Coaxial Coils?

[Tinkerer]

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.

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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

Hi Aziz,

An approximate balance is easy to obtain. The perfect balance takes more precision. I found that a large RX generally leaves more residual voltage. A small RX less. Do you think this is proportional to the RX target signal amplitude?

There are other factors that increase the residual voltage, among them the length of the untwisted or "un-coaxed" leads of the coils. My best has been about 3mV with a 300V Flyback, with a PI configuration of about 1A coil current.

The induction balance adjusting circuit helped a lot with that.

Increasing the coil current there is more residual.

What other factors have an influence on the perfect balance?

Tinkerer

[Aziz]

BTW,

I have put the coil comparison graph into the other thread.
here: http://www.geotech1.com/forums/showp...8&postcount=22

I'll respond later.
Cheers,
Aziz

[Aziz]

Hi Aziz,

An approximate balance is easy to obtain. The perfect balance takes more precision. I found that a large RX generally leaves more residual voltage. A small RX less. Do you think this is proportional to the RX target signal amplitude?

There are other factors that increase the residual voltage, among them the length of the untwisted or "un-coaxed" leads of the coils. My best has been about 3mV with a 300V Flyback, with a PI configuration of about 1A coil current.

The induction balance adjusting circuit helped a lot with that.

Increasing the coil current there is more residual.

What other factors have an influence on the perfect balance?

Tinkerer

Hi Tinkerer,

I didn't understand your first question.
But generally spoken, a bigger RX coil has more capacitance and has more flux detection area for EMI noise as well. A good shielding of all coils is strongly recommended.
Note: A coil is also a loop-antenna. It's generally a very inefficient EM wave antenna. Good shielding will make the loop-antenna more inefficient.

BTW, I don't need a perfect balance. I can live with high residual voltages as well. It shouldn't be so much high, that the amplifier comes into the saturation region (causing non-linearity to the system). An amplifier gain of 50 - 100 should be enough and there is much room for induction balancing.

"What other factors have an influence on the perfect balance?"
- Big coils, more copper -> more self eddy current induction in the wires (-> use Litz-wire!)
- Big coils, more shielding-> more eddy current induction in the shielding material
- More capacitance -> more capacitively coupled signals from the TX (-> good shielding of all coils minimizes this)
- Coil leads (inductively or capacitively coupled to the RX coil leads)
- Metal parts in the coil (bolts, nuts, coil stem, etc.)
- Stability of the coil (temperature effects, mechanical shocks, ...)
...

Well, I haven't built a bigger IB coil yet. I'll see it.

Aziz

[Aziz]

Hi all,

I am keen on testing a big coaxial IB coil just to see, how it faces with problems like:
- IB stability issues (thermal, mechanic, shocks, usage in the field, etc.)
- high mineral grounds
- depth performance (it's made for the large targets only, should perform for the small targets very poor in the near distance range)

I'll try a rectangular 40 x 60 cm coil.

BTW, the anti-interference coil configuration (RX-, TX, RX+) should be very interesting as well. It has of course a much higher axial thickness.

Cheers,
Aziz

[Aziz]

Hi all,

the co-axial IB coil is progressing slowly.

But the TX coil is finished (shielding as well, made out of cheap loudspeaker cable).
A wooden frame for the coils are also finished.
It will be a 60 x 40 cm coil with a desired thickness (height) of approx. 15-25 cm.
I will make the IB coil RX+, TX/RX- version first.
Later the RX+, TX, RX- version (anti-interference co-axial coil). Note, this coil type needs more height space.

I am waiting for the next release of my IB detector (now release 5). So I can finish the coil and test it then.

This coil will be a large target ripper on hot mineralized soils!!!

Aziz

[Kjamesgraham]

I've been thinking about making a 10 inch coax for my gmt

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-.

So the two rx+ and Rx- should be the same value. Rx- doesn't function as a bucking coil. Rx would be what ever value the receive would resonate at (for lack of a better word). Both coils would be equal to Rx needed.

