Electrical Interference Essay by First Texas Lead Engineer Dave Johnson

Electrical Interference

First Texas Products & Fisher Labs August 2009

Dave Johnson, Chief Designer @ FTP & Fisher

Dave Johnson, Chief Designer @ FTP & Fisher

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.

Two metal detectors of the same model in the same environment may be affected differently, because of minor differences in operating frequency or because the controls have been adjusted differently.

Common sources of electrical interference

Common sources of electrical interference include: overhead electric power lines, underground power lines, other metal detectors, telephone lines carrying electronic data, computer systems, electric fences, old CRT-based televisions, cell phones, thunderstorms, fluorescent lights, metal vapor lamps, military aircraft with electronic warfare countermeasures turned on, electric motors, VLF military communications systems, and automobile ignition systems. It will sometimes be the case at home, in the showroom, or in an urban environment that there are several different sources of electrical interference present simultaneously.

All metal detectors generate a certain amount of electronic noise internally. On most metal detectors, especially the higher performance models, the sensitivity can be adjusted high enough to “work into the noise”. This is not a defect, but an intentional design feature. Experienced users striving for maximum depth often adjust the machine to “work into the noise”, and then they “listen through the noise” for the sound of real targets.

Is electrical interference a bigger problem than it used to be?

Stricter regulations have cut down on interference from electric light dimmers and auto ignition systems. However there has recently been a proliferation of VLF-UHF wireless communication systems (cell phones, Bluetooth, Wi-Fi, etc.) which often affect metal detectors. Overall, the potential for electrical interference is greater than it was a few years ago.

Also, modern high-end metal detectors are a lot more sensitive than older units, which increases their vulnerability to electrical interference. Engineers are working on ways to reduce that vulnerability, but the battle will never be won because metal detectors are by their nature designed to detect magnetic fields, and electric current always produces magnetic fields.

Metal detectors which operate in the time domain (pulse induction, “BBS”) have gained some popularity in the last few years, and these types tend to be more vulnerable to electrical interference than the more traditional machines which operate in the frequency domain. Pulse induction machines are mostly used for gold prospecting outside of urban areas so their vulnerability to electrical interference is not usually regarded as a serious liability.

What can a user do about electrical interference?

All metal detectors are equipped with a sensitivity control, or with other controls (for instance gain or threshold) which have the effect of controlling sensitivity. The primary reason metal detectors provide sensitivity control, is so the user can reduce sensitivity in order to eliminate response to electrical interference. Some users are reluctant to reduce sensitivity out of fear of “losing depth”. Well, you’re going to lose some depth, but you can still search. If you give up and walk away, you lost 100% of your “depth”. The sensitivity control (or its equivalent) is your first line of defense against electrical interference.

Many midrange and high end metal detector models have a feature called “frequency shifting”. This can be used to reduce or eliminate certain kinds of electrical interference, especially from other metal detectors. It is usually (but not always) effective in dealing with power line interference. It is not effective against electrical interference from thunderstorms, electric fences, or auto ignition systems. For additional information, consult the user’s manual for that particular model.

Many metal detector models have both a discrimination mode, and a motion all metals mode (often called “autotune mode”) which has a slower, smoother response than the discrimination mode. Electrical interference is often more controllable in the all metals mode than in the discrimination mode.

In the discrimination mode, setting the discrimination level into the foil region will usually reduce electrical interference problems: however there are many different discriminator designs out there in beeperland, and this trick doesn’t work for all of them.

In the discrimination mode, you’ll often find that although the machine may be chattery if the searchcoil is not in motion, once you start sweeping over the ground, the signal from the ground will suppress the electrical interference chatter except for an occasional pop or click.

When you’re doing “air testing” (demo’ing the unit indoors), you may find that changing the orientation of the searchcoil makes a big difference in electrical interference pickup.

If you carry a cellphone or other “high-tech’ electronic equipment with you while metal detecting and run into problems with electrical interference, try turning off the electronic equipment (all the way off, not just standby) and see if that solves the problem.

