One turn coil never crosses 0 while balancing

[green]

The thread is about samu not being able to balance his figure8 Rx coil made with 5mm diameter wire (reply #1). I was assuming the imbalance might be caused by the 5mm coil wire (high conductor, maybe 75us tau). Was wondering why the suggestions to use a low conductor (less than 1us tau) for R to balance the coil. Maybe the 5mm coil wire isn't causing the imbalance? Maybe using 5mm wire to make the coil is a problem?

Don't have any answers, just questions. Just trying to learn something.

If the 5mm wire is causing the imbalance, a piece maybe 10mm long should be able to balance the coil?

[Skippy]

"I'm confused"
It's pretty VLF-specific. The imbalanced coil will have some sine-wave coming off it, that could have a phase relationship with the TX coil voltage that's anything from -180 to +180 degrees. You could resolve the signal into 0 and 90 degrees ( quadrature synch detectors for example ). Then you would see that shifting the coil about physically would almost exclusively affect just one of these. And placing a 'very low conductor' target ( like the wire loop ) in various locations would largely affect the other quad signal. This in theory allows independant nulling of X and R null levels, and achieving a good null is reasonably easy.
However, if your 'very-low conductor' is just an average target, like a US 5c coin, it would allow nulling of one component, but also affect the other ... so you'd then have to move your coils a bit to re-null things. Then probably fine-tune your 5c placement, and re-adjust again the coil geometry ( or use small ferrite bits, for example.)
So Samu can move his coil geometry to minimise one phase component .... but needs a new tool in his toolbox to adjust and minimise the other phase component. That 'tool' is preferably a low-conductor non-ferrous one. But it may need to be a 'large' low-conductor target - thankfully adjusting the wire-loop and/or the position of it gives plenty of tinkering range.

[samu]

I finally managed to balance the coil thanks to all your suggestions. The tin loop worked great!. I had to use also a piece of steel (bar) but it end up at about just 50mV!. If I made another coil like this, I would use the proposed geometry by Skippy and probably it will lower the initial unbalance.

If the 5mm wire is causing the imbalance, a piece maybe 10mm long should be able to balance the coil?

About the wire causing the imbalance, I have used it before without issues, I think that the geometry was not as good as it should be, that was the problem.


Thanks again for all yoour support, without you, it would have been impossible!

[Carl-NC]

Thanks for the update, glad it worked out!

[Skippy]

Here is some maths related to using 'very-low-conductivity' targets to null out the Resistive part of an unbalanced null coil:
The idea of such targets is that their phase lag is sufficiently low angle that they produce a strong Resistive response, and a much weaker Reactive one.
To see how much Resistive/Reactive effect is actually observed, I went through the maths. What matters is the ratio of 'adjustment target' frequency to machine frequency. Here are phase lags, and ratio of Resistive to Reactive nulling effect, for various targets:
(Target freq / detector freq) : phase lag ( degrees ) : (ratio of Resistive voltage to Reactive voltage change)
1 : 45^o : 1 to 1
5 : 11.3^o : 1 to 0.2
10 : 5.71^o : 1 to 0.1
15 : 3.81^o : 1 to 0.067
20 : 2.86^o : 1 to 0.05
25 : 2.29^o : 1 to 0.04

So for example using Samu's 13kHz detector. He makes a wire loop with 260 kHz target frequency. His target/detector ratio is 20. And his Resistive nulling effect is 20 times that of the Reactive. So if he reduces a 100mV Resistive null to the lowest he can, he will offset the Reactive null by just 5 mV. This should be mostly be reduceable with minor geometry adjustments or small ferrite bits, and re-adjusting the Resistive null again should be unnecessary.

Using a target whose freq is only 10 times ( or less ) than the detector freq is going to be more troublesome, due to the way it affects both parts of the null. So keeping it 20+ times higher is best.

