Dual coil for mpp

[green]

Hi godigit, The boys came for some fishing. Haven't had a change to reply after a full days fishing for a week.

Thanks for the testing. You are getting a better damped waveform below 10usec than I get. Have you determined if the lead wire has an effect(twisted pair vs your cable)?

Have you compared your SRF and inductance measurements with mine?

[godigit1]

Hi Green,

Awesome hope you guys had allot of fun and some good eats.

Ive been under a work crunch sorry for my delay we have a big storm coming I had to finish up and close the work site for the winter.

Ive started Srf testing the coils but so far just with the live probe loop pick up method 1 and 10x.

I was having trouble coupling to the dual field if you remember the waveforms were damped looking compared to yours.
Ive switched to one of the 30 awg test coils and have excellent coupling now.
Im getting slightly higher #s than you originally got and one thing that is happening now as far as coupling goes is,
On 1x or 10 x probe settings I get no srf frequency change only a change in signal level.
Thinking if the probe is actually isolated from the coil during the test this should be the case so I think I have good coupling by that???

So far on your coil here is what Im getting,
My meter measures inductance of both coils seperatly at 118uh you had calced 115uh
Both coils together Im getting 290uh

Small coil SRF
you had measured 6.67 Mhz, Im getting 6.94 mhz
on the large coil Im getting 5mhz and you had 4.65mhz
both of my #s are about .250mhz higher Im not totally sure why maybe just coupling or one other factor maybe to consider is Humidity.
The coil has dried out a bit here in the high desert. The coil package has warped a little just like furniture does when moved from humid to dry inviroments.
Just a thought.

Combined the two coils Im getting a SRF of 4.386 you had 4.16mhz close to that .250 difference Im getting again.

I would like to hear your thoughts on that as well?

On the twisted pair I damped with my shielded cable and switched to the twisted pair. There was no real speed differance I could see, but the noise level of my test area as compared to the shielded cable was really high.
Im post # 249 at the bottom Is the damping shot with the twisted pair. The shots above are with the belden cable. the shielding is exceptional.

I still have quite a bit of testing to do and thinking another thread might be in order for the srf testing. Ill try to be timely.
Im finally seeing your point about the actual capacitance of the coil being set to the highest SRF readings, its taken me a bit of learning and rethinking things.
Thanks for your patience over the years.

[green]

Thanks for the testing. Some thoughts, maybe correct maybe not. Increasing capacitance or inductance decreases SRF. Connecting a scope probe across the coil to see SRF on the scope increases capacitance(capacitors in parallel), decreases SRF. Think placing a loop on the test coil(scope probe connected to probe ground lead) has little effect. Positioning the test coil near the exciting coil increases inductance, should decrease SRF but SRF increases. Capacitance decreases more than inductance increases(capacitors in series)? Maybe a reason spiral wound coils have a high SRF(each winding adds inductance, capacitance between windings in series, decreases capacitance)? Thinking minimum coupling is best, scope set to 2 or 5mV/division and distance between coils as far as possible still getting good scope signal to read SRF.

Missed the twisted pair picture #249, thanks. I don't see the noise when I try it but you still get a good damped wave at 5us where I can't get below 10us.

[dbanner]

The coupling coefficient (k) between the two coils works out to be 0.22 (inductors in series aiding)

Mutual inductance (M) is 27uH : 290-( 118+118 )/2 transposed from L=L1+L2+2M

I can't say anything about the coupling given the approximate 2:1 ratio of the diameters of the outer and inner coils. But I wonder about making the inner inductance higher than the outer or the opposite.

Green, how did you arrive at 118uH(115uH), did you target around 300uH and just picked a value of something less than 150uH based on the fact that there would be some mutual inductance aiding?

[green]

The coupling coefficient (k) between the two coils works out to be 0.22 (inductors in series aiding)

Mutual inductance (M) is 27uH : 290-( 118+118 )/2 transposed from L=L1+L2+2M

I can't say anything about the coupling given the approximate 2:1 ratio of the diameters of the outer and inner coils. But I wonder about making the inner inductance higher than the outer or the opposite.

Green, how did you arrive at 118uH(115uH), did you target around 300uH and just picked a value of something less than 150uH based on the fact that there would be some mutual inductance aiding?

Made my coils to match godigit's replies #77 and #78 except for wire size. I used AWG28, he used AWG30.

[dbanner]

Made my coils to match godigit's replies #77 and #78 except for wire size. I used AWG28, he used AWG30.

OK. I was wondering if the coupling between the coils could be increased a little, say fron 0.22 to closer to 0.4
Stronger coupling could translate into a more sensitive coil. Perhaps? Something to look at.

One way of doing so would be the change the ratio 2:1, but I don't think this is wise.
Another way would be to increase the inductance of the inner coil relative to the outer coil, while still maintaining total inductance near to 300-350uH. Maybe using 100uH for outer coil and 150uH for inner coil, something along those lines. Increasing the mutual inductance might have overall benefits to the sensitivity to small gold.

[godigit1]

I mentioned this in post # 253

Since then Ive tried changing coil spacing or coil coupling within the two coils by moving them close together. No real effect but I know its a bit different than changing actual coil size. I think changing inductances would do more for adjusting sensitivity to large or small targets. One other consideration is maybe stronger coupling may increas capacitance between the two coils. Just thinking!!
This coil has many possiblilties for experimentation, Its been a eye opener for me so far.
Thought I was done with the dual field a while ago but all the SRF testing has me deep in it.
More on this later as a Litz coil is in the works making two different outer coils.

