concentric coil size for small targets

[Koala]

Time to make a new coil for my Bandido

On fairly iron free fields what would be a good coil size for tiny 15-25mm paper thin silver coins.

I know there is a big difference between a 6" and 10" coil on a large pre decimal UK penny but very little on tiny targets.

Feel its already passed the optimum as if the coin is on its edge its almost undetectable with the 10"

[Sean_Goddard]

Have you looked at an elliptical coil? I made a 3 X 10 "knife" coil which worked very well, unfortunately it got lost in the move

[Skippy]

I would say 8" would be a reasonable choice, I'm assuming you're also after cut halfpennies and farthings too, if they are there.

[Koala]

I would say 8" would be a reasonable choice, I'm assuming you're also after cut halfpennies and farthings too, if they are there.

Most interesting things tend to be quite small. There is only so many pre decimal copper pennies one person can collect. Would love to find a cut quarter.

Since using the 6" coil pin heads and beads have been more abundant but I struggle on low find areas compared to others.

[Skippy]

You've also got to consider these coins are often bent, cracked, chipped, clipped, all of which make them 'higher frequency' targets. A cut quarter penny has a -3dB / corner freq of about 25KHz, so your machines operating freq of 10K isn't the best for the smallest targets. It's noticeable that people running higher freq rigs, like 18KHz ones, find small hammered coins more readily.

[silverdog]

Most interesting things tend to be quite small. There is only so many pre decimal copper pennies one person can collect. Would love to find a cut quarter.

Since using the 6" coil pin heads and beads have been more abundant but I struggle on low find areas compared to others.

If your finding pin heads and beads your not missing much, just not walking over those hammereds at the moment
Just stick with it the finds will come

[FatBob]

I like a 3 1/4" coil for the tiny stuff;

http://www.ebay.com/itm/3-Coil-for-F...p2056016.l4276

[Skippy]

Hmmm....I would imagine the 4.8KHz (?) operating freq of that detector would make it less than ideal for many tiny objects. What sort of items are you looking for with it, Bob?

For info, I use the 5" DD coil on my Fisher F75 (13Khz) a lot, particularly on crop stubble fields, and it's no slouch at finding small stuff at depth. So don't be fooled into thinking small coils are shallow.
If you fancy an experiment, a fair approximation to a 'cut quarter penny' can easily be made. Cut a 7.5mm x 7.5mm square from an aluminium drinks can. Cut from the middle of the can (half way up) where the metal is thinner. This ID's fairly close, and it's 'depth of detection' is comparable, too.

[Koala]

Thanks for the replies. Still fancy making a new coil to compliment the 6" I have built.

Think I will try a 8" be interesting to compare side by side on different targets and the same detector.

[Koala]

Just been doing some testing and was a bit surprised.

Using a 6*6mm and 10*10mm squares cut from aluminium pop can.

Bandido 6mm = 6cm and 10mm = 10cm with a 5.7" coil

IDX. 6mm = 3cm and 10mm = 4cm approximately so poor I did not measure acutely. 10" DD

however 6mm = 5.5cm and 10mm = 9.5cm with a 8" concentric

So IDX with 8" and Bandido with 5.75" are very close even though the IDX is less than half the frequency.

[Koala]

Just had a look at an old 6000/D series 2 on GEB max it out preforms my IDX (using the same coil) and exactly matched the result from the Bandido

[Koala]

Built a 8.5" concentric.


A little disappointed with its depth on larger coins. Only seeing an 1/2" to 1" improvement over the 5.75". Pound coin and pre decimal penny.


However there is still an increase on the 10*10mm square of a good inch. Surprised at this. Thought it might have been passed optimal.



Need to do some more test might be able to squeeze a bit more out of the new coil yet.

[Koala]

Been really struggling to get good performance out of this coil.

Tried different ground options with Rx and Tx


Tried slightly under mulling and over nulling nothing seems to work


tried moving a piece of ferrite around the coil. When its bang in the middle of the coil it comes alive. Adding 2". Also each target fades out nice and gradual just going fainter until no signal. Where as without the ferrite as you increase the depth you get hit and miss beeps as you swing.


Cut half 8"
Pre decimal penny 12"
Tiny 15mm hammered 8"
Modern 50p and £1 13"

Pretty much matches a T2 on air tests in a noisy house each maxed out on sensitivity and a bit flaky.