Another dumb question..but you guys are used to them

[Midas]

About diluting the TX field strength:

With the fields of coil 1 and coil 2 aiding, how does this affect the field strength? It should compensate for some of the dilution.

With the fields aiding, there is also a large area of enhanced angled field vectors that increase the response of coins in the vertical position.

Looking at the ergonomics, it gives space for the feet of the operator and the weight is closer to the body.

Tinkerer

Well my intuition says no. Both coils must operate independantly. Move them closer together to try and get some advatage from the TX of the opposite coil will only be detrimental as the singal cancels in the opposite RX coil. Howver inuition is no way to operate, somebody should get the Aziz symbol projected onto the clouds of Gothem city. Perfect time for a simulation.

That's in a case of using 2 quadrant detection with discrimination. In case you go a step forward and make a full 4 quadrant Rx, you'll have both coils giving sensible output, discrimination, and even pinpointing by means of a sharp minimum right in between the coils.

I'd say this construction deserves making a 4 quadrant Rx.

BTW, a regular 2 box uses a Tx at nearly orthogonal position, and quite away from the business area of the Rx coil. That would account for much more dilution than this one. Besides, you can also introduce some slow motion compensation, GEB, doodahs and doohickeys of nowadays rigs, and get a very decent deep scanning device - with pinpointing.

Maybe I'll have to play a bit with 4 quadrant discriminator. The rest is easy.

You've lost me Davor. 4 quadrant, all TX and RX in series? That would only seem to exacubate the problem. The problem with the 2 quadrant isn't that only one coil gives 'sensible output' they both do, just one is opposite of the other. The problem is you essentially have two independant detectors running of one TX.

Midas

[Davor]

The problem is you essentially have two independant detectors running of one TX.

Not exactly. You have both coils connected in series and supplied to the same Rx, thus obtaining balance. It is just that one of the coils, being in reverse, will supply a target response in counterphase. There is a well known "double tone" problem with DD coils in case you reverse phases, and you don't get tone in a middle where it is supposed to be. To make it work as it is supposed to, you need a 4 quadrant receiver.

Let me explain what I mean.

In a normal 2 quadrant receiver you have +I (GEB channel) and +Q (Disc channel) for Cu, and +I -Q for Fe.
Now, with reversed Rx phase you have -I -Q for Cu, and -I +Q for Fe.

What is important here is that vector of, say Fe, in both cases lays on a same line that goes through the coordinate system origin. However, in a 2 quadrant receiver a negative I is ignored, hence there is no sensible information coming from a coil in counterphase.

[Midas]

Not exactly. You have both coils connected in series and supplied to the same Rx, thus obtaining balance. It is just that one of the coils, being in reverse, will supply a target response in counterphase. There is a well known "double tone" problem with DD coils in case you reverse phases, and you don't get tone in a middle where it is supposed to be. To make it work as it is supposed to, you need a 4 quadrant receiver.

Let me explain what I mean.

In a normal 2 quadrant receiver you have +I (GEB channel) and +Q (Disc channel) for Cu, and +I -Q for Fe.
Now, with reversed Rx phase you have -I -Q for Cu, and -I +Q for Fe.

What is important here is that vector of, say Fe, in both cases lays on a same line that goes through the coordinate system origin. However, in a 2 quadrant receiver a negative I is ignored, hence there is no sensible information coming from a coil in counterphase.

OK I think I see where your coming from now. But its not what I was talking about.I think we assume too much from each other. I know I'm shocking for it. I've been coming at all this from a PI perspective where being counter phase isn't really a big issue, and to be honest I know very little about VLF. But I wasn't talking about either specifically.

All I was trying to convey was:
Simply a fundamental property of the coil, the TX is an ordinary coil split in half and spread over twice the area, therefore its only going to have half the magnetic field density at a given power level than if you left it together.

Midas

[Davor]

Very true. However, Tx coils being with the Rx coils are much closer to the business area, and this loss is greatly overcompensated - if you compare this with a two box concept. Being apart, these coil pairs posses a level of orthogonality, so you actually get the best from the both worlds.

It would be very nice if Aziz steps in with his nice software and compares these two cases, 2box and bicycle.

