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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.

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4 Dave’s Beach Hunting 101

Dave is one of the DetectorStuff forum moderators and an active poster.  He is an avid beach hunter and member of CRABS (Carolina Relic and Beach hunting Society).   I read this post (originally seen HERE) and saw some cool tips and beach hunting information…since detectorstuff is about “learning”, I asked Dave if we could post it here on the “main” site.




There’s allot of different styles, some work better than others, some work one day and not the next, some just don’t work on certain beaches or for certain people but here’s mine.

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Carl Cladoff’s Beach Hunting Tips…Cuts

Carl Cladoff

Carl Cladoff

DetectorStuff Member Carl Cladoff recently responded to my inquiry about reading beach cuts…and how to find the goodies therein.  His response (originally seen HERE) give great insight into the awesome dynamics of surf and wind…and how they factor into whether you finish a hunt with treasure…or nothing!  Thanks for allowing me to publish this Carl!  -Mark

Yo Mark!  I usually refer to those ‘sand cliffs’ you see primarily in the fall and winter as a cut…terminology may vary depending on where you are…

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Discrimination Mode "Sensitivity" and "Depth" in Single-Frequency VLF Metal Detectors

*Used with permission of Fisher Labs*

Dave Johnson, Chief Designer @ FTP & Fisher

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.)

“Sensitivity control” A control labeled “sensitivity”. It actually controls either gain or threshold, or a combination of both, depending on the machine. If both, the higher settings vary threshold and the lower settings vary gain.

“[Audio] threshold control” Determines the signal strength level corresponding to the threshold of audibility. A negative threshold setting is used to suppress signals by a fixed amount so that only signals stronger than that amount will be heard. Negative threshold settings are used to silence internal “circuit noise” and electrical interference. Machines which have no threshold control have an internal threshold which allows silent operation, or a control labeled “sensitivity” which actually controls threshold. …..Some models allow positive threshold settings. In most cases the positive range controls the loudness of a minimum detectable signal, a separate internal threshold determining what will or will not be detected.

“Gain control” This makes signals bigger or smaller. High gain settings make signals bigger, and therefore signals which were originally weaker can more easily exceed the audio threshold, and be heard. If the gain setting is too high, electrical interference or internal circuit noise may cause constant audio chatter. ….Lower gain settings reduce the size of signals, so that relatively weak unwanted signals (electrical interference, deep iron fragments, aluminum foil shreds, etc.) can be silenced.

The effects of discrimination “Discrimination” between different metal objects is done using a different set of signals than the ones used for detection. Since the overall purpose of discrimination is to eliminate response to certain classes of metal objects beginning with metallic iron, magnetic iron minerals in the ground will tend to make nonferrous metal signals look more like ferrous, increasing the probability of their being rejected by the discrimination circuit. Some machines provide data on the amount of iron mineralization, so with experience you can estimate the depth of effective detection and discrimination on that site.

Interactions between controls There are many types of discriminators, all of which have some effect on “air test sensitivity”. The most common pattern (nearly universal in older all-analog machines) is that “air test sensitivity” decreases slightly as discrimination is increased. Most of our recent designs do discrimination entirely in software, where control settings are actually data which don’t necessarily do the same things to signals that circuit components used to do. In the case of the T2 and F75, those differences were confusing to some users. In general if a T2 or F75 seems too noisy, the solution is to set the discrimination level to the iron range; and if that doesn’t do the job, also reduce the sensitivity setting. In the F70 and F5 which are more recent designs, the interactions between control settings and signals are even more complex, but we did a better job of hiding the details thereby giving the user an improved sense of predictability.

Electrical interference In many (probably most) machines, elimination of electrical interference is best achieved by setting the discrimination level to the top of the iron rejection range, then reducing the threshold setting (if threshold control is available; it may be labeled “sensitivity”). Even if the machine chatters in air, while actually in motion searching over the ground it will usually quiet down, except for occasional random pops which don’t sound like targets. (Frequency shifters found on some machines are beyond the scope of this essay.)

How much depth should I get? There is no single answer to that question. In a few places, you may get in-ground depth almost as good as in an air test. There are also a few places with so much iron or salt mineralization that most detectors are not even usable. In most soils, the best discriminators will usually detect coins to a depth of 7 inches or more, and will usually provide usable discrimination and target ID to a depth of 5 inches or more. However, a particular target may not be detected or may be misidentified because of the proximity of rocks or other metal targets, disturbed soil caused by digging, peculiarities of the target, or suboptimal user technique for that target in that setting. On sites where the discriminator doesn’t provide enough depth for your purposes, search in the all-metals ground balanced mode if the machine provides one.

Copyright First Texas Products and Fisher Labs File: sensitivity and depth last update 18 Sept 08

What Kind of Detector Should I Buy?

