X-terra 705 -coil frequency

Gday all


ive got an x-terra 705 and it came with a 10" DD 18.7 khz eliptical. Im after a mono coil and wondering what i should go for... like what frequency. this is for coin and relic detecting. Should i go high, medium or low frequency? I was thinking a medium but then saw there is a 9" and a 6" and then got really confused. Ive just been using the eliptical that came with it and done extremely well the past couple of days but i really need something waterproof.

Im not quite sure what the different frequencies mean. Depth, right? But i really have no idea about coil sizes and their differences.

thanks again guys!

Hello TheH0ward!

Yes, Frequency can generally can equate to depth with VLF detectors. The coil you already have is the ideal main-use coil and the preferred search-coil for most users. There are two types of detectors commonly used to find gold: Pulse Induction detectors (PI) and Very Low Frequency detectors (VLF). The PI detectors include the Minelab SD, GP & GPX series as well as the Garrett Infinium and a few others from other manufacturers. PI detectors are preferred by many for the goldfields because they go deeper than VLF detectors and can handle the mineralized soil better and with more stability. They are more expensive though. But a few VLF machines (including the X-Terra 70 and the 705) can also be used on the goldfields.

The X-Terra is a VLF detector and the X-Terra 705 and the earlier X-Terra 70 are one of the few capable of penetrating the mineralized soils of the Australian Goldfields with any degree of success. They won't go any where near as deep as a PI but they are a little easier to use and have arguably better discrimination capabilities. PI detector output a strong pulse of energy at extremely fast pulse speeds. But you can't buy different frequency coils for PI detectors., only for the VLF detectors. The VLF detectors come in two varieties: Multi-frequency detectors like the BBS and FBS types as well as single frequency types such as the X-Terra detectors. For gold hunting, the X-Terra detectors are possibly a little harder hitting with all the energy directed into a single frequency compared to the diluted signals that the multi-frequency detector coils. The High Frequency Elliptical coil that you have is close to 20 khz (it's 18.75khz) which means that it is a good overall frequency for different sized nuggets at different depths. With VLF detectors, you could run a lower frequency coil for more depth and ground penetration or you could run a higher frequency (like the 60khz on the Minelab Eureka Gold detector) to find finer specimens and small gold. No detector can reliably find the extremely fine Alluvial gold that you might otherwise find in a gold pan. But those smaller coils are extremely sensitive and can certainly find the coarser stuff with a larger surface area.


My own 6" HF DD coil on my X-Terra 70 will give a GPX-4500 with a 6" Goldstalker or a 12" Nugget Finder coil a run for it's money in a shallow creek bed. It's kind of nice to see these respected coils on a GPX compare so closely with the X-Terra coil like that. But at depth, the SD/GP/GPX's will probably win out with any coil.

For the X-Terra, there are just a few options for you to choose from: Double-D coils and Double Concentric coils. Both come in the High Frequency type (18.75khz) although you may find that Double-D coils are best for mineralized ground. Monoloop coils are similar to the Concentric coils in that they produce a cone shaped field below the search coil. Double-D coils produce more of an axe-blade shaped signal. But Mono-loop coils (that transmit and receive from the same coil winding) are not available for the X-Terras. The Monoloops are generally only found with PI machines like the GPX and the SD/GP series machines. The nearest thing you can obtain to a Monoloop for your X-Terra is a Double Concentric or "Concentric" Coil (see example in the below pic - right hand side). This coil transmits a signal into the ground from the larger, outer coil and then receives the signal again from the target via the smaller, inner loop.

The only coil that was deliberately made for underwater applications on the X-Terras (the coil is waterproof) is the 6" coil... the one you are asking about is a HF coil and is also a DD coil. The coil you have is 'water resistant' so it can handle a torrential downpour in a forest but not so much the pressure of under water immersion. Be sure to keep your control box and cable connection dry. I think retail in Australia for the 6" HF DD coil is around $185 or $175. It is VERY SENSITIVE out of the water and some like to use it on the beach. The downside is that the coil is so small that you have to swing carefully in order to overlap your sweeps... plus it can beep on little bits of ironstone on the ground so you may need to adjust the sensitivity if working away from the creeks etc. The upside is that this coil can be immersed in water and is very useful in finding nuggets under water or in wet mud.

I'd even venture to suggest that an X-Terra 70/705 with a 6" HF DD coil is easily as sensitive (or possibly even more so) than a GPX with a similar sized coil. I've found both large and small gold in creeks with the X-Terra 70 using a 6" HF DD coil. It's also a very impressive coil on the beach and in the parks. It should give you a little more penetration in the ground than your larger 10x5" DD coil but the 10x5 will have a wider field. I find the 6" coil frustrating to use outside of a creek myself but sometimes it's just phenomenal.



These are your main choice of coils for the X-Terras. There's another coil (not shown) that is a little larger called the 10.5" HF DD round coil.


If you have an X-Terra 705 and you're thinking of the 6" HF DD coil, it's a superb addition to your nugget hunting tools. I really LOVE this coil and still take it with me when working areas that don't require a PI machine.

Cheers,

Marco

Thanks heaps for the response Marco. And thank you for being patient with my questions. hey, im learning!
Yes i have been absolutely cleaning up at the beaches since these storms we have had. ive easily found more coin with my 705 in the 3 weeks ive owned it than what i found with the poor little 30 ive had over 18 months.
i just couldnt seem to find if the eliptical coil that i have was waterproof or not, so i was a bit weary using it at the beach, only because the sea has been terribly rough and big swells washing up all the way to the collapsed dunes.
Im still 'preparing' for my gold find, lol. As you can imagine from having an Xterra 30 coin hunting then jumping to a 705 in prospecting mode, well to say the least, it was confusing. As you know with a 30 there is either a signal or there isnt, but with the 705 its an array of different sounds in prospecting mode. So much for me to learn. Ive had a few bits of ironstone scream at me which got me excited for 5 minutes. Oh and then there was that little piece of ... well it kind of looks like silver but i really dont know what it is. It wasnt rusted or corroded but gee it gave a lovely scream.

Well thanks again much appreciated!