Coaxial Coils?

[Aziz]

Hi all,

I am just wondering, what happened to the coaxial coils (see below)? They seem for a quite long time not much popular anymore. What's the reason?

Aziz

Source:
Coil Basics
by Carl Moreland

Can be found on Geotech somewhere.

[Carl-NC]

Garrett used them in their early VLFs, and C&G Technology used them, too. They are much thicker than coplanar coils, and tend to be heavier. Also harder to build, as the IB happens in 3D, whereas coplanar has a simpler 2D balance. This means more expensive.

However, they work great under powerlines and along metal fences. In fact, we had a utility customer who needed to detect right up to a chainlink fence, and no regular coil would do it. So we built him a special coax coil for his MXT, and it worked perfectly.

[Aziz]

Thanks for your feedback Carl.

I've never seen a coaxial coil in the coil market yet. The coil thickness can be lowered to a reasonable limit I think. Or it could be built as a rigid frame coil (less weight).

This coil is much simpler to build when comparing with concentric coplanar coils. But is still a challenge when comparing to other simple IB coils (OO, DD).

Well, the depth performance is dependent on the architecture & geometry of course. The anti-interference configuration (RX+, RX-, TX) could be interesting when increasing the amplifier gain (compensation for the depth loss). Provided that, there is enough CMRR achieveable.

One interesting similar configuration however outperforms every IB coil for a comparable amount of copper (wire) and coil coverage area. It's either not known yet or other facts didn't lead to their success.

Well, I probably will try one and want to see how it's performing.

BTW, I think, I saw similar or equal coil designs in the UXO detection publications.

Cheers,

Aziz

[WM6]

The coil thickness can be lowered to a reasonable limit I think.

Yes, Aziz, but only with reasonable limit of coil diameter, otherwise lost in sensitivity/directivity are significant. Except if you use TX with smaller diameter, but in this case we get hybrid between Coax and Concentric coils. In fact Coax and Concentric coils are pretty the same things, only tuned on different way (by Coax with coils distance).

Try some of your simulations.

[DOOLEY]

hello all , thought i'd jump in and through in my two peneths worth ,

tried this configuration last year after finding it on the web , it's also used as a sensor head for detecting flaws in metals without intrusion .

experimented on the bench using 3 x 6 inch matched coils , one TX two RX , mounted on an empty blank cd container , noted that increacing the distance between them reduces depth , ballance is not as easy as two oo coils , and deviation for same test piece is about half that of oo , so gave up with that idea.

also tried flat ooo setup with TX in the middle , no increace in distance , and harder to setup / ballance.

the last wacky idea was 3 x RX coils layed flat in a triangle , and a single TX coil right in the middle , steady hand required to ballance that up !!

result , not alot different to oo , just larger catchment area.

[Aziz]

Try some of your simulations.

Hi WM6,

I did dozens of them last days.

Therefore I was quite positiv surprized about the magnificient depth performance and the ease of building such coils. Balancing is made by purely varying the distance of one coil, which makes up the thickness of the coil at the end.

It would have a better response to highly mineralized soil as the transmit coil is quite above the ground. The hot rocks/ground won't be magnetized much and could reduce the ground noise.

Unfortunately, the thickness of the coil is related to it's diameter. A bigger coil needs more thickness of course. Could be a problem in the terrain usage.

Nevertheless, the coaxial coils can be made to outperform every IB coil for a comparable given inductivity and coil coverage surface. But you need to modifiy the architecture slightly to reduce the thickness of the coil.

---------------------------------------------------------------
RX- (less turns, very close to TX, RX- & TX fixed tightly together)
TX
..
Gap for fine balancing
..

RX+ (more turns)
---------------------------------------------------------------
Ground

Cheers,

Aziz

PS: Let's do some pancakes now.

[Aziz]

BTW,

the other configuration (TX-,RX, TX+) is also possible of course.

------------------------------------------------------
TX+ (more turns)

..Gap..(balancing)

RX
TX- (less turns, very close to RX, fixed tightly together)
------------------------------------------------------
Ground

Cheers,

Aziz

PS: A message to the patent trolls. It's a prior art.