[green]

If I place a US copper penny (70us tau) over one of the figure8 Rx coils, separate Tx coil causing a target signal I can cancel the signal by placing another penny over the other Rx coil with my PI. Probably any target with the same tau (70us) would work. Can't cancel the signal with ferrite and a wire loop (.5 to 1us tau). What is different with a VLF detector that can it cancel the penny with ferrite and the wire loop?

Some possible differences. Most VLF use a DD or concentric coil, not figure8. Could coil type be the reason?
Some VLF's are single frequency, some multi frequency. Would multi frequency cancel with ferrite and the wire loop?

samu replied he used a steel bar to help cancel the offset signal (reply 33). Maybe the metal inclosure (reply 1) was causing more of the problem than the 5mm coil wire?

[Skippy]

Quote:"What is different with a VLF detector that can (make) it cancel the penny with ferrite and the wire loop?"
VLF's don't just measure the time-constant, they wouldn't be able to discriminate if they worked that way. They measure the phase too, hence to get the '1 cent' cancelled out, it's necessary to match the time-constant AND the phase with an opposing phase.

Your question about coil-nulling a multi-freq coil is a very good one..... which probably has a very complex answer.

[Skippy]

@Green: After some thought, I see what may be the problem may be.
Q: Where are you placing the ferrite and wire loop, in order to attempt cancellation of the 1c coin? I think this could be the key. If you are placing them above the sweet-spot of the second RX loop, then I think it's possible you can get a balance. If you're trying to place them close to the loop with the 1c above it, you're not going to succeed. Whereas with a VLF, I think it should be possible to place the ferrite/loop 'somewhere' about either loop to cancel the 1c.
I'll try and set up my F75 for some tests in the next week, to see how nulling a DD coil may relate to your 8-coil.

[Carl-NC]

VLF's don't just measure the time-constant, they wouldn't be able to discriminate if they worked that way. They measure the phase too, hence to get the '1 cent' cancelled out, it's necessary to match the time-constant AND the phase with an opposing phase.

Not exactly. The Tarsacci measures time constants and discriminates just fine. Phase is directly related to tau by phase = arctan(2*pi*f*tau) so you could discriminate either way.

The difference is that in a VLF coil the phase can be quadratured into an X and R signal. If you tape a penny to the coil you get an RX signal with a certain phase. You can cancel the X component of that phase with a ferrite, and cancel the R component with a loop of wire.

In PI you have exponential responses that are all purely resistive signals. When you tape a penny to the coil you get a particular exponential. Adding the ferrite (which is essentially an instantaneous decay) does not help. And adding a target with a different tau will subtract from the penny but never in a way that nulls the penny.

So even though phase and tau are closely related they behave differently in circuitry. To throw in another twist, the Tarsacci uses a square wave (voltage) drive and an IB coil so that the target responses are exponential but contain both reactive and resistive components. I've not tried it but it seems that ferrite could cancel the reactive component but my thought is that the resistive decay is still unique to the tau of the target.

[green]

@Green: After some thought, I see what may be the problem may be.
Q: Where are you placing the ferrite and wire loop, in order to attempt cancellation of the 1c coin? I think this could be the key. If you are placing them above the sweet-spot of the second RX loop, then I think it's possible you can get a balance. If you're trying to place them close to the loop with the 1c above it, you're not going to succeed. Whereas with a VLF, I think it should be possible to place the ferrite/loop 'somewhere' about either loop to cancel the 1c.
I'll try and set up my F75 for some tests in the next week, to see how nulling a DD coil may relate to your 8-coil.

Hi Skippy
Interested in what you get with the DD coil.

I've been placing the offset target (penny) on Rx coil A. Null target (ferrite and wire loop or other correction target) on Rx coil B. Couldn't adjust null with ferrite and wire loop. Easy to adjust null with like targets. Thought zinc penny and copper penny with would null better than it does since tau's are close. Attaching pennies chart I did awhile back.

First two scope pictures, Tx off. Third picture, Tx on then off.