For some time now I'm thinking of all the coil possibilities using a 4 quadrant receiver. I have an idea already, but not the time or resources to pull it through at the moment. If there are some other tinkerers interested in pursuit of this idea, I'll join and share my models. In fact, it is just incredible how little tinkering is required to make it tick.

I'd say that EMI cancellation alone is worth the trouble. You can expect some 0.1μT√hz at VLF bands of background noise (see www.adv-geosci.net/14/69/2008/adgeo-14-69-2008.pdf ) and at least some of it may be cancelled by a smart coil configuration.

In case of PI you also get EMI, EF, and great deal of ground cancelled this way. Yes, I think it is worth it.

[Davor]

I just remembered another coil that may benefit from 4 quadrant Rx: DOD.

BTW, I tried mikebg's suggestion regarding a small piece of ferrite to null a coil - as seen in his depiction of a DOD coil. Works as advertised. There is no difference in the final result between coil squeezing, small loop adjusting ... and simple ferrite positioning

[Aziz]

Hi guys,

sure, I can provide you nice magnetic fields.
Please specify the exact parameters of the coil configurations.

But it will take some time as I have not much time right now.
Aziz

[Davor]

There is not much to specify. Just model two Tx circular coils D=50cm with one diameter free space between them, wired parallel in phase and inside each of the Tx coil put one Rx coil at D=48cm, and wire the Rx coils in antiseries. Please do the magic you did for the DD configuration before :wink: :nudge:

I think it is important to wire Tx coils in parallel so that voltage is the same on both of them regardless of the ground proximity, skew etc. Thus the Rx coils will see the same voltage as well and maintain the balance better. Right?

[Sergey_P]

and maintain the balance

http://md4u.ru/download/file.php?id=2845
…
http://md4u.ru/download/file.php?id=12219
http://md4u.ru/download/file.php?id=12249

[Davor]

?

[Sergey_P]

differential coil with the balance of the soil

[Aziz]

Tandem Coil Magnetic Fields

Hi all,

this is the tandem TX coil in in-phase and anti-phase configuration (50 cm coil diameter, 50 cm distance, 1 A coil current). I didn't put the RX coils so you don't get confused. Both configurations were normalized to approx. 300 µH total coil inductivity.
Cheers,
Aziz

[Tinkerer]

Thank you Aziz.

This is very interesting. It shows how the in-phase will give more depth and the anti-phase will better find the targets that are hard to find with the single coil, due to their shape and orientation.

With the anti-phase coils, an orthogonal RX coil above the center of the 2 TX coils might give interesting results.

Tinkerer

[Sergey_P]

>RX coil above the center of the 2 TX
RX is located on the equipotential surface of the TX
>in-phase will give more depth
the best =>

[Davor]

...I didn't put the RX coils so you don't get confused...

Please do With in-phase and counterphase multiplied, we'd get a real picture of what this setup does. E.g. points where these fields are orthogonal will give zero response in Rx.
This looks very promising.

@Sergey_P, please note that the single largest constraint to go any deeper is Rx sensitivity. With natural and artificial noise sources you are limited by EMI, various noises, and the ground signal much more than you can gain by aiding the field. In fact, you can get the most field by a simple circular coil, just like with the monocoil - exactly like in your example above.
To put things in perspective, with voltage gain of *10 (or 20dB) you obtain ~40% increase in depth. "Normal" VLF has sensitivity at ~0.5μV with a 15mH coil, and it is far above the thermal noise floor. Going lower than that make little sense because of the chatter thet is a result of excessive noise. So the major constraint is additional noise on top of the thermal noise. High energy natural sources of noise are usually of impulse type, and I expect to see that 20dB reduction with this coil configuration.

Ergo, to go any deeper with a coil of the same size you need every means available to reduce noise. I can imagine:
- EMI (and ground signal) fighting coil configurations
- better balance methods
- target response discontinuity somewhere on a coil system to aid motion detection and/or pinpointing - a sharp signal change will pass through motion detection even at a very reduced sweeping speed AND improve resolution for small targets

I think the bicycle coil has it all. Guess the coils distance can be optimised for depth.

[Sergey_P]

please note

Iknow , read=> http://md4u.ru/viewtopic.php?f=77&t=3866&start=0 (IB) http://md4u.ru/viewtopic.php?f=77&t=6694 (PI, GB no)
and see =>http://md4u.ru/download/file.php?id=1276 (PI+IB+GB coil)