OK, that thing on the end of a metal detector rod that goes over the ground, what do we call it? Inside that shell there is a coil of wire, some might call it a loop of wire. So you tend to hear two terms. White's Electronics liked calling them "loops" for many years but now most companies refer to them as "search coils" or simply "coils".
Search coils are nothing more than an antenna built to both transmit and receive a signal. As such they are usually tuned to work on specific detectors at specific frequencies. Single frequency machines need a coil designed to work at the specific frequency they operate on. There is a bit of wiggle room, like plus or minus a couple frequencies, but not much.
There are some detectors that operate in multiple frequencies, and they need special coils also. Coils made for the White's DFX, for instance, are tuned to work at 3 kHz or 15 kHz, or both. The Minelab X-Terra series is unique in that they operate at a single frequency, but that frequency can be changed by changing the coils. The detector "talks" to the coil, finds out the frequency the coil was made for, and switches to that frequency. The coils are specially made for the X-Terra.
Manufacturers will often try to leverage a set of coils by making a series of detectors that all operate on the same frequency or frequencies. This is very common in coin detectors. More specialized detectors have coils that work just on that particular model and not even other detectors made by the same manufacturer.
The thing is you really need to know what detector you have (manufacturer and model) to get a coil for it. Coils almost never are interchangeable between makes, so don't think about using a Tesoro coil on a Fisher. But even if you are talking one manufacturer, only a few models can interchange coils. So the White's M6, MXT, and DFX can all swap coils. But gold machines in particular tend to run at higher frequencies, so only coils made for a White's GMT will work on a GMT.
Fisher is owned by First Texas as is Bounty Hunter and Teknetics, so they are a special case where a few coils can be swapped between brands, because they are all really First Texas detectors sold under three different brand names. Like GM and Chevy, the parts are often the same.
Manufacturers do not always make all the coils end users want, and so aftermarket coils are common. The same rule applies however - be very sure of what detector a coil is meant to work with.
Whenever checking out a detector, check out how many coils are available for it. Some nugget detectors have only two or three coils available, including the stock coil. Some have more, and more is better, as it gives you more options. For many, the only coil they will ever need is the one that comes on the detector. I'm just the opposite. I invariably run coils that are either larger or smaller than the stock coil, depending on what I am doing, so coil availability is a big deal to me. It should be to you also.
Tech notes -
Coil Basics by Carl Moreland
About Search Coils by Dave Johnson
Search Coil Field Shape by Dave Johnson
~ Steve Herschbach
Copyright © 2010 Herschbach Enterprises
Metal Detector Search Coil Basics Video