Measuring Antenna Capacitance

Posted: 1/1/2017 3:48:07 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"Have tried to approximate the dependences by analytic functions,  in all the distance range, but without success."  - ILYA

Ya, I've spent more time looking into C analytics than I care to admit.  The solutions they show you in school textbooks have tons of symmetry (concentric spheres, infinitely long coaxial conductors, etc.).  I'm certainly not the best at math, but it seems even those who are the best can't do this stuff without FEA simulation guiding them to the right expressions, even for what seems doable like parallel circular plates.  I think much of the complexity has to do with the charges on a conductor being free to move around.  Papers will come up with a near case and a far case, and often a surprisingly good fit is just adding them.  And then there's the question of mututal vs. intrinsic vs. total seen by the antenna, what are they really solving for?  (Ask a group of physicists to calculate the capacitance of the Earth and you'll get many different answers, all of them adamantly defended by their proponents.)  Simulation, backed up by real measurements such as you and I have done, are the only things I care about at this point.

Posted: 6/17/2017 9:51:26 AM
feliperodrigosek

Joined: 4/27/2017

WOW! awesome results here.

I am doing a similar experiment, and I am really struggling on calculating the static capacitance. I have seen a lot of charts and results, but I don't know how to get the capacitance.

 

Thanks ;)

Posted: 6/17/2017 10:07:17 AM
feliperodrigosek

Joined: 4/27/2017

I am avino trouble calculating the mutual capacitance. 

Please help

 

Thanks

Posted: 10/20/2017 12:01:49 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Human Body Model

ILYA, I'm curious.  I assume you're simply grounding the "hand" in your experiments?  This is what I'm doing in my simulations, but my real data uses my real hand (my body in a chair, on a rug, other hand holding plastic cased but grounded frequency counter).  What about the body resistance?  Using a DMM between my moistened hand and foot I measure somewhere around 1M Ohm (very approximately!).  Shouldn't the test setup take this into account?  And how about the intrinsic C of the body?  Seems like an accurate simulation should be modeling these distributed capacitances and resistances.

I bring this up because I believe the resistance is easily seen in any Theremin setup, though it's quite obvious in an oscillator that (1) doesn't have an EQ coil and (2) that accurately tracks phase (no phase error, like my digital Theremin oscillator).  I say (1) because EQ coils do a sort of offset resonance that normally affects amplitude, and (2) because even small phase error can cause a quite large amplitude change in high Q resonance.  

As my hand approaches the antenna the oscillation amplitude goes way down.  If my body were just C to ground this couldn't happen, clearly my body R is damping the oscillator Q.  I'm wondering what effect this has on the oscillator frequency.

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