not use your search coils without coil covers (skidplates). Protective covers are inexpensive (usually $15) compared to the price of a new search coil. Make sure you remove the coil cover and clean the inside occasionally.
If the dirt and soil minerals along with it accumulate inside the coil cover in large amounts, they will be capable of altering the ground balance tuning and detector's performance. To solve this problem, I hermetically sealed the coil covers attached to the coils when I got my detector new and have never faced this problem again since then (you can read all details on page 20 - "Sealing Coil Covers Against Dirt and Moisture").