figure 8 coil????

[cr15]

I am hearing rumours of a coil in a figure of 8 configuration with reportedly phenomenal depth capabilities. 2 feet!!!! Has anyone heard of this one?

[Esteban]

Nexus

Here "8" coil:

http://www.garysdetecting.co.uk/nexus1.htm

[Tim Williams]

Esteban Carl has a PI with that type coil or like it here.
May be same layout I'm not sure.

http://www.thunting.com/geotech/page...p/twinloop.pdf

Tim

[Carl-NC]

Those are not Figure-8 coils, they are "OO" coils, or "double-O". They are the same as "DD" or "double-D", only the coils are round, not flattened.

The only Figure-8 coil I know of is the Bigfoot coils made for White's detectors.

A Figure-8 coil has either the TX coil or the RX coil folded into an "8" shape. If the RX coil is folded, then you can get excellent ground cancelling, and noise immunity.

- Carl

[cr15]

The Nexus seems to match what I heard. Very impressive! Due only to round instead of D shaped coils do you think? Some experimentation needed here!

[cr15]

Actually Carl, I must confess to not being a new member - just lost my original login. remember the Radiometer Ferret? Good hunting Dude. Cris.

[Carl-NC]

Hi Cris,

Good to see you drop by. I don't think it's the OO vs DD that makes the Nexus more sensitive. I think Georgi has just done a far more careful job getting a precise balance on the coils, which most manufacturers intentionally do not do.

- Carl

[Geo]

Hi Cris,

I don't think it's the OO vs DD that makes the Nexus more sensitive. I think Georgi has just done a far more careful job getting a precise balance on the coils, which most manufacturers intentionally do not do.

- Carl

I believe the same plus an input stage with low noise and high gain. Why not???

[Arnie S.]

Hi Cris,

Good to see you drop by. I don't think it's the OO vs DD that makes the Nexus more sensitive. I think Georgi has just done a far more careful job getting a precise balance on the coils, which most manufacturers intentionally do not do.

- Carl

Carl,
Could you answer a few questions on the OO coil?

What do you mean by " getting a more precise balance"?
I saw a reference in a posting that the coil makers do not "tune" {if that is correct understanding} their coils on purpose. Are they factory detuned to limit radiation? Could you give a breif anatomy of a VLF DD or OO description and what guidelines to follow to make/experiment with one? It would make sense to have the coil resonate to the machine's output f, and the recieve tuned also- have I assumed that is the case.
thank you,
Arnie S.

[Carl-NC]

By precise balance, I mean the induction balance. That's the amount of receive signal you get in the absense of a metal target. If I'm not mistaken, most coils are tweaked to give a little bit of signal, so that the demodulators will have something to work with.

Tuning is different. It usuallt refers to the receive coil be tuned to the transmit frequency. Again, I think most narrow-band coils are intentionally tuned off to one side of the TX frequency, so that a predictable no-target phase comes out of the demod stages.

What we really need is a test circuit that illustrates how all this works. That's been on my "to do" list for about 3 years.

- Carl

[Qiaozhi]

By precise balance, I mean the induction balance. That's the amount of receive signal you get in the absense of a metal target. If I'm not mistaken, most coils are tweaked to give a little bit of signal, so that the demodulators will have something to work with.

Tuning is different. It usuallt refers to the receive coil be tuned to the transmit frequency. Again, I think most narrow-band coils are intentionally tuned off to one side of the TX frequency, so that a predictable no-target phase comes out of the demod stages.

What we really need is a test circuit that illustrates how all this works. That's been on my "to do" list for about 3 years.

- Carl

Actually, it's not the precision of the inductance balance that makes the Nexus so sensitive, it's because the coils are in full resonance. This is quite difficult to achieve as the search head has to be constructed very carefully and is labour intensive. i.e. it takes a long time and a lot of careful work to build these coils. That's why mass produced metal detectors do not use full resonance coils. Have a look at Georgi's website for a more detailed explanation of the Nexus -> http://www.nexusdetectors.com/

[Arnie S.]

