How metal detectors work. Fisher Research Labs F75 SPECIAL EDITION

F75-MANUAL(122811).qx 12/14/12 9:03 AM Page 3

F75

Comprehensive Operating Manual & Guide to Metal Detecting

How Metal Detectors Work

Most hobby metal detectors use VLF Induction Balance technology. Here's how they work.

The searchcoil (also called search head or loop) contains two electrical induction coils which are like antennas. One coil transmits a rapidly alternating magnetic field, illuminating the region surrounding the searchcoil. If metal is present, its electrical conductivity distorts the magnetic field. If iron metal is present, its magnetism also distorts the magnetic field, but in a different way, allowing the metal detector to distinguish between ferrous and nonferrous metals.

The other coil is a receiving antenna which detects changes in the magnetic field caused by the presence of metal. Electronic circuits amplify this weak signal, analyze it to determine the changes which occur as the searchcoil sweeps over the target, and then convey the information to the user in the form of a visual display or audio tones. Most modern metal detectors perform many of these tasks in software running on an internal microcomputer.

The iron minerals which are present in most soils also distort the magnetic field, obscuring the weak signals of small or deep objects. This can cause the object to go undetected, or to be misidentified when it is detected. Much of the technology that goes into modern metal detectors is devoted to the task of eliminating the unwanted signals from iron minerals in the soil, while not losing the signals from metal objects.

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This device has been designed to operate with the antennas listed below, and having a maximum gain of 3 dB. Antennas not included in this list or having a gain greater than 3 dB are strictly prohibited for use with this device. The required antenna impedance is 53 ohms.

10COIL-F70, 11COIL-F75, 5COIL-F75, 6COIL-E

The following statement is not relevant to metal detectors but is printed here to satisfy legal notification requirement:

“To reduce potential radio interference to other users, the antenna type and its gain should be so chosen that the equivalent isotropically radiated power (e.i.r.p.) is not more than that permitted for successful communication.”

The following countries allow free movement of this product within the European community:

FIN,LVA,SVN,SUI,BIH.

Comprehensive Operating Manual & Guide to Metal Detecting

Table Of Contents

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Quick-Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Mechanicals

Assembly Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7

Mechanicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Armrest adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Using Headphones (not included) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Introduction to the F75

General information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-12

Menu System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Ground Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14-16

Motion All Metal Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Static All Metal Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Discrimination Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19-22

PinPoint Feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

LCD Visual Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24-26

Numeric Target Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Probable Target Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Target Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Depth Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Confidence Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Fe

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O

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Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Battery Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

G.B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..26

Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

Frequency Shifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

Capabilities & Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28-30

Tips & Techniques

Search Techniques

How to sweep the searchcoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

Pinpointing targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

Estimating target size and depth . . . . . . . . . . . . . . . . . . . . . . .31-32

False signals and chatter . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33-34

Adjusting Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Tips on ground balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Detecting Activities

Coinshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Relic Hunting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37-38

Gold Prospecting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38-39

Cache Hunting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

Shallow Water Hunting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40-41

Salt-Water Hunting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

How metal detectors work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Back Page

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