Gonna say 4, but depends on the size of silver and depth your detecting and the type of soil conditions or if your detecting high mineralized soils or sandy areas but since I don't own one, thinking it should ring in at VDI 31-40 high conductivity for Minelab 900 what most are indicating on the internet, but the lower the better for frequency is the case for most detectors with low kilohertz.
The Tesoro Cortes, Deleon, and Silver come in at 10Khz machines, but even a Garret Ace 150 at 6.5 Khz can do a good job vs any other on the market, other then the high end multi frequency out their which are their claim for fame.
Single frequency options with your Multi Frequency, never owned one...like the Deus machines that has two range including the single four lower Khz settings for Silver.
I imagine lots of machines are doing a great job...but strictly on single frequency machines on finding silver I imagine they are picking up just as equally well, if you have your machine in Mulit frequency bouncing around just think like a point to point wireless shot how access points work....think it like an antennae transmitting and the other receiving just like your concentric coil you want more of a directional sector antennae over a a patch panel or omni directional antennae and the distance incluing in the soil to low conductivity, I think a single direct non changing frequency settings you can home in better, knowing you wont have to deal with emi it deals with it better... just makes sense to me with the years detecting with Single vs just Multifrequency machines. with respect to the coils though being best here at say 4.5-7 inch coil is going to do a good job finding small silver small to medium and large coins a 8" 10, 11, or 12" coil, anything highter will work on big silver dollars, but I just think it will miss on small silver deeper and the 4.5 to7-8 are going to be better cause the single stength your unit can work to penetrate are gonna perform well in soil with low to modeate electroconductivity mineralized with grassy condition (normal) soil conditions its concentric for me.
Since the concentric coils work well for silver penetraiting and zeroing on the coins, you wanna sweep low and slow , just listen cause as you know silver when heard will ring in with a load sound here so you should'nt even be looking at your display just know its silver with the right descrimination, you will notice even better with some Grey Ghost proper headphones it will hit those silver targets better with concentrics over DD on single frequency settings, as silver is the most electrically conductive seeking treasure object.
Yeah 'I believe' Concentrics win over DD coils and monolithics come in a close 2nd. So not sure what range that comes in on...from what I can see VDI is 3-4 inches deep for Silver on Tesoro display units likely on air tests but I know some guys are gonna jump in on this thread, now after me saying this with their multifrequency coils dd's just please switch to concentric and turn off the multi and try single frequency s and listen carefully and slower down then at this level being single and concentric then lets here some of your comments then based on the Multi Frequency machines and the dd coils they are packaged normally with...Pick up yourself a small concentric and in single freq mode let me know what your comaprisons are ...but if you compare to Tesoro units and any other lower end machine just on silver and the cheaper ones I think you can do just a well as an expensive unit, but if you have a higher end advanced machine your good as well if not better just grab the right concentric you will be finding more silver on single mode. Just my .02, but remember I never tryed a blended machine don't have to I have both and tested and know if I combined them in one machine I think I would be just switching away to single anyways if others chime in cause they have experience on this then I would like to hear what you have to say, then one day I may just purchase a Deus II and try it for myself.. over say Minelab unless I get a good buy on one I will try likely an X-Terra 705.
9"" Concentric 3 kHz Coil Fits X-Terra series (except X-Terra 305) , Suitable for most X-TERRA series metal detectors. This low frequency (3.0kHz) lightweight waterproof Concentric coil is ideal for highly conductive targets such as silver and copper coins, large rings and relics.
Thanks for listening
