20" and 25" NF coils

HI
I use an 18" NF round mono on my GPX4000 and am very happy with how it runs and what it finds.

I would like to search deeper still for bigger nuggets, and am considering the 20" or 25" NF round mono.

What could users of these coils advise me on what I could expect in stability and performance?

BillJ

Ive got the 20" round NFA and its the quietest mono coil ive got.

only 2nd hand information here.

guy i know runs the 25" NF. loves the coil and uses it over the longer grass in a way to compensate for a smaller coil that would run closer to the ground. he has had a lot of success with the coil.

don't forget to run an extra long shaft.

we did an air test between the 25" and the 18" coil on a 7oz specimen. 18" picked it up at about 1m. 25" picked it up at about 1.2m.

hope this helps.

Thanks for the advice guys. I think I will get a 25 ". Very impressed with NF coils

Did you end up getting it? How have you found the coil. I'm looking into it but the young fella at Bendigo gold says that anything over 20inches in mono is noisey on the GPX's.

The 20" and the 25" both ran well on all my GPX's and handle the hotter ground fine because of their large footprint.
The 25" is a coil you won't use as often because of the large size, where a 20" is a bit more versatile. The 25" is the one for outright depth I feel though and what I use if depth is my goal.
Don't forget a composite upper shaft is a must for large coils as the coil will sense the standard aluminium one and balance it out with a loss of performance.
Cheers.

Yes I got a 25" NF as I found the 18" to be a great coil, and wanted to go deeper for bigger.

I upgraded the 4000 for a 5000,  and both the 18" and 25" run well on it, using a composite upper shaft, no problems.

I use the 25" only occasionally on areas where I think there may be bigger gold, say  half oz or more, and to date have not found anything decent with it.

You need fairly flattish areas without fern roots or scrub to swing it properly, or you need to keep it above the ground a bit..
The 18" I paid off in the first 2 pieces I found with it, but I am still trying with the 25", and I am very positive of getting something decent in the future.
Recently I tested the 25" on a piece of gold about 4g and it was a bit vague in the center of the coil, but okay around the edge up close.
I have tested it on a piece of lead about half an OZ, buried about a foot down, it gave a good dig signal, similar to the 18"
As they say, the 25 is a bigger coil for bigger gold down deeper, and that's why I got it.
Here's hoping!

Agreed,but nothing that the 20 inch NF couldn't handle.The moment you jump from a 20 inch to 25,the size of the nugget would also have to triple in size to notice a significant depth advantage over the 20 inch.I would only recommend the 25 inch coil for potato size slugs and above.

Cheers Kon.

Kon I would of said 3.814697265625 not triple.

Woops I meant 1.953125 Area of 20" = 400 unit 25" 625 units Area ratio 625/400 =  1.5625

target area = 1.5625  Target Width = 1.25 units



 Vol=1.25³  ............ =  1.953125 There for weight = 1.953125  bigger at depth of Coil Diameter



That is 1.00 oz @ 20"   would sound like 2.00 oz @ 25"  or am I wrong again.     

geof_junk wrote:Kon I would of said 3.814697265625 not triple.

Woops I meant 1.953125 Area of 20" = 400 unit 25" 625 units Area ratio 625/400 =  1.5625

target area = 1.5625  Target Width = 1.25 units



 Vol=1.25³  ............ =  1.953125 There for weight = 1.953125  bigger at depth of Coil Diameter



That is 1.00 oz @ 20"   would sound like 2.00 oz @ 25"  or am I wrong again.     

Might want to check those figures... I think you might be just slightly out.

Just an example there Geof . You can go as large a winding as the machine is capable of running,but that don't mean it will give you the power to drive the transmit field any deeper.There is a cut-off point to all coils used on any particular model PI or VLF. If say a 2 ouncer,is being picked up at a depth of say 18 inches with an 11 inch mono,don't expect the 25 to go 30+ inches over the same size/shape target,a couple of inches above maybe,but not double that of what the 11 inch mono is doing.Now if the 2 oz were a 10 oz slug,this is where you'll see the 25 inch mono go significantly deeper over a piece that size,than the 11 inch coil will.
Mathematical formulas might work perfect on paper if everything were to be equal,but nothing is equal in the gold game,for there are one to many hidden variables one must take into account before trying to work it out accurately on paper.

         Cheers Kon. 

You are correct Kon, and that was the point. Beginners don't know the what size of the coil has on their possible finds. How often do you see them with a big coil looking for gold and most of the nuggets are small. Or looking for small gold in deep ground with a small coil. They have a standard 11" coil that is the best in most cases. It is discussion like this that helps them understand. That is the area of the coil and the target that play the most in selecting a coil. Also as the distance increases away from the coil the magnetic field dies of at power of 2 and the same happen to the induced field from the target coming back resulting in a very quick cut off of detecting depth. Eric Foster one of the pioneer of PI has worked out the optimal coil size of a coil is that which finds the desired target to a depth of its diameter. Meaning if you find it deeper you should up your coil size (get deeper nuggets and cover more ground) and if you find shallower you should get a smaller coil and get more smaller nuggets.     

I know that if I find some deep targets, then I eventually use the largest mono coil I can on that area.

On our hill patch, I had done an area several times with the 14x9, 17x11 and 16 round in enhance timings on the 4500. I  then ran over it with the 20 round and got some deeper bits including a 27 grammer from 20". The next trip I ran the 25 over it in enhance and scored 2 x 12 grammers down around 18" that I missed with the 20. So  for giggles I ran the 20 back over the same spot in normal timings and put up with the feedback to score an 8 grammer at around the 17" mark, just 18" from one of the 12 grammers.

One coil won't get them all, just as one timing won't. Deeper bits are usually bigger bits and respond to a larger coil, whereas they most likely won't to a smaller coil. You need to remember that as you go up in coil size, you allow more EMI in, so the detector settings will have to be reduced  to compensate. I also see a lot of new operators who want to get the biggest coil imaginable, but my advice is to get used to the detector and coils you have before being baffled by the sounds a large coil can make. Happy hunting.

ps don't ask me about DD's as I just don't use them.

Qld Sandy wrote:The 20" and the 25" both ran well on all my GPX's and handle the hotter ground fine because of their large footprint.
The 25" is a coil you won't use as often because of the large size, where a 20" is a bit more versatile. The 25" is the one for outright depth I feel though and what I use if depth is my goal.
Don't forget a composite upper shaft is a must for large coils as the coil will sense the standard aluminium one and balance it out with a loss of performance.
Cheers.

composite upper shaft ? This is the first time I hear about it. Where can I buy it?