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The Tek Files

Question and Answer Session with FT-Fisher Engineers David Johnson and John Gardiner

First of all, I want to thank Tom Walsh, Mike Scott, Tricia Richardson, Dave Johnson and John Gardiner for their help in arranging this interview. I think it’s a fair statement that First Texas – Fisher is one of the most exciting metal detector companies on the planet right now, and as such, has piqued the interest of the hobby detecting world in a way that hasn’t been seen for quite a while.

I had the idea for this interview months ago, before I had started this web site. There was quite a buzz in the forums when rumors started of a new Dave Johnson design coming from First Texas under the Teknetics moniker. After the subsequent release and success of the T-2, the hobby community was set on fire with excitement over the prospects of “things to come”. Another wave of excitement hit when it was learned First Texas had acquired Fisher, and that wave turned into a tsunami with the release of the F-75. Mr. Johnson was quick to point out that the T-2 and F-75 were team efforts, not solo projects, and that Engineer John Gardiner was key to the success of both machines.

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Are metal detectors safe for people with pacemakers?

The question of whether pacemakers present a safety issue in relation to metal detectors is one which we have paid close attention to over the years. We are not aware of any report of a hobby type metal detector ever interfering with a pacemaker or other electronic medical device, or having any other adverse health effect. 

The magnetic field to which a hobby metal detector exposes the user in normal use is much weaker than the geomagnetic field which already surrounds us, many times weaker than the electromagnetic radio waves near a broadcast transmitter or CB radio or cellphone, and weaker than the magnetic field of walkthrough security metal detectors such as are used in airports and courtrooms.

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What are “processes”?

There are a number of ways to process target signals in order to extract target presence and target type information.  We call these different ways “processes”.

One example of a “process” familiar to many metal detector users is the classic second derivative (“two-filter”) discriminator circuit first made popular during the 1980’s.   Nowadays some metal detectors process the signal in this same way, but in software by “crunching numbers” rather than by using analog computation circuits.

Another process familiar to many, is the peak sampled second derivative process, patented in the 1980’s and used on many Bounty Hunter products as well as the Fisher CZ’s.  This process is usually associated visual target ID systems.

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About Field Testing

There are basically four types of field testing, with some overlap between them.

1. Evaluation of conceptual and product prototypes in connection with research and development engineering, working with engineering department.
2. Evaluation of products during the process of product release, working primarily with engineering department.
3. Evaluation of products during the process of product release, working primarily with marketing department.
4. Evaluation of products during or after introduction, primarily for the purpose of publishing a report in a magazine or other public media.

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Electrical Interference

Because of the high sensitivity of modern metal detectors coupled with the proliferation of sources of electromagnetic interference, you are likely to encounter electrical interference at times during the use of your metal detector.  It is important that you recognize electrical interference when present, and take appropriate measures to deal with it.  This will prevent you from giving up on a worthwhile site unnecessarily, or from sending in for a repair a machine which is working properly.

Symptoms of electrical interference

Electrical interference can cause a metal detector to “chatter” spontaneously, to lose sensitivity for no apparent reason, or to cause periodic audio “wobble” or slow waves of spontaneous sound.   What you’ll hear will depend on what model of metal detector you’re using, what operating mode you’re using it in, how you have the adjustments set, and what the source of the electrical interference is.   The most common manifestation is spontaneous chatter.

All metal detectors are susceptible to electrical interference, but they vary in what kinds of electrical interference affect them.  In a given environment some metal detectors may be affected by electrical interference whereas others may not.

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Discrimination mode “Sensitivity” and “Depth”

Some metal detectors are “more sensitive” than others, and “how deep” a particular metal detector can detect a specific metal object depends on many variables.

“Air test sensitivity” refers to the maximum repeatable detection distance achievable in air using a standard metal test piece (typically a US nickel coin), with the searchcoil that’s standard with that model, in a location without electrical interference, the machine adjusted to just barely eliminate background chatter. If ground balancing is available on the machine, it must be done using ferrite. …….. A properly done “air test” provides an indication of a machine’s potential to “go deep” on buried coins. Because of interference from magnetic iron minerals in the ground, actual detection depth will usually be much less than what’s achieved in “air test”. (NOTE: for maximum depth on buried objects, search in the all-metals ground balanced mode, which is much less affected by iron minerals.)

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About Search Coils

This essay pertains to induction balance searchcoils (“loops”) of the types most commonly used on hand-held hobby-type metal detectors.

Searchcoil shape   Most searchcoils are round, but some are elliptical. This refers to the overall shape, not to the type of coil construction.  In general, elliptical coils provide a broader sweep pattern over the ground, and narrower target response for better pinpointing.  Round coils are easier to design and less expensive to manufacture, which is why they're the most common.

Searchcoil type of construction

The words "concentric" and "DD" (or "double-D") refer to the type of coil construction internally.

Most searchcoils (whether round or elliptical) are of concentric construction. A concentric searchcoil has a large transmitter coil, and a smaller receiver coil in the center, usually in the same plane.

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Metal detecting technologies for gold prospecting

This essay pertains to modern name-brand metal detectors designed and marketed specifically for gold prospecting. There are two basic metal detection technologies in common use for gold prospecting -- single-frequency VLF induction balance (“VLF”), and pulse induction (“PI”).  

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More Articles ...

  1. The History of Metal Detectors, with Emphasis on Gold Prospecting
  2. First Texas Products & Fisher Chief Designer’s favorite machines