[godigit1]

Hi Green,
I finally finished Srf testing of your dual field coil and the other three test coils we exchanged.

I was going to start a new thread but since 3/4 of the testing is on a dual field I'm going to put it here.

I used 5 different methods of Srf testing all as defined earlier in this thread.

On my probe loop testing I feel I have isolated the probes, I can add capacitance between the probe ground and tip with no change in SRF, Also can move the probe in and out of range with no change except in signal strength. By that I think the probes are truly isolated.



This is the Capacitance testing of each stage of assembly.



The final Srf # When testing on a Mpp if you notice are different with the inner coil damped.
I went through this multiple times always with the same results.
My explanation I think is that almost half of the capacitance of the coil is isolated once the inner coil is damped.

Does any one else have a possibly reason for this?

I think you are 100 Percent correct when you have stated you thought the highest SRF was the true SRf.
Of the test methods used only 2 are capable of measuring true unloaded SRF in my opinion.
That is the probe loop method and the dip meter the #s are almost exact.
There has been some debate weather these #s or method is correct?
After recent posts from other respected forum members like Eric Foster I think there is no more debate as to the probe loop pick up method.


"
Well, I am happy with 1% accuracy. I did some tests today and found that the scope leads did have a small effect, particularly on the x1 setting. x10 is better and best is the x100 high voltage probe. My initial method was to insert a 47K resistor in series with the coil and drive from a signal generator with the scope connected to the live coil end. The signal generator was then tuned to give a peak waveform on the scope, and then the period was measured with the cursors. A better way is to have a small PI coil driven by the PI Tx and Rx and have it near to the unconnected coil under test. The ringing of the unconnected coil will appear on the output of the Rx amplifier where it can be measured without the effect of scope probe capacitances."

Thank you and everyone once again for helping me learn this has been a blessing for me.
Jim

[green]

Hi Green,
I finally finished Srf testing of your dual field coil and the other three test coils we exchanged.

I was going to start a new thread but since 3/4 of the testing is on a dual field I'm going to put it here.

I used 5 different methods of Srf testing all as defined earlier in this thread.

On my probe loop testing I feel I have isolated the probes, I can add capacitance between the probe ground and tip with no change in SRF, Also can move the probe in and out of range with no change except in signal strength. By that I think the probes are truly isolated.

[ATTACH]48592[/ATTACH]

This is the Capacitance testing of each stage of assembly.

[ATTACH]48591[/ATTACH]

The final Srf # When testing on a Mpp if you notice are different with the inner coil damped.
I went through this multiple times always with the same results.
My explanation I think is that almost half of the capacitance of the coil is isolated once the inner coil is damped.

Does any one else have a possibly reason for this?

I think you are 100 Percent correct when you have stated you thought the highest SRF was the true SRf.
Of the test methods used only 2 are capable of measuring true unloaded SRF in my opinion.
That is the probe loop method and the dip meter the #s are almost exact.
There has been some debate weather these #s or method is correct?
After recent posts from other respected forum members like Eric Foster I think there is no more debate as to the probe loop pick up method.


"
Well, I am happy with 1% accuracy. I did some tests today and found that the scope leads did have a small effect, particularly on the x1 setting. x10 is better and best is the x100 high voltage probe. My initial method was to insert a 47K resistor in series with the coil and drive from a signal generator with the scope connected to the live coil end. The signal generator was then tuned to give a peak waveform on the scope, and then the period was measured with the cursors. A better way is to have a small PI coil driven by the PI Tx and Rx and have it near to the unconnected coil under test. The ringing of the unconnected coil will appear on the output of the Rx amplifier where it can be measured without the effect of scope probe capacitances."

Thank you and everyone once again for helping me learn this has been a blessing for me.
Jim

Thanks for the testing.

I think you are 100 Percent correct when you have stated you thought the highest SRF was the true SRf

I think your not seeing a change when changing spacing between coils is a good indication the readings are correct. I have tried two coils of the same size close and saw maybe a 20%+ increase in SRF so I think using 2 or 5mV/division(probe X1) and adjusting coil distance at maximum distance still getting a good SRF reading is best. Moving closer and not seeing a change is a good check.

[surfdetector]

Denots, respondents are discussing 2 entirely different solutions. Let's start with your questions, assuming your intention was an IB coil.

1. You can easily build a concentric IB coil for a PI detector. Most people use DD instead because concentric has a stronger ground response in nasty ground, and nasty ground is often where PIs are used.

2. Large coil = TX, small = RX and bucking.

3. You would typically wind the TX just like a mono coil, i.e. 300uH and 1-3 ohms or so. The RX has a little more leeway, but 1mH might be a practical max. Then you also have a bucking coil wound on top of it. There is lots of info on these forums for how to do this.

The other solution is a non-IB "dual field coil," where you wind large and small mono coils and wire them in series. The trick is that the inner coil needs its own damping resistor, and the overall connection also needs a damping resistor. This may require trial & error.

If you want "easy" I would suggest the dual field coil.

How is the ground response of the Dual Field compared to that of the DD coil?