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Digger_O'Dell
08-19-2015, 02:33 AM
Hello all!
Since I now got stuck working 84 plus hours a week for an indefinite term, I am thinking about purchasing a new DD coil for my X-Terra 705. I already have the stock 7.5 KHz concentric and the 18.75 KHz DD as part of thee package I bought. Being that Minelab has a very limited selection, and coils being pricey I was looking at the Coiltek 15" series. As I'm usually searching more open areas for coins, I was thinking the larger size would be a big advantage. But once again there is the question of which frequency to go with. I watched the videos doing the air depth tests, and all 3 frequencies were about the same within maybe 1/2 inch. Personally I have been leaning towards the 3KHz coil. So here's what I got out of the videos and literature:

The 3 KHz coil seems to be more reactive to higher conductive metals such as silver, which is what I am seeking most. But it is supposedly disadvantaged with metals such as iron or gold, and I would really hate to miss out on a gold find if possible. But, the advantage here also is supposed to be greater depth in the ground over the other 2, and better resolution around nails and such-or so I hear. It's also said it's not the best at small targets-whatever that may mean. My 7.5 KHz concentric has picked up a partial rusty staple at 6 inches-how small do I need to go? lol

The 7.5 KHz coil is supposed to be a good all around coil that will get both high and low conductive metals equally. So either gold or silver would be picked up pretty much the same and other than a DD configuration with the advantages there, it wouldn't be that much different from the concentric I already have.

The 18.75 KHz coil I couldn't justify as I already have the Minelab DD coil in this frequency, just a bit smaller. I like this for trashy areas where I can more easily pick between the garbage, and for highly mineralized areas such as on the black sand beaches near me. The only problem is it is not a waterproof coil, so have to be careful.

Overall, I really don't see any real advantage of one frequency over another in most cases, only at the far limits of detecting is where one seems to make any difference over another, depending on the target. So, my question is is anyone can shed more light on this topic of frequency choice, and if anyone has any experiences they can share comparing one to another. Thanks!

Here's the link to the coils: http://www.coiltekmanufacturing.com.au/products/coin-relic-and-treasure-coils/15-all-terrain-x-terra/

del
08-20-2015, 02:51 AM
if the frequency is not an issue where your at , how about coil size . small and large coils do have their advantages in their respective environments .

Dan

Digger_O'Dell
08-20-2015, 03:52 AM
if the frequency is not an issue where your at , how about coil size . small and large coils do have their advantages in their respective environments .

Dan

The question is what frequency is the most advantageous using the 15 inch coil. The use would be mostly open parks, yards, fields, and sometimes beach areas. Basically asking if the 3KHz is really any better with coins than the 7.5 KHz, or is there a big risk of also missing gold (rings, etc.)? Also, is there really any depth advantage with different frequencies as often claimed, or is it just the bigger coil diameter?

MangoAve
08-20-2015, 06:41 AM
...The frequency and coil size or shape are different aspects. The coils size just means the detect area is larger and something like 20% increase in depth. I calculated it out once, but from an 8" to a 10" is an increase of approx. 20% increase in depth and 56% area. The shape whether it's elliptical or DD or CC just means the shape of the detect pattern. I am not quite sure why the stock is a CC on most machines when those who get introduced to a DD never go back. Maybe they are just cheaper to make. I would suggest the 15" if ONLY using for open fields. They have vastly more area and therefore the activity is dispersed throughout the field. Not centralized like around a house. Parks are somewhat centralized areas where people who visited had congregated (whom once owned the stuff you are trying to find), but there is also a lot of trash you want to see through. Always think about the detect pattern. Two objects side by side are easier for the DD to see yet two objects front to back might actually be slightly harder to distinguish. When you are talking about high frequency and relating it to gold.... that's mostly gold flakes which are smaller in size or tiny gold chains. The performance on using a 3kHz on gold rings should not be degraded, or very little you probably wouldn't notice. Most rings aren't deep, correct? But I will say on frequency.. the machine's pickup input stage is usually a fixed gain and so is the output stage. (the signal received by pickup coil and the signal emitted by output coil respectively) The attenuation of the actual signal (or how deep it can go) is halved when the frequency is doubled. I believe the attenuation equation is on wiki if you want to plug in the numbers. Keep the attenuation coefficient the same (it's not quite a variable even tho each TYPE of soil IS different...it would still be the SAME soil under the coil at the same spot no matter which type of frequency you use). Set the attenuation number equal and it just comes to a proportion of frequency 1 and how deep it can go to frequency 2 and how deep that can go, to be at the same amplitude.