Fisher F19 Owner's Manual page 32

ELECTRICAL INTERFERENCE (EMI)
EMI is usually easier to control in the All Metals Mode than in the Discrimination
Mode.
On the F19™, setting the discrimination level between 20 and 30 will usually
reduce interference. In the Discrimination Mode, the detector may chatter if the
searchcoil is not in motion, but once you start sweeping it over the ground, the
signal from the ground will usually suppress the EMI chatter, except for an
occasional pop or click which will not sound like a real target.
If you are conducting an indoor demonstration, you may find that changing the
orientation of the searchcoil will reduce the pickup of EMI.
If you carry a cell phone or other high-tech electronic equipment while metal
detecting and you encounter EMI, try turning the device off and see if that solves
the problem. Turn such devices completely off, not just into standby mode.
When working near overhead power lines, you may get the best results right under
the power line and the worst results when standing at a 30° to 45° angle to the
power line.
Many sources of EMI are intermittent. You may find that an area which is difficult
to search at one time of day may be easier after 5 PM, or on weekends. Power
lines are usually quietest late at night, and on weekend mornings.
Small searchcoils usually pick up less EMI than larger searchcoils. On a site with
severe electrical interference, a small searchcoil is often a better choice than a
large one.
Distinguishing Electromagnetic Interference from other problems
EMI will often vary as you walk around from place to place, and it will also vary with
changes in the orientation of the searchcoil. This is almost never the case if there
is a functional problem with the metal detector itself.
The most common cause of a noisy metal detector, where the problem is not EMI,
is a defective searchcoil. If striking the searchcoil with your hand causes the noise
to start or stop, the problem could be a defective searchcoil. Also, a defective
searchcoil often causes noise of a more intermittent nature than electrical
interference.
The second most common cause is a loose searchcoil connector. The noise from
a loose connector will usually be very erratic or intermittent in nature. Try
unplugging the searchcoil and then reconnecting it to ensure the connector is firmly
installed.
If you use a searchcoil cover, dirt or water can accumulate inside it, and movement
of those materials can cause false signals while sweeping the searchcoil.
Searchcoil covers must be periodically removed and cleaned.
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