Advanced PI coils

[Davor]

Yes, there are issues, and they are fixable. A centre tapped bifilarly wound Rx coil with balanced Rx input providing equal impedances for both ends and some reasonable CMMR should perform as good as a well shielded single ended coil. There are a few mechanisms at play in such configuration. Distributed capacitive coupling average of this coil is equal to the ground, hence it does not produce any differential mode signal if subjected to electric field of any kind. This kind of arrangement works fine in professional microphones for ages.

A step further is making a Tx coil balanced, in a way that distributed capacitive coupling average also becomes equal to ground. It is never equal to ground in a single ended configuration. This would make an IB configured coils double balanced. This is good because there is always some capacitive coupling between Rx and Tx regardless of shielding.

IMHO troubles that still happen WITH shielding on are mostly related to disobeying a common ground single point rule. This tends to create various unintentional antennas all over the place. But not in my rig. My common ground point is a coil connector. All other ground connections are in a star configuration. My rig's case is plastic and there are no problems with static anywhere. I do however have problems with sea water and shore line, but I expect to fix it with a differential coil.

I have a solution for everyone expecting problems with EMC tests: just place some source of light or a quartz watch on your rig and declare it a lamp or a wrist watch Point is that faced with the impossibility of making the CFL's and wrist watches obey any EMC regulations in any form, shape, or flavour, regulation is a bit simplified: lamps and wrist watches are exempted of EMC tests. At least in Europe.

[Ferric Toes]

Multiple coils are not required if you squeeze a coil into an ellipse or oval - comes to the same thing.Shielding is related to the common mode signal that develops with capacitive coupling to the electrostatically charged objects in the surroundings, such as wet grass etc. IMHO shielding is not required if you wind your coil bifilarly and with a centre tap. My own coil works that way and I'm happy with it. It has no shield. I call a class of coils without shield "FKK coils" but very few people here regard them seriously. To reap a full potential of such coils you'd need a balanced Rx input.

Your best bet with any coil is using shields. A proper shield has very little to do with the coil's magnetic properties.

Wet seaweed is the worst!

There are still good reasons to shield a coil array until a detector using a bifilar coil technology undergoes some serious testing in a variety of environments. A wet salt beach with rock pools and seaweed is one environment that PI's have excelled on for a few decades. Original machines with sample delays of 25uS and upwards could get away without a shielded coil, but as delays shortened to 15uS and then 10uS the need for a shield became necessary. This is of course when using a mono coil, or even a TX with coplanar RX. One example of this is the AN19/2 PI mine detector made by Schiebel in Austria. This uses the latter coil configuration and was originally supplied with unshielded coils. Fine on dry soil and OK on freshwater wet soil, but unstable on a wet beach. Later the coils were replaced by shielded ones. I have an AN19/2 with unshielded coils and have tried to use it on the beach, so I experienced the problem first hand. I have no information on the circuit except that it is a bipolar TX, and comparing the sensitivity with other PI's, it seems to be sampling at around 15uS. If anyone has schematic information, I would be most interested. The unit is now replaced by later models.

Another problem that can occur on conductive ground is what I believe to be induced polarisation. This has only been noticed on a PI detector that has a metal case. If the case is grounded to the electronics and the coil is unshielded, then touching the case with a finger, or any contact with the skin, gives a large spurious response with a long decay. It seems that the detector/body/wet beach electrolyte/unshielded coil make a nice leaky capacitor.

A different situation that occurs in a dry, hot climate is the buildup of static charge on the plastic coil housing by scrubbing on the ground.. This has the potential to spark over to the nearest conductor in the coil housing. If it is to the coil (minimally insulated) Litz wire or to the coil/cable terminations then it can make annoying noises in the phones, or worse. If the coil has a shield grounded to the electronics, then this is the target for any discharge which is safely conducted away. The question is where does it go as the detector ground is "floating", unless you drag a metal chain behind you which is connected to detector ground. You might think this is an unlikely scenario, but I have experienced this in an industrial application at a polythene bottle factory where hot bottles from a molding machine were bouncing along a conveyor belt from which you could draw 1/2in sparks. Occasionally it sparked to the detector coil, which when protected by a properly grounded shield was immune to the effect.

Next, and important in these regulatory days, is EMC. Your detector whether PI or VLF is supposed to comply with certain regulations, particularly with regard to emissions. I personally was involved in emissions testing of industrial PI's and compared a shielded with an identical unshielded coil. The detector with unshielded coil would have failed, but shielded it passed by a very safe margin. It is not necessarily the TX waveform that is the source of the emission, but it could be another device in the electronics, even the clock generator with fast edges that leak into the coil circuit by way of internal capacitance or poor layout on the circuit board.

Eric.

[Sergey_P]

d/h

There is no difference, moment reduced ~ three times.

More moments are involved in compensation - more attenuation of ground
(Focused logging device has more than 3 coils ... 7 coils provide almost complete suppression of the environment)

[ppodg]

Yes [ATTACH]22373[/ATTACH]

No difference: sum(e(d)) < E(d), because the really important factor is d/h, but you are loosig depth using small coils.
The resultant s/n does not seem to inrease...

[Sergey_P]

more copper.

a little more than usual. This sensor has equal moments which is not quite optimal for suppressing the signal ground, but has similar sensitivity to the target

[Davor]

I wouldn't expect much of a difference on any realistic terrain. Maybe a little more over water. IMHO too many coils means only lots more copper.