[Carl-NC]

---------------------------------------------------------------
RX- (less turns, very close to TX, RX- & TX fixed tightly together)
TX
..
Gap for fine balancing
..

RX+ (more turns)
---------------------------------------------------------------

This will work for achieving IB, but it will negate the two advantages of the coax arrangement: interference cancellation, and the edge-null for hunting next to metal posts. Both of these require equal RX+ and RX-.

[Aziz]

This will work for achieving IB, but it will negate the two advantages of the coax arrangement: interference cancellation, and the edge-null for hunting next to metal posts. Both of these require equal RX+ and RX-.

Yes, that's right.

But the asymmetrical configurations decrease the coil thickness. You can decrease it more at the cost of depth performance of course.

I like the coil configurations, as these are definitely very easy to build (for me). And I can build much bigger IB coils (as tight frame coil with two tires).

Aziz

[WM6]

I like the coil configurations, as these are definitely very easy to build (for me). And I can build much bigger IB coils (as tight frame coil with two tires).

Aziz

Hi Aziz, look at this hidden testing plant of such coaxial (two man operating) coil.
It can completely eliminate all Ferro particles and hot soil to 1.7 meters of depth - dream for you:

[Aziz]

Hi Aziz, look at this hidden testing plant of such coaxial (two man operating) coil.
It can completely eliminate all Ferro particles and hot soil to 1.7 meters of depth - dream for you:

Hi WM6,

this ain't efficient enough!
But goes into the direction of what I plan to do.

It's quite difficult to get the appropriate plastic rods. So I want to do it with wooden rods first (quite cheap and easy).

I'm investigating how to make a thread in the wood for the plastic bolts. It's obviously possible to glue a metal bolt into the wooden hole and screw it out when dried (epoxy glue). This gives a tight thread. If the metal bolt is oiled first, then this would give a much better thread I think (easy to screw out).

The wooden rod is a distance spacer. Two rectangular wooden frames with tight fixed coils will be assembled together. The distance spacers give you some amount of adjusting range to get the balance of the coils. This could be done with some flexible (soft) gum or tight XPS foam.

Two plastic tires will be mounted to the frame coil (pushing and pulling the coil).
That's it.

Aziz

[Rov]

Hi Aziz,can we see some pics and results of your dozen coil builds please?with your scale off results per coil build,will be very interesting for all,for sure? this is interesting? Thanks!Rov

[Aziz]

Hi Aziz,can we see some pics and results of your dozen coil builds please?with your scale off results per coil build,will be very interesting for all,for sure? this is interesting? Thanks!Rov

Hi Rov,

sure. Why not?

I'll put the mono coil, DD coil, concentric coplanar (0.5 R, standard), concentric coplanar (0.8 R, optimized) and the coaxial coil together in an excel comparison chart. You can then see the distance gain in a graph. But this will take some time.

-------------
Oh, boy! Gluing the metal bolt with epoxy ain't good. I can not screw it out anymore. It's working for small glued parts only. I'll try the wax instead of oiling the screw. When heating the bolt, one should get out it easier.
Next try.

Cheers,

Aziz

[maikl]

Hi Aziz. Why do a combination of three coil (TX-RX-TX or RX-TX-RX). Is not it enough combination of the two coil (TX-RX or RX-TX). The smaller dimensions, lighter weight, easier to balance ...

[Aziz]

Hi Aziz. Why do a combination of three coil (TX-RX-TX or RX-TX-RX). Is not it enough combination of the two coil (TX-RX or RX-TX). The smaller dimensions, lighter weight, easier to balance ...

Hi Maikl,

these versions wouldn't work in our IB configuration. The RX coil would induce a much higher voltage from the TX coil. That's the reason, why we need bucking coils (either RX or TX) to cancel the high induced voltage. We just want a few mV (or just a few tens of mV). Perfect IB would mean: 0 V induction from the TX coil.

--------------------

BTW, the thin wax film on the metal bolt makes a perfect thread for the plastic bolt's. After heating the metal bolt, it can easiy be screwed out. I have still taken the fast drying epoxy glue as this becomes quite stiff and strong. In the wooden distance rods, I can make perfect threads now.

Aziz