This question is much easier to answer nowadays because virtually all modern detectors are 'motion' or VLF discriminator types. Most of them are also fitted, as standard, with search coils of around 8" diameter, which is generally regarded as the optimum size for coin hunting on sites with moderate amounts of metallic litter. Using an incorrect sweep-speed will also affect depth, but the following figures are achievable with a correctly used detector:
5p-10p sized targets - 4 to 8 inches
2p-50p sized targets - 6 to 12 inches
Fruit jar lid sized targets - 8 to 16 inches.
Many other factors can affect your detector's depth, but the same factors will affect all other detector depths. The two most important things for you to do today are to become familiar with your machine, and to use it at the proper sweep-speed.
Some of the better 'all purpose' detectors can make a pretty good job of most treasure hunting assignments, but there are certain machines specifically designed for such tasks as underwater work and gold nugget hunting. These specialised types are less adaptable to general purpose work.
Are detectors with lots of knobs better than those with just a few knobs?
All detectors fall into one of two classifications; 'turn on and go' or 'do it yourself'. The first group have either a preset ground adjust, or an automatic ground adjust. They do not require any further adjustment by the operator because the electronic circuitry takes care of eliminating mineral effects. The 'do it yourself' group usually have a multi-turn ground adjust knob which must be set to the correct position by the operator. Such machines are generally capable of just a little more depth provided they are set correctly, but they yield much poorer results when incorrectly adjusted. Both 'turn on and go' and 'do it yourself' units can be very effective.
What kind of detector should be used for [a] relic hunting and [b] coin hunting?
An all purpose detector will do both jobs, although a larger coil might be an asset whilst relic hunting. For coin hunting, a motion discrimination detector is virtually essential if you wish to avoid digging out every bit of iron you find in the ground. Other features, such as notch discrimination, target identification and coin depth indication can be regarded as optional extras.
What's the difference between concentric and wide scan coils?
A concentric coil is better than a wide scan at discriminating, but it is much more affected by the ground you are hunting. A wide scan coil is less affected by the ground and it can be a superior coil in areas of high mineral concentrations and on salt water beaches.
Which coil size is best?
For an all purpose detector, the standard coil supplied with the unit is usually the optimum size for most hunting. In extremely littered areas, a smaller coil (4" or 7") will usually bring better results; even though the smaller coil has less depth on coin-sized targets, your results will be better because good targets are less likely to be masked out by bad targets lying next to, or over them. When searching in relatively clean and litter free areas, a larger coil (10.5" or 11") will usually yield better depths and a wider area of coverage because masking of good targets is less likely to be a problem.
How much discrimination should I use?
If you are serious about finding gold rings and gold coins, use as little discrimination as possible. Most gold items are rejected at about the pull-tab level of discrimination, so by eliminating pull-tabs, you are also eliminating all the gold targets. Even if you use a notch discriminator to reject pull-tabs, you will lose all those gold targets which have the same phase response (or 'electronic fingerprint') as the pull-tabs.
A truly serious hunter, and one who has been successful over many years, will have dug many pull-tabs, but that is why he has also found most of his gold targets. With today's motion machines, it is pretty easy to get rid of most of the iron objects, but those iron objects could be masking good targets beneath them.
What is the purpose of notch discrimination?
Notch discrimination can be used either to reject a narrow band of targets (=notch reject), or to respond to a narrow band of targets (= notch accept). It is usually used to reject pull-tabs while still finding 5p and 10p coins and those gold rings which do not have the same response as pull-tabs. The notch level control generates a 'window signal' whose width is set by a small component on the PC board. This 'notch window' can be moved up and down the discrimination range until it properly covers the desired range of target response.
If the detector is being used to eliminate the response to the pull-tabs, you must remember that any good targets which have the same phase response as pull-tabs, and which therefore fall within the same window, will also be eliminated. Such good targets consist mainly of gold items and rings.
The 'notch accept' feature can be used to tune the detector to a particular type of item, such as a known type of ring. The detector will then only find items which fall within that narrow notch window.
Does using notch discrimination cause loss of depth?
