PI Mono Coil - measurement of Shield capacitance, it's very low, is this correct?

[Polymer]

I have made a new monocoil for the MPP E using a different construction method than before.

Whilst going about this I have taken measurements to see what is happening along the way to resonant frequency etc.

The coil:
19 Windings, bundle, round ended rectangular, 36.5x15cm, mean diameter as a circle 28cm. Rdc 1.8 Ohm.

Coil_WR1, measured directly at coil, coil shield NOT connected to GND: 312uh, 125pF

Coil_WR1, measured directly at coil, coil shield connected to GND: 312uh, 136pF

Coil_WR1, with 2m cable & coil-shield/cable braid connected to GND 312uH, 322pF



The calculated capacitance per meter of cable is (322pF-136pF)/2m=93pF/m.
That's in the same ballpark as the technical cable info. So I believe this to be correct.


The shield capacitance shows as only 11pF.
This seems low, considering that this capacitance often seems to be a problem when reading about it in this forum.

Have no experience with this yet, so no idea if this is plausible?

I would also like to measure the total capacitance of the coil setup including the Mosfet.

Would I have to disconnect the 1k resistor leading to the preamp when doing this?


Tried measuring with the 1k still on board - the SRF value was horrible.
Didn't note it, thought something is very wrong with my measurement. Now I'm asking ...

[waltr]

What material did you use for the coil shield?

If Aluminum or copper then I guess C is too low.
But if Graphic then due to higher R the C measured may be correct.

[green]

I have made a new monocoil for the MPP E using a different construction method than before.

Whilst going about this I have taken measurements to see what is happening along the way to resonant frequency etc.

The coil:
19 Windings, bundle, round ended rectangular, 36.5x15cm, mean diameter as a circle 28cm. Rdc 1.8 Ohm.

Coil_WR1, measured directly at coil, coil shield NOT connected to GND: 312uh, 125pF

Coil_WR1, measured directly at coil, coil shield connected to GND: 312uh, 136pF

Coil_WR1, with 2m cable & coil-shield/cable braid connected to GND 312uH, 322pF



The calculated capacitance per meter of cable is (322pF-136pF)/2m=93pF/m.
That's in the same ballpark as the technical cable info. So I believe this to be correct.


The shield capacitance shows as only 11pF.
This seems low, considering that this capacitance often seems to be a problem when reading about it in this forum.

Have no experience with this yet, so no idea if this is plausible?

I would also like to measure the total capacitance of the coil setup including the Mosfet.

Would I have to disconnect the 1k resistor leading to the preamp when doing this?


Tried measuring with the 1k still on board - the SRF value was horrible.
Didn't note it, thought something is very wrong with my measurement. Now I'm asking ...

How do you measure coil inductance and capacitance? I excite the test coil floating so the shield should be connected to one of the coil leads not ground. Rd and input resistor should be disconnected when measuring SRF with coil connected to MPP. The series diode isolates most of the Mosfet capacitance.

I measured a 20cm spiral coil. Graphite shielding added less than 20pf

[Polymer]

What material did you use for the coil shield?

If Aluminum or copper then I guess C is too low.
But if Graphic then due to higher R the C measured may be correct.

My shielding is silver plated (um) polyester mesh. I think I've posted a photo of it somewhere.

[Polymer]

How do you measure coil inductance and capacitance? I excite the test coil floating so the shield should be connected to one of the coil leads not ground. Rd and input resistor should be disconnected when measuring SRF with coil connected to MPP. The series diode isolates most of the Mosfet capacitance.

I measured a 20cm spiral coil. Graphite shielding added less than 20pf

I use the parallel tuned oscillator circuit and excel to calculate L & C. The SRF is measured with the oscilloscope.
It's well documented in a thread here somewhere. I use a 170pF +-0.5% mica-silver capacitor as parallel reference.

I will disconnect the 1k to the preamp and see what results I get, with and without diode.

[Polymer]

Thanks for all your replies.

Since there is a lot of air space between shield and coil I am now sure the low value is true.

Test with MOSFET etc. still to be done.

[andon80]

good afternoon, friends.
how do I measure coil capacity.
I haven't tried many LCRs.
can you help me with the drawing by explaining it to me ? .
thanks

[bbsailor]

good afternoon, friends.
how do I measure coil capacity.
I haven't tried many LCRs.
can you help me with the drawing by explaining it to me ? .
thanks

Measuring coil shield capacitance is done like this.
1. Measure the coil self resonance without the shield.
2. Measure the coil self resonance with the shield.

The difference is called distributed capacitance. If you look up this term on the web, you will see calculations that look like a graduate course in physics. The easiest ways is to just measure the resonance before and after the shield is added.

Based on my research in writing Making a Fast PI Mono Coil article in the projects section of this forum, only about 20 to 25 percent of the coil to shield capacitance or distributed capacitance acts to reduce the resonance.

I hope this helps simplify this matter.

Joseph J. Rogowski

[andon80]

thanks bbsailor
for the help.I want to measure with LCR meters.
I got xjw01 lcr but I can't measure it I don't know how to do it
capacity is measured with lcr
I bought this LCR with capacity measurement, but I do not know where I am making mistakes
thanks
and for this

[Altra]

thanks bbsailor
for the help.I want to measure with LCR meters.
I got xjw01 lcr but I can't measure it I don't know how to do it
capacity is measured with lcr
I bought this LCR with capacity measurement, but I do not know where I am making mistakes
thanks
and for this

The way I do it is disconnect the coil winding from the shield lead. Connect one lead from the capacitor meter to the shield and the other lead to one side of the coil. In the photo the blue lead is the shield drain and black are the coil. Not sure if this is the proper way? Need to verify by doing a self resonance test.

[bbsailor]

Altra and andon80

The full coil to shield capacitance is measured between the shield and on one coil lead is called distributed capacitance but its full value does not lower the coil self resonance. Based on the dielectric constant of the wire insulation, spacer materiel between the shield and the coil, its dielectric constant and the shield area around the coil, the actual reduction of the self resonance will be less than the full coil to shield capacitance. See more details in my Making a Fast PI Coil article where I disclose my own measurements and techniques to measure.

Joseph J. Rogowski

[andon80]

Altra and andon80

The full coil to shield capacitance is measured between the shield and on one coil lead is called distributed capacitance but its full value does not lower the coil self resonance. Based on the dielectric constant of the wire insulation, spacer materiel between the shield and the coil, its dielectric constant and the shield area around the coil, the actual reduction of the self resonance will be less than the full coil to shield capacitance. See more details in my Making a Fast PI Coil article where I disclose my own measurements and techniques to measure.

Joseph J. Rogowski

Thank you very much.friends
I was looking for this
I got it with the coil shield.
how can I measure without a shield?
you can show this as a drawing.
thanks

[andon80]

The way I do it is disconnect the coil winding from the shield lead. Connect one lead from the capacitor meter to the shield and the other lead to one side of the coil. In the photo the blue lead is the shield drain and black are the coil. Not sure if this is the proper way? Need to verify by doing a self resonance test.

Thankyou Altra