When working near overhead power lines, you’ll often find that you get the best results right under the power line, and the worst results at about a 30 to 45 degree angle away from the power lines. When you read reports of people bragging that their metal detector worked great right under power lines, this is often the situation. The person doing the bragging often couldn’t get out from under the power lines because of weeds, right-of-way fences, etc. and so may have been unaware they didn’t have a magic metal detector in their hands.

Many sources of electrical interference 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 early weekend mornings.

Small searchcoils usually pick up less electrical interference than larger searchcoils. For a given size searchcoil, concentrics usually pick up less electrical interference than does a DD. However, the differences are not great. …….On a site with really bad electrical interference, a small concentric searchcoil may be the best choice.

Distinctive characteristics of certain Bounty Hunter, Teknetics, and Fisher models

In general, it can be said that the “hottest” models experience the most frequent problems with electrical interference, and that the least sensitive low end models have the fewest problems.

The T2 and the F75, when used in discrimination mode, usually provide better electrical interference rejection at low discrimination settings, than at high discrimination settings. This contrary to the pattern of most discriminators.

The JE discrimination process (often called “jewelry mode”) in the F75 and F70 is more vulnerable to electrical interference than the other discrimination processes. However if you are searching for small jewelry on a site where you’re experiencing electrical interference, you may choose to search using the JE process anyway, and to control electrical interference by other means (or just listen through the noise).

Among our medium to high performance metal detectors (as of August 09), the ones which are overall probably the easiest to use in environments with electrical interference, are the F5 and the Tek Omega.

The Gold Bug II discrimination system was designed specifically for gold prospecting away from sources of electrical interference, and to be used for checking a target, not for searching. It is not like a conventional discriminator. By design, it tends to run noisy, and the user interface provides no convenient means for cutting chatter from electrical interference. If you are experiencing electrical interference with it, just sweep it over the ground, and the signal from the ground will usually suppress most of the chatter unless the source of electrical interference is unusually strong.

A few years ago before Bounty Hunter came under present ownership, one Time Ranger version had some sort of antenna mounted on the control box which was supposed to suppress electrical interference. We don’t believe it was ever useful, and it was eliminated from all later revisions of the Time Ranger.

Distinguishing electrical interference from other problems

The loudness of electrical interference will usually vary as you walk around holding the machine, and also it will usually vary with changes in the orientation of the searchcoil. This is almost never the case if the problem is in the metal detector itself.

The most common cause of a “noisy metal detector” where the problem is not electrical interference, is a defective searchcoil (including cable and connector). Searchcoil manufacturing is as much art as science, and although we wish we could say that searchcoils never go bad, sometimes they do. In many cases a defective searchcoil is intermittent which can be determined by giving the coil a whack with your hand (not with a hammer!). If whacking the searchcoil with your hand fixes the problem at least briefly, the problem isn’t electrical interference. Also, it is often the case with a noisy searchcoil that the noise has a sporadic character, and is almost as bad with sensitivity adjusted to a low level as it is with sensitivity adjusted high. Electrical interference sometimes exhibits this characteristic, but is usually more continuous and exhibits a greater sense of proportionality with respect to control settings.

The second most common cause of a “noisy metal detector” is an internal calibration which has drifted over time. This is rarely seen with the most modern designs which have few calibration adjustments because nowadays so much of the function is in software. However some of the most popular machines are ones which have been around a long time with a proven track record, and some of these older mostly circuit-based designs have a number of internal calibration adjustments to maximize performance.

The third most common cause of a “noisy metal detector” is dirt or water in the searchcoil cover (“scuff plate”). If you use a searchcoil cover to protect the searchcoil from abrasion, it should be periodically removed and cleaned. Dirt or water in the searchcoil cover can move around while you’re sweeping, causing false signals.

If your complaint is “noisy metal detector”, before you decide it needs to be repaired, please first make sure that the problem is not electrical interference, and verify that you are using the machine properly. The factory can fix the machine only if there’s something actually wrong with it.

* * * * * * * * *
File: Electrical Interference last update 4 Aug 09 DEJ copyright First Texas Products – Fisher Labs

Leave a Reply 0 comments