Parallel plate capacitance to co-planar capacitance

Thread Starter

richeekdey

Joined May 23, 2018
2
Hi all,

This is for my master thesis.

So in my previous experiment, I made a parallel plate capacitor by putting 2 copper plates parallel to each other (the third figure in the image). I then hooked up the AD5933 evaluation board (which is an impedance analyzer) and then I put in things in between and observed the dispersion in biological materials (fruits and veggies).


My task now is to "open" up the plates to they're co-planar. I have done that and placed an active shield at the back of the plates to direct my electric fields. However, now I see no observable change on the impedance analyzer when I bring myself or fruits/veggies close to the fields. I do "feel" the field since I sense a bit of warmth closer to the plates. I observed the oscilloscope which is hooked up to the end of the receive plate and I don't see much of an observable difference, rather I see a very small change.

PS...even in my parallel plate configuration, the change in the oscilloscope observed is very minimal.

My question is NOT about why I am NOT getting any change. I know the reason, I would just like to validate it before I take it up with my superiors.

I think the reason that I am NOT observing an change of impedance is because a) the impedance analyzer requires a calibration for the open-plate setup ie: the capacitance across the open plates is obviously NOT the same as the capacitance across the parallel plates. And change of capacitance = change of impedance = new calibration range needed. And b) also, co-planar capacitance is complex and has to be simulated and only then will I arrive at a value.

So would it be right to think that I first need to analytically look at the open-plates and then based on some simulations, apply the formula/value to calibration for the impedance analyzer.

Another fact is that the impedance analyzer cannot measure below 1kOhm. This value can actually be lesser; but in my case I'm using the evaluation board so its limited to 1kOhm - 1MOhm. But this of course comes in WHEN I know the capacitance of these co-planar plates.
 

OBW0549

Joined Mar 2, 2015
3,011
So would it be right to think that I first need to analytically look at the open-plates and then based on some simulations, apply the formula/value to calibration for the impedance analyzer.
I don't know what would be appropriate for your impedance analyzer; but Googling on the phrase "capacitance of co-planar plates" resulted in a bunch of information that looks relevant, including this.
 

Thread Starter

richeekdey

Joined May 23, 2018
2
I've looked into that already. I've also read a few books and articles about co-planar capacitance. So it is indeed my conclusion that a simulation is necessary. A straight-forward formula is not the best idea. I know the formula mentioned in the link, I saw it last week myself; but I've been advised by my superior to simulate it because that way we have a better idea of the field lines. And the formula is for a "strip" co-planar. Mine is 2 plates. They're to plates of 9cm*9cm each.

They were placed parallel and now I've opened them.
 
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