A metal detector with an 8" coil normally radiates an electromagnetic field in a rather billowed configuration (with cancer sores here and there) both on the top of the coil and on the bottom too. In normal instances though the image when viewing iron filings dropped on top of a piece of paper over the side of the coil reveals a weird pattern somewhat resembling two very large (each around 4 feet in circumference) radiated fields, back-to-back, with the negative voltage/EMF field on the top of the coil windings, and the positive voltage/EMF on the bottom of the coil windings. The field becomes wider and wider as the EMF field weakens radiating outward, much like waves do on water.
Now this means also that since the negative field is the weaker force (not to be confused with the "weak force", as in quantum mechanics) it will produce a lesser degree of useability and intensity than the positive field on the bottom side. The unuseable EMF for smaller items will still radiate a total field size of 8 feet + for a larger item. If in doubt, try the coil out on a car body or steel fence and you will quickly learn that there are more than even these two parameters that I just mentioned regarding total radiation fields. With some detectors though the outer field area crosses over each other as far as readability or detectability, but this is due to recover speed and many other factors, including voltage group clusters. This crossover of readability for small items is not as true with multi frequency detectors though. But of course multis only utilize one frequency at a time anyway, as most techs and all EE's already know.
In normal cases this all roughly equates to total use as is comprised of a voltage gradient of maximum useable voltage for small items extending outward from the side of the coil, roughly equating to slightly 20% lesser than the total area of the localized (useable, readable voltage) small item manageability from TOP of field to BOTTOM of field. This means that the search field is greater from top-to-bottom of useable EMF than it is from side-to-side. In other words the useable for (MOST) coin-sized objects field in taller than it is wide, by around 20%.
In short again, if a well-made detector has an 8" coil it normally has the ability to detect (in air, with zero interference) a coin-sized object often exceeding 10". This is not always true though, I had a Sov that although it worked as it should it still got only 10.5" (in air) with it's 10" coil and so did an SE.
Keith Wills is the first person who designed a discriminator (in around 1984) that had the ability to accomplish this using a ground balance discriminator. In fact, quote: "if a Compass metal detector cannot read 11" on a coin sized object in air, I (Keith Wills) did not modify that circuit at the factory". Keith Wills can now be found at "East Texas Metal detectors". He is the best detector repairman on the planet and was one of the chief techs at Compass Electronics before it closed down. He is one of the most important reaons why we now have such fine phase-shift, etc instruments available today. Before he re-engineered metal detectors it was (past tense) true that most detectors only got depth equal to width of detection. He changed the rules though.
Records show that in mild to medium soils the Naut II's get 4 out of 5 Civil War hunts while the Tejon got 1 out of 5.
The air depth on the Tejon with 8X9" coil was 14" for a nickel, 13" for an 8" Compass R&C, 11" for my Fisher cz-70 w/ 8" coil. The Tejon and Minelabs sucked in bad ground though. REALLY sucked. I sold them both.
Hope this helps.
LL
Hi Goodguy, Long time no talk, how's life on the other side of charter membership ? 