That’s a goooood question! There are lots of people who have been in the hobby for many years who still try different machines on a steady basis. (I’m one of them 🙂 ) There are others who have found a detector they are comfortable with, and have stuck with it, having no desire to change. That’s fine too!

If you’re a person considering getting into this great hobby, I suggest finding someone who already has a metal detector and asking them if you can try it. If they will let you, spend some time throwing coins on the ground, listening to the sounds the detector makes. If the person is REALLY generous maybe they’ll let you borrow it for a few days to make sure this hobby is really one you would like.

Now, you’ve taken the above suggestion and tried out metal detecting, and you think “This is a hobby for me!”… What next?

Well, it’s time for you to buy your own. There are lots of good metal detectors out there. The huge variety is due to different preferences and needs. Generally, there are three “financial” categories of metal detectors.

  • First is the “bargain” or entry level machines. They are the least expensive, and generally offer the least number of options, or somewhat subdued performance. These usually run in the $100 to $300 range.
  • Second is the “mid-level” detector. These are a middle of the road machine, usually offering higher performance or more features than the entry level, but not quite as much as the next category, the high end detector. Here you’re looking at the $350 to $600 dollar range (give or take). For the most part, these detectors have very respectable performance and offer enough to satisfy even the avid hunters.
  • Last, is the “high end” detector. These are the pinnacle of current hobby detectors. They usually offer the best a company has in performance and user options. Now, I know you’re thinking “THAT’S WHAT I WANT!”, please consider that a lot of new users have bought this type right from the beginning and felt frustration when trying to learn so much at one time. Other new users have bought these and been just fine… Here you’re looking at the $700 to $1200 dollar range. That’s a lot of cash!

Ultimately the choice is yours, but my suggestion is either the mid level detector or certain entry level machines for first time hunters. Why? Well, the first thing that causes new detectorists to “fall out” of the hobby is frustration. Frustration of not understanding what the detector is “telling” you, frustration of not finding good stuff every time you dig, frustration from the weight or ergonomics of your detector.

A good entry level machine will let you do what you want….find cool stuff! The definition of “good machine” in this context is one that comes from a reputable company. (Fisher, Bounty Hunter, Garrett, White’s, Minelab, Tesoro, etc.) I, of course, have my “favorite” companies for my own detectors, but I’m not going to suggest them, because I feel that would be unfair to you as a new person to the hobby.

There are many good metal detecting sites where people in this hobby talk about their detectors…here are a few.. this one of course 🙂 ,,, . Most people on these sites are polite and friendly and will be more than willing to offer suggestions and advice to new users. However, be aware, just like some folks like Chevy more than Ford (or vice-versa) there are some who will swear by a certain brand of detector. Take it all with a grain of salt, accumulate all the info. you can, and visit a local detector dealer to check out what they have. DO NOT succumb to strong sales tactics (ie: “Oh, you don’t want that cheap detector! Buy this one *they point at the most expensive one in the store* You’ll like it MUCH better!” 🙂 ) You will also find detector website “sponsors” to be a good moral choice. By “moral” I mean they are the one’s who pay the websites for advertising. As such, the sites are available for reading and information due to their contributions. Without them, little to no info.

Buying used is another option. The sites I listed above will usually have a buy/trade/sell forum where hobbyists swap around detectors. Caution, of course, is advised, and be aware most companies do not allow transferable warranties.

Once you’ve found a good general purpose metal detector, and not paid a fortune for it 🙂 after a while, you may think “Hmmmm, I really like this hobby! I think I’m gonna stick with it!” At this point you’re ready to “consider” buying that “high end” machine you’ve been lusting over 🙂 You should know enough about the hobby by this point to understand what it is you want out of a detector. Who knows? You may find enough with that entry/mid level detector to PAY FOR that top end machine!

Welcome to one of the greatest hobbies in the world!

Garrett Guide to Beach Hunting – Search Coils and Other Equipment

Most search coils are submersible. All of those manufactured by Garrett can be submerged to the connector, but always check with the manufacturer when uncertain. Be careful of water in the detector stem since not all units have a plug to prevent water from running into the control housing. To be safe, immediately after using a detector in water, drain the lower stem. IF water is not drained, it may flood the instrument the first time the search coil is placed higher than the control box.

For greatest success on the beach always use headphones. Of course, most veteran treasure hunters use headphones no matter where they are searching. They are especially necessary on the beach where wind, surf and “people” notice will mask detector signals and cause many good targets to be missed. Any type headphone is better than none at all, but the best are those with ear cushions and adjustable volume controls. Coiled cords are recommended along with right-angle plugs.