Yes it can cause a small loss of depth, for two reasons. Firstly, the notch discriminator adds a slight amount of capacitance to the normal discriminating circuit, and this slows the detector's response to targets. Sweep-speed then becomes more critical when seeking deep targets, but if the detector is used at the correct sweep-speed, the loss of depth will not amount to much.
Secondly, Signal strength diminishes at the top and bottom edges of the notch window. As the target approaches the response cut-off of the filters used, its signal weakens. Setting the top and bottom edges of the notch window will cause some loss of depth on those targets. However, a properly adjusted notch window will not cause a great deal of depth loss.
How do I adjust the sensitivity control to get maximum depth?
First find out which modes are affected by the sensitivity control on your detector. Do it by turning your machine according to the instruction manual, and then waving a good target over the coil with the sensitivity control set first at the maximum, then at the minimum position. If the detecting distance alters, then the sensitivity control is functioning.
When using the detector on site, set the sensitivity control as high as ground conditions will allow you to use the machine without too many false signals or spluttering noises. Too many spurious signals will make hearing the weak responses of good, but deep targets difficult.
How does target ID work?
If you have any kind of discriminating metal detector, you already own a 'do it yourself' ID machine. By increasing the discriminate level until the target just disappears, you can tell what the target is. All of this is done automatically and very rapidly by a target ID detector as you swing the coil over each target. Basically, the electronics measure the phase angle of the target at the moment the signal occurs. It generally takes only one pass over the target to get accurate identification, although the earliest models required several swings, which is why they were know as 'pump up' ID detectors. However, on older sites, many ancient coins and artefacts may show as junk or reject signals. ID detectors therefore are best suited to searching for modern coins.
Is manual ground adjust better than fixed ground adjust?
A fixed or automatic (ie factory preset) detector will always give good performance, no matter what type of ground you are hunting on. A manually adjusted machine may increase the detection depth by a small amount, but only if the adjustment is very accurately carried out. Incorrect adjustments may give horrendous results. For the average user, a preset or automatic detector's performance is superior to the results that might be obtained with an improperly adjusted manual machine.
Are battery test readings accurate?
Only if the tests are done while the batteries are delivering the proper amount of current to the detector. Some cheap battery testers, and even some more expensive meters, may give erroneous results when used to test batteries that are not delivering current during the test. An audio test is much more reliable since the audio circuitry loads the battery to the detector's full power while the test takes place.
By how much will the use of headphones increase battery life?
Since the speakers in most detectors are of 8 or 16 ohms impedance, while most detectors use a resistor of approximately 100 to 200 ohms to limit the sound output in the headphones, the current necessary to drive a set of headphones is considerably smaller than that required to drive the low impedance speaker. Using the figures just given, a set of headphones would increase battery life by 2 or 3 times. However, since the electronic circuitry in the detector is always operating when the machine is switched on, even though the detector may not be making any noises, the savings on battery life may not be as big as the above numbers seem to indicate.
Why does my detector sometimes detect rocks and tree roots?
The detection of rocks can be due either to your detector's ground adjust not being set correctly so that typical 'hot rocks' are ignored, or to the rock that you have just found being a truly positive reading mineral sample. Hot rocks are iron ores or magnetites, which are 'negative' with respect to normal ground signals. Their intensities can vary considerably, which makes setting the detector to get rid of all of them a little tricky.
Early motion detectors usually 'beeped' at hot rocks, but switching to all metal yielded a 'null' response. Since the hot rock was negative in all metal and also negative in discriminate, both signals were the same, and the detector said 'good target' even though it was really bad. Today, most manufacturers set the ground adjust so that the filtered all metal signal responds in a positive fashion to hot rocks. Therefore, the signals are different, so the detector doesn't beep at negative hot rocks.
However, minerals come in many different forms, and some of them are detectable. Tree roots can also absorb various chemicals and end up being electrically conductive. Sometimes, all you can do is grin and bear it.
Will meter detectors find coins deeper than non-meter detectors?
Not generally. It takes some kind of circuitry to drive the meter, and if that circuitry has more gain than that which drives the speaker, it may be possible to detect deeper with the meter. But virtually all manufacturers realise that if they can get deeper performance, they will add the extra gain to the audio stage as well.