Thank you Carl and Qiaozhi,
I am wondering if I could make such a coil for my "off the shelf" unit as the symmetry looks good as does having the coil at resonance- Cannot work the Dx on my bedsprings ( it's a futon, K). There are two board adjustments to the unit and one is "phase". Had it in the shop for warranty and the tech optimised it for the coil sent. Other part is a trimpot for " threshold". Wish I had a schematic. Not being an engineer, I am at my limits but still willing to experiment. The rx/tx coils at resonance makes sense for best performance and I saw on the England site for Nexus, that each unit varies in F of operation. Could the tx/rx coils be peaked on the board- this would make some sense for fine tuning, rather than fight coil windings. Could a home-brew coil be made with this symmetry work and be peaked in the box getting improved performance with DD guidelines? {VLF}
thanks much
A.S.

[Qiaozhi]

Thank you Carl and Qiaozhi,
I am wondering if I could make such a coil for my "off the shelf" unit as the symmetry looks good as does having the coil at resonance- Cannot work the Dx on my bedsprings ( it's a futon, K). There are two board adjustments to the unit and one is "phase". Had it in the shop for warranty and the tech optimised it for the coil sent. Other part is a trimpot for " threshold". Wish I had a schematic. Not being an engineer, I am at my limits but still willing to experiment. The rx/tx coils at resonance makes sense for best performance and I saw on the England site for Nexus, that each unit varies in F of operation. Could the tx/rx coils be peaked on the board- this would make some sense for fine tuning, rather than fight coil windings. Could a home-brew coil be made with this symmetry work and be peaked in the box getting improved performance with DD guidelines? {VLF}
thanks much
A.S.

To get the kind of sensitivity available with the Nexus you need to have the tuning capacitors built into the search head. Otherwise you wouldn't be able to change coils because you'd need to retune the detector each time. As you can see from the Nexus website, there are both 9" and 20" coils available. There is also a 4" coil, but it's not shown on the website. Not sure why. Perhaps it's not an official product yet, but there is a photo of the prototype on the Finds page.

[Arnie S.]

Am somewhat confused, it seemed to me the 20" unit was a special order and not an upward compatable addition to the smaller stock coil ( 9'?,10? 12?) combination. I am assuming that this unit has interchangeable coils, (does it?)how can it be so tight if the units shipped vary in Xmt freq? For the unit to have interchangeable coils, there can be no variation from the fundamental.
Each unit is 'custom' and operates within a small base range f. Sounds like there is some operator tuning going on.

I think I would be inclined to peak the transmit coil to maximise power transfer and take the hit on the receive. I disagree (respectfully) that the tuning has to be done at the coil, I would tune the whole system of the load/transmittion line; though I have, tuned antennas for contesting at the feedpoint. This would optimise one coil, yes- and each other coil would not match as well, true. Would I get better performance matching the line to load? Xl and Xc are = on xmt. How about just using a field strength meter to tune the load? Switched l or c could be available for different coils at the box, like an antenna switch later on. Just wondering. your thoughts please,
A.S.

[Unregistered]

Am somewhat confused, it seemed to me the 20" unit was a special order and not an upward compatable addition to the smaller stock coil ( 9'?,10? 12?) combination. I am assuming that this unit has interchangeable coils, (does it?)how can it be so tight if the units shipped vary in Xmt freq? For the unit to have interchangeable coils, there can be no variation from the fundamental.
Each unit is 'custom' and operates within a small base range f. Sounds like there is some operator tuning going on.

I think I would be inclined to peak the transmit coil to maximise power transfer and take the hit on the receive. I disagree (respectfully) that the tuning has to be done at the coil, I would tune the whole system of the load/transmittion line; though I have, tuned antennas for contesting at the feedpoint. This would optimise one coil, yes- and each other coil would not match as well, true. Would I get better performance matching the line to load? Xl and Xc are = on xmt. How about just using a field strength meter to tune the load? Switched l or c could be available for different coils at the box, like an antenna switch later on. Just wondering. your thoughts please,
A.S.

Little help.
When coils are tuned in full resonance having the tuning caps in the control box instead on the coil assembly will result in the coil cable acting as additional antenna, because it becomes part of the resonance circuit.
Bad idea.