Kenny;')

[Kjamesgraham]

Hello Aziz,

I finished my coil winder with tensioner. I will be winding the recieve coils for the -rx tx +rx arrangement. I've calculated where I want recieve to resonate at. To get the advantage of this arrangement -rx must = +rx and since -rx is in series with +rx with the caps across both...I'm not sure about the math yet. If you could point me in a direction I would appreciate it.

The reason I want to try this arrangement is that the BLM in my area has put up a chain link fence to protect some old Yucca's. There is also some transformers etc that are putting out some very strong low freq emi. I was successfully finding nuggets ( > than match head size). It's noisey the fence wanders out in the desert. Most of the search area is close to these structures. The pulse detector is getting its *** kicked, the GMT has the most promise.

I would like to experiment with a few coil sizes. I will share what I do and how effective these coils will be in these conditions.

ps. I wouldn't take advantage of anyone's experience or talent that would shorten the learning curve. We could work out compensation.

Thanx

Kenny

[Aziz]

Hello Aziz,

I finished my coil winder with tensioner. I will be winding the recieve coils for the -rx tx +rx arrangement. I've calculated where I want recieve to resonate at. To get the advantage of this arrangement -rx must = +rx and since -rx is in series with +rx with the caps across both...I'm not sure about the math yet. If you could point me in a direction I would appreciate it.

The reason I want to try this arrangement is that the BLM in my area has put up a chain link fence to protect some old Yucca's. There is also some transformers etc that are putting out some very strong low freq emi. I was successfully finding nuggets ( > than match head size). It's noisey the fence wanders out in the desert. Most of the search area is close to these structures. The pulse detector is getting its *** kicked, the GMT has the most promise.

I would like to experiment with a few coil sizes. I will share what I do and how effective these coils will be in these conditions.

ps. I wouldn't take advantage of anyone's experience or talent that would shorten the learning curve. We could work out compensation.

Thanx

Kenny

Hi Kenny,

have a look at the "Top-Hat" coil design on this forum (do a search please). It's a fully anti-interference IB-coil design.
Make the inner coils (Rx+/Rx-/Bucking coil) small (1/3 to 1/2 of the outer coil size). Make the co-axial height at least half of the diameter of the outer coil size (better the full diameter height).
RX+ and RX- are connected in series (but anti-phase to cancel EMI noise).

Cheers,
Aziz

[Kjamesgraham]

Hi Kenny,

have a look at the "Top-Hat" coil design on this forum (do a search please). It's a fully anti-interference IB-coil design.
Make the inner coils (Rx+/Rx-/Bucking coil) small (1/3 to 1/2 of the outer coil size). Make the co-axial height at least half of the diameter of the outer coil size (better the full diameter height).
RX+ and RX- are connected in series (but anti-phase to cancel EMI noise).

Cheers,
Aziz

Thanx Aziz I did find the link to your coil. I think I'll twist up an 8 inch (tx) version of it. I have an Idea on how to set it up so I can swing it.
Will add pics later. Thanx again (Holly Moe-lly what a coil)!

[Dave J.]

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.

Back in the early 1980's we made an approx. 5 inch diameter searchcoil in this configuration for the 1260-X. Its depth was very nearly that of the 8 inch coplanar. Customers loved it, but it was hard to manufacture and to keep stable, so when the 1265-X replaced the 1260-X we stopped making the coaxials, and the 1265-X got the 4 inch coplanar "hockey puck" coil.

--Dave J.

[Aziz]

Back in the early 1980's we made an approx. 5 inch diameter searchcoil in this configuration for the 1260-X. Its depth was very nearly that of the 8 inch coplanar. Customers loved it, but it was hard to manufacture and to keep stable, so when the 1265-X replaced the 1260-X we stopped making the coaxials, and the 1265-X got the 4 inch coplanar "hockey puck" coil.

--Dave J.

Hi Dave,

that's interesting. Pity, that the co-axial coils weren't developped since then. The idea was good and it is very obvious now, that the coil technology had some merit. I think the co-axial coil technology has gained more attention (not only due to the demining industry using them for a long time now (co-axial frame coils)).

BTW, the Top-Hat coil is one of the best IB AI coils ever (regarding performance). And one can build and balance it quite easily. But it isn't looking good. It's looking very strange.

Cheers,
Aziz