[Sergey_P]

Multiple coils are not required if you squeeze a coil into an ellipse or oval - comes to the same thing

No you're wrong. Several coils form part of direct and inverse zones. This allows you to better compensate for the signal from the ground

[Davor]

Multiple coils are not required if you squeeze a coil into an ellipse or oval - comes to the same thing.

And when you say business as usual, Is faraday shielding still important or can it be eliminated if coils are set and tuned correctly in a true anti phase state??

Shielding is related to the common mode signal that develops with capacitive coupling to the electrostatically charged objects in the surroundings, such as wet grass etc. IMHO shielding is not required if you wind your coil bifilarly and with a centre tap. My own coil works that way and I'm happy with it. It has no shield. I call a class of coils without shield "FKK coils" but very few people here regard them seriously. To reap a full potential of such coils you'd need a balanced Rx input.

Your best bet with any coil is using shields. A proper shield has very little to do with the coil's magnetic properties.

[Sergey_P]

If +/- parallel to ground

Yes

[sido]

For this very reason I'm about to make a 4 quadrant adapter. In the meantime it is good to know that there are some other possible solutions for 2 quadrant detectors as they are right now.

Davor, there was a rumor quite a few years back here in Australia regarding a detector that was developed and using such multiple coil configurations. It was told that it used more that 8 coils in its construction for a PI detector.

Never was released or seen by anyone.

The 4 quadrant adapter will be interesting to analyse if can be built on a horizontal plane for ease of construction. Wish you luck there Davor.

Time is my limiting factor ATM or else i would be constructing different complex coils.

Sid

[sido]

Hi Davor, Thanks for a very informative reply.

Hi sido,

in all these cases the coils have some inductance, even if wound in counterphase. It is EMI and ground response that is cancelled with these arrangements. The great grandfather of them all is a DDO coil which is a true differential coil, and it has a phase reversed under one of the D-s. This particular coil is otherwise perfectly balanced, has perfect ground suppression, and perfect EMI suppression. Only thing missing is a receiver that can use it properly.

So the DDO configuration requires a dedicated receiver.

So, for all these coils it is business as usual, capacitors and all. The only way of destroying most of the inductance is winding a bifilar coil and connecting it in counterphase - which is exactly the case with centre tapped coil and it's ability to divert a common mode noise to the ground.

So for a given detector with certain coil specs can be built if the coil parameters are used according to your example and Aziz's Top hat example plus calculations for all coils presented here?

And when you say business as usual, Is faraday shielding still important or can it be eliminated if coils are set and tuned correctly in a true anti phase state??

These are all IB coils. Unfortunately all of these coils except Davor-Sergey configuration are reversing phase at odd places.

I like this coil configuration as the coils are practically on a horizontal plane if constructed, which could ease in building of such a coil. As you say though not a true reverse phase configuration yet, according to Aziz's sims and observations so far.

These IB coils can be used for PI in case Rx is connected to a separate coil, and if they are wound to inductance as specified for your rig. In case you insist on a monocoil, you can try figure 8 coil which cancels EMI, but nothing else. It will not go as deep as a simple mono, but may come handy in industrial areas with strong EMI. It will not create any problems with phases because it is both Tx and Rx reversing the phase simultaneously, but for that very reason there is no ground signal suppression. Only EMI.

Correct, using of figure 8 coils is a compromise and depth is decreased.Ground suppression as you say is not taken care of with such coil, therefore you still require some form of Ground balancing circuitry.

Will be easier to test these coil parameters given on a basic PI such as the Surfpi with two coil configuration, having TX and separate RX.

Cheers Sid

[Davor]

Hi sido,

in all these cases the coils have some inductance, even if wound in counterphase. It is EMI and ground response that is cancelled with these arrangements. The great grandfather of them all is a DDO coil which is a true differential coil, and it has a phase reversed under one of the D-s. This particular coil is otherwise perfectly balanced, has perfect ground suppression, and perfect EMI suppression. Only thing missing is a receiver that can use it properly.

So, for all these coils it is business as usual, capacitors and all. The only way of destroying most of the inductance is winding a bifilar coil and connecting it in counterphase - which is exactly the case with centre tapped coil and it's ability to divert a common mode noise to the ground.

These are all IB coils. Unfortunately all of these coils except Davor-Sergey configuration are reversing phase at odd places.

These IB coils can be used for PI in case Rx is connected to a separate coil, and if they are wound to inductance as specified for your rig. In case you insist on a monocoil, you can try figure 8 coil which cancels EMI, but nothing else. It will not go as deep as a simple mono, but may come handy in industrial areas with strong EMI. It will not create any problems with phases because it is both Tx and Rx reversing the phase simultaneously, but for that very reason there is no ground signal suppression. Only EMI.

[ppodg]

http://www.md4u.ru/viewtopic.php?f=94&t=8150
[ATTACH]22364[/ATTACH]

If +/- parallel to ground

[sido]

Hi all.

To adapt the coils presented so far for Vlf detectors do i still need to use a capacitor for the TX frequency and only TX. Where as RX obviously has been taken care of in its anti interference configuration and therefore does not require a capacitor.
Am i correct?

To build such coils as presented above, can they be used on existing VLF and PI detectors and how do you go about it? Do you build according to original coil specifications?

The reason i ask is i want to build a very large VLF coil and would love to have the ground and noise cancelling capability of the coil designs presented.

Just not sure how you go about adapting it to existing detectors?
Great work

Thanks Sid

[Davor]

For this very reason I'm about to make a 4 quadrant adapter. In the meantime it is good to know that there are some other possible solutions for 2 quadrant detectors as they are right now.