The first metal detectors were developed almost a century ago. They became practical for hobbyists with the advent of cheap transistors, and their technology has been advancing ever since. Here you'll learn how detectors work and the types and features available today.
Metal detectors work by sending out radio waves and measuring the differences in response caused by metals of different sizes, depth, and types. The original basic technology is referred to as "BFO" for Beat-Frequency-Oscillator. Actually, these use two radio-frequency generators (oscillators). The first is set to a set frequency. In the second the frequency is largely determined by the detector coil. As that coil nears metal objects, the frequency shifts. A little more circuitry converts this to an audio tone. When you hear the pitch rise, you've found some sort of metal... or other conductive material.
In the 1960's the founder of Garrett Metal Detectors concluded that these basic technologies weren't good enough, and he began a long series of innovations. His competitors licensed some of his patents, and followed suit with their own research and development. So now we all benefit from advanced high-performance detectors.
The earliest changes improved the stability of the oscillator and core, so that constant adjustment wasn't needed. Further developments improved sensitivity, allowing the detection of smaller or deeper finds. More importantly, key innovations made it possible for the detector to measure the type and depth of the metal. This was a huge leap, and saved tons of time otherwise wasted in digging up trash, making detecting a more fun and less frustrating hobby.
This continued research & development led to two completely new types of metal detectors. The old BFO types are much less effective and are rarely used except in very-low priced "toy" models. So what are the new types?
VLF Detectors
"VLF" stands for Very Low Frequency. They're the most common type of detector in use today. Here the electromagnetic waves are way down at 5,000 to 30,000 cycles per second ("cps" or "Hz"). This technology uses two coils; a transmitter coil and receiver coil. The receiver coil ignores the transmitted signal and instead picks up a signal produced by the target's response to the transmitted waves. Not too surprisingly, the strength of the signal received is a pretty good indication of the depth of the metal. What's interesting is that the "phase difference" (time lag) between transmitted and received waves is a good indicator of the type of metal. Because of this, VLF metal detectors can discriminate between different targets such as coins and junk such as aluminum cans. It's this discrimination that makes these detectors so popular.
PI Detectors
A Pulse Induction (PI) detector works more like radar. A sharp pulse of radio waves are sent out, and the returning signal monitored. Normally the received signal gradually fades away. If metal is nearby the time for this signal to fall to zero is slightly reduced. This technology is less effective at target discrimination, but nonetheless better under certain conditions. This includes deeper targets (such as items intentionally buried to hide them) as well as in mineralized soil and sea water.
Choosing Your Detector
Unless you need the specific features of a PI detector, you'll be better off with a VLF model. Here's a couple of features to look for.
Ground balance - All detectors need a little adjustment for different ground conditions, such as soil type and moisture level. A detector without any balance adjustment won't be all that useful. Manual balance is OK, but an automatic balance features lets you focus on your hunting rather than twiddling an adjustment.
Notch filters - Many VLF metal detectors offer "filters" to ignore certain phase shifts, that is to reject certain metals. They're often marketed using names like "iron rejection" or "trash rejection." This feature can really make your hunting go faster and with fewer disappointments
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