How do I overcome interference from other detectors when I am at a rally?
Interference occurs when two detectors of similar frequencies operate in close proximity. The nearer the frequencies, the further apart the two detectors will interact. Crystal controlled detectors are especially prone to this problem because the crystals are very accurate. The only solution in that case is to fit a 'frequency shifter box' or to get further away from the interfering machine.
Are 'audio enchanters' any good?
They operate by amplifying weak signals and attenuating strong signals. Thus, they tend to make all signals sound alike. They can be helpful when you are hunting in TR discriminate, or in all metal mode, or if your detector has an 'audio threshold'. However, on 'silent search' detectors are less effective, though they do limit the sound in your headphones when you pass over a piece of surface junk. This can add to your comfort during extended search periods.
What is ground cancelling, and how can I adjust my detector to the right point?
Think of ground cancelling as being exactly the same thing as discrimination. If your ground control is set too low (counter-clockwise) the detector with 'reject' the ground. If your ground control is set too high (clockwise) the detector will 'find' the ground by beeping as the coil approaches the ground.
Your aim is to set the ground control so that the detector remains neutral to the ground, or doesn't see the ground at all. It will then detect as deep in the ground as it does in the air. To accomplish this, first tune the detector to a threshold tone while holding the coil in the air. Then lower the coil to the ground and listen. If the threshold dies away, turn the ground control in a clockwise direction. If the threshold tone gets very loud, turn the ground control counter-clockwise. Next, raise the coil, retune to a threshold tone and repeat the above operations. When you get it right, the sound will change very little as you lower the coil.
Why does my detector find large deep nails and rusty iron even when set to reject pull-tabs?
Unfortunately, most motion detectors can be fooled by large rusty items, especially if those items are circular, such as iron washers and steel bottle caps. Sweeping the detector faster will help a lot on the steel bottle caps, and it will help some on the washers. Fortunately, large pieces of iron will be heard as much 'broader sounding' targets than non ferrous items in the all metal mode. Experienced motion detector users rarely dig large pieces of iron.
Does the moistness of the soil affect detection depths?
A lot of metallic targets corrode when lying in damp soil. This can cause the target to appear much larger than it really is. When the soil dries out, the corrosion may not affect the detector and the target will seem more like its normal size. Gold doesn't corrode in the ground, and silver doesn't corrode nearly as much as copper, brass and bronze. Iron and steel, of course, rust in moist ground, and can cause some really horrendous false signals.
Is depth sacrificed for accurate identification in a target ID detector?
Virtually all target ID detectors can find targets much deeper than they can identify them. Discrimination only requires one reference signal, which yields a positive signal for good targets and a negative one for bad targets. To identify the target requires some very special and complex electronic circuitry which does not have the ability to reach the same depth as a simple discriminator. If manufacturers limited the depth of their ID machines to the depth of the identifier circuit, they would sell a lot fewer detectors.
How should I adjust the discriminate level on my detector to achieve maximum depth?
For maximum depth, set the discrimination level as low as possible. To get the absolutely best depth, set the discrimination level at the point where you get a broken signal, rather than no signal at all, from the type of object you want to reject. This will give you some additional signal on all good targets that are heard.
By how much, and in what ways, is a £500 detector better than a £300 detector?
The £300 detector will probably have the same depth as the £500 machine, but it will not have all the features of the more expensive unit, such as a meter, depth measuring ability, notch capability, target ID, multiple discriminate levels, surface blanking, and the other 'bells and whistles' that can raise the cost of a basic machine. But unless you really want those features, and understand that you will consume a lot more batteries by powering them, why pay for them? Although some manufacturers may put less gain in their cheaper models, you should expect to find only fewer features on your £300 detector.
What is the best operating frequency for finding gold?
Typically, most VLF detectors, made today operate in the 5 to 15kHz range, while gold detectors typically operate at about 20 kHz. The increase in frequency gives a minutely greater response to tiny bits of gold, but the increased response to small targets results from them having about ten times as much gain as general purpose detectors. Increasing the frequency much above 20kHz gives a tiny bit more response in air tests, but the increase is lost as soon as the target is laid on the ground.