Since large cushioned headphones can become hot and uncomfortable, smaller versions are available. Even though these light models with smaller ear coverings mask out less noise interference, they can be used effectively.

Since almost any kind of digger can be used in loose beach sand, many beginners overlook the importance of a digging tool. Why, some even use their hands! I strongly counsel against this for several reasons-first and foremost being the abundance of broken glass. In fact, I strongly recommend gloves, at least for the hand that does any digging.

Another reason for not depending upon hands as a digging tool is that the beach hunter cannot always expect to find targets in soft beach sand. I prefer two types of diggers: a heavy-duty garden trowel and a light-weight pick with a flat blade on one end. Just a quick whack with the pick, and I usually have my treasure. Of course, pinpointing is essential before digging is attempted. The hobbyist should begin with a trowel or small shovel and graduate to a pick-type digger with a long handle when pinpointing improves. The long handle permits uncovering targets without having to kneel on the ground.

Scoops are reasonably good in dry, loose sand. A quick scoop, a few shakes and there’s the find. In wet sand, however, scoops are just a waste of time. It takes too long to work damp sand out of a scoop, except in the water where onrushing surf can help clean wet sand from the scoop.

Occasionally, a strong, thin digger-like screwdriver is needed. A good percentage of my finds are buried in roots beneath trees and tree stumps. Digging can become difficult within a complicated root structure, and a strong, thin rod is needed to loosen the soil and make a hole from which the find can be recovered.

And speaking of holes, let’s talk about them. Some treasure hunters leave the holes they dig. Don’t you! Always, without exception, fill every hole you dig. It doesn’t take much time, and you are doing it for the sake of our hobby. Plus, you don’t want someone to step in one of your holes and twist an ankle. I have filled so many holes that I do it automatically. Even in deserted mountainous and desert areas, I kick dirt into the holes I dig.

Other gear needed for beach hunting includes an assortment of pouches, a secure pocket for storing especially good finds and a pocket for storing especially good finds and a place for personal items. If you’ve hunted for treasure at all, you probably already have some ideas about recovery pouches. Let me offer just a couple of suggestions for the beach:

  • Place all detected items in a pouch; carefully inspect your finds occasionally and discard trash properly.
  • When I find an especially valuable article, I return to my vehicle or camp to stow it properly.
  • Use care in handling rings with stones. Often, mountings corrode during exposure. Examine jewelry with your pocket magnifier; when a mounting shows corrosion, handle that ring with extra caution.
  • A fastener on a pouch is not a necessity on the beach unless you lay your pouch down carelessly or let it bounce around in your car.
  • Pouches should be waterproof to prevent soiling your clothes and sturdy enough to hold plenty of weight.

Many pouch styles can be mounted on a belt. I often wear a web-type belt carrying a canteen and an extra pouch or two.

Concerning clothing, the best advice is to dress comfortably. But, protect yourself against the elements you’re sure to encounter on the beach. Obviously, you’ll want to keep warm in the winter and cool in the summer, but I caution you to shade exposed skin areas to protect against sun and windburn. In warm weather I wear shorts or lightweight trousers, a light (but long-sleeved) shirt, socks and comfortable shoes or sneakers. I wear a wide-brimmed cotton hat with some sort of neck shield. Even when hunting only on the beach, I’m always prepared to get wet. Sometimes an attractive low place in the sand will be yielding recoveries, and I must be prepared to follow it right into the water.

Garrett Guide to Beach Hunting – Using the Metal Detector

What sort of metal detector should be used on the beach? This is a good question for the veteran hobbyist who lacks beach hunting experience as well as the novice. Knowing what to expect on the beach is helpful and knowing where to find it is even more important. Without the proper equipment, however, such information is irrelevant at best and essentially useless.

First, comes the choice of a detector. While the sand on most beaches looks innocent enough, the “wrong” type of detector can spell trouble for a beachcomber. Depending on ground mineral content, some detectors are practically worthless, others so-so, and yet others perfectly suited. A quality automated VLF detector with discrimination is the best choice. These instruments ignore iron magnetite (black sand) and salt minerals, and they permit discrimination to be adjusted.

This is good news for most hobbyists since the automated VLF detectors such as those in the Garrett Freedom and AT (all terrain) series are among today’s most popular models. High quality manual adjust VLF detectors such as the Garrett Master Hunters are equally suitable for beach hunting, especially when operated semi-automatically in their Discriminate Mode, which I will discuss shortly.

So named because they operate in the Very Low (radio) Frequency spectrum of 3 to 30 kilohertz, VLF detectors generally ignore minerals, including salt water. Some instruments have an internal switch that cuts out salt minerals. Automated models can be operated from zero discrimination through pull tab rejection.

Now, that doesn’t mean a hobbyist without an automatic VLF detector can’t search beaches effectively. Neither does it mean that a veteran treasure hunter has to leave at home that “Old Faithful” BFO or TR detector that has already found so many coins, rings or nuggets.

On iron mineral-free beaches such as those of Florida, a BFO, TR or most any of the later designs works well. When a BFO or TR features a discriminating mode, water-saturated sands can be worked easily. With discrimination control set near bottle cap rejection, salt minerals in the water are eliminated from detection.

Manual adjust VLF’s give good depth in most beach sands. Unless the circuits are “automated,” however, performance may be somewhat limited on beaches with heavy iron mineralization. A VLF with TR discriminating mode should be set at approximately the bottle cap setting. Of course, that setting imposes limitations, especially if the hobbyist decides to advance the setting to pull tab rejection and dig mostly coins. Many veteran beach hunters are probably aghast at that recommendation, since few of them use any discrimination unless the beach is a veritable “junk yard.” And, it is true that a hobbyist using discrimination is more likely to miss some valuable treasures than one who is “digging all targets.” That’s a fact of life; but, there are times when I believe that discrimination is needed.

Pulse induction models such as the Garrett Sea Hunter underwater detector operate nearly flawlessly on all beaches. Giving good depth, they are a pleasure to use. Generally, they are heavier because of extra battery requirements and the heavier case needed by submersible/land models. One shortcoming of pulse detectors is that small iron pieces, especially nails and hairpins, may not be rejected.

On beaches where black sand (iron magnetite) is present, the choice of detectors is narrowed considerably. BFOs and TRs are out of the question because they cannot cancel the effects of the natural iron. Pulse induction detectors ignore it as do both manual and automated VLF models.

They hobbyist who likes to hunt with a BFO or TR instrument should take it tot the beach and use it over wet sand with discrimination set near bottle cap rejection. If the detector’s audio cannot be “smoothed out,” black sand is probably the reason; another instrument may be required. I urge all beach hunters to consider purchasing one of the late model automated (sometimes called “motion”) discrimination detectors. Even better are the environmentally protected units which permit hunting in rain or splashing surf.

When a detector is not protected by its manufacturer against the environment, it is necessary to cover its control housing with a plastic bag. This will protect the detector from mist, rain and blowing sand, and offer less opportunities for ever-present beach sand to work its way into the electronic controls.

Fill Your Holes

Regardless if you’re new to the hobby, or an old pro, FILL YOUR HOLES! One of the fastest ways to generate some negative PR is to leave unfilled pits in school yards, parks and home yards. I even kick the sand back in when digging at the beach!

Imagine for a moment you know very little about metal detecting. Someone… (friend or otherwise) comes up to you and says “you live in one of the oldest houses in town! can I metal detect your yard?”. You think a moment, then say “well, sure…I guess” (not really sure what that means)… The next weekend that person shows up, grabs this strange looking device out of his pickup truck, and hangs shovels and other digging implements from his belt! You’re thinking “what’s up with the digging stuff?” Next, you see this person swinging this silent weedeater around the yard, then stop, stoop over and start DIGGING! You’re thinking.. “I hope he doesn’t kill the grass!”. In a moment, the fellow stands up, rubs the dirt off his hands and starts doing a jig in the yard. You’re thinking “he must have found something good! Maybe he’ll come show it to me!”… next the fella looks cautiously over each shoulder, then subtly slides the mystery find into his pocket. You’re thinking “well, maybe he’s too busy having fun.. he’ll show it to me before he leaves”. Next the detector guy picks up his stuff, and uses his foot to sort of push the dirt he dug up in the general direction of the huge hole he made. He then stomps on the hole, and starts swinging again. You grab your binoculars, and peering out of the window see that there’s this ugly brown dirty patch where there used to be gorgeous grass! You think, “well, it’s too late now…but I’ll know better NEXT time!….there won’t be a NEXT time!” Needless to say, when the detector dude gets ready to leave, you, the homeowner, asks “Did you find anything good?” Your friend the detectorist says.. “Naw, not much. Just mainly old pieces of junk!” He volunteers to show you what is in his apron, which is pulltabs and rusty nails. However, he neglects to show what got “slid” into his pocket.

Your metal detecting friend drives off into the sunset and you look over your once lush and beautiful yard… it now looks like someone has been lobbing mortars over enemy lines. The holes that did get “filled” are just dirt depressions… You think “never again!”

Here’s an excellent how-to video by SouthCarolinaTeacher.


Now do you see why I say it’s important to fill your holes? 🙂 ….And fill them properly? We’ll have another article soon for proper recovery techniques that don’t “burn your bridges” when it comes to having hunting privileges.

Any other tips or pointers (or gripes 🙂 ) on digging? Post them below in the “comments” box!