How do you reduce the capacitance of a thin copper tube antenna without reducing its size

Thread Starter

lygion

Joined Dec 30, 2023
1
Hi all :)

I have a straight piece of 3/4 inch (~10mm) diameter copper tubing that is around 40cm long. It’s being used as an antenna as part of a Theremin circuit I’m building; essentially this means that this copper antenna is acting as a capacitor to ground, and has its own (tiny) capacitance of order pico-Farads.

Right now my various antennae of similar sizes all have capacitances greater than 15pF (which I’m measuring using another inductor-capacitor resonance circuit). I need an antenna capacitance of less than 10pF for my circuit to work optimally. However, to make the instrument playable, I also need the antenna’s size to remain relatively fixed, even though making the antenna smaller would be the cleanest way to reduce the capacitance.The dimensions of the antennae are based on those stipulated in the manual I’m following (EMTheremin, Moog, 1996), and other Theremin designs. Even though I’ve made the antennae following the manual’s dimensions etc., mine seemingly have higher values.

Moving the antenna away from all nearby metal conductors, circuits, etc. works to reduce the effective capacitance by a few pF, which is great, but it's not quite enough for what I’m looking for. Increasing the length of the wire that connects the antenna to the circuit also decreases its capacitance by maybe 1 or 2 pF, however, if you make that wired connection too long you actually end up increasing the capacitance and I think you also introduce other undesired effects.

TL; DR: Long story short, I need to know if there is some practical/workable way to make the capacitance of this copper tube smaller (by just a few picofarads!!). Perhaps one could use a dielectric in some way, inside or outside the tube? Or the addition of another object/material near the antenna? There are probably a good few ways to do this but if anyone had any practical suggestions or ideas I would be very very grateful!
 

nsaspook

Joined Aug 27, 2009
13,443
One way that might be too complex here, is to have active driven shield (triaxial connection) over the part of the conductor not used for control, with the end-effector exposed.
https://www.mdpi.com/1424-8220/23/19/8319
https://www.ti.com/lit/an/snoa926a/snoa926a.pdf

In this demo I used a active shield plate to reduce sensor plate capacitance and direct the e-field of a sensor to detect hand movements.

Looks like they have been used with a Theremin.
http://www.thereminworld.com/Forums/T/29524/plates-shielding-spread-spectrum-etc
 

MisterBill2

Joined Jan 23, 2018
19,054
The capacitance of a conductive object in free space is proportional to the surface area . Possibly covering the tube with an insulation wrap might help. Something like the clear film wrap used on some foods for storage. Simple enough to try.
 

nsaspook

Joined Aug 27, 2009
13,443
The capacitance of a conductive object in free space is proportional to the surface area . Possibly covering the tube with an insulation wrap might help. Something like the clear film wrap used on some foods for storage. Simple enough to try.
True but it you have equal charge (expressed as a voltage between ground and the potentials) between two conductive objects the net electric field is zero (if perfectly balanced) between them and the effective capacitance is zero between them.
The result of this is that no charge is stored in the inner dielectric, and the capacitance seen by the center conductor is essentially zero at low frequencies.
1713574658015.png
https://www.belfuse.com/resources/w...axial-cables-for-low-current-measurements.pdf

The same basic physics is used in a source dark space shield to direct the flow of ion and electrons to the center of a plasma arc chamber instead of the much closer edges.
https://plasmaterials.com/dark-space-shields/
1713575479238.png
 
Last edited:

MisterBill2

Joined Jan 23, 2018
19,054
True but it you have equal charge (expressed as a voltage between ground and the potentials) between two conductive objects the net electric field is zero (if perfectly balanced) between them and the effective capacitance is zero between them.
The result of this is that no charge is stored in the inner dielectric, and the capacitance seen by the center conductor is essentially zero at low frequencies.
View attachment 320339
https://www.belfuse.com/resources/w...axial-cables-for-low-current-measurements.pdf

The same basic physics is used in a source dark space shield to direct the flow of ion and electrons to the center of a plasma arc chamber instead of the much closer edges.
https://plasmaterials.com/dark-space-shields/
View attachment 320340
How does this explanation help the TS find a solution to the problem?????
 

nsaspook

Joined Aug 27, 2009
13,443

Ya’akov

Joined Jan 27, 2019
9,248

dovo

Joined Dec 12, 2019
72
Hi all :)

I have a straight piece of 3/4 inch (~10mm) diameter copper tubing that is around 40cm long. It’s being used as an antenna as part of a Theremin circuit I’m building; essentially this means that this copper antenna is acting as a capacitor to ground, and has its own (tiny) capacitance of order pico-Farads.

Right now my various antennae of similar sizes all have capacitances greater than 15pF (which I’m measuring using another inductor-capacitor resonance circuit). I need an antenna capacitance of less than 10pF for my circuit to work optimally. However, to make the instrument playable, I also need the antenna’s size to remain relatively fixed, even though making the antenna smaller would be the cleanest way to reduce the capacitance. The dimensions of the antennae are based on those stipulated in the manual I’m following (EMTheremin, Moog, 1996), and other Theremin designs. Even though I’ve made the antennae following the manual’s dimensions etc., mine seemingly have higher values.

Moving the antenna away from all nearby metal conductors, circuits, etc. works to reduce the effective capacitance by a few pF, which is great, but it's not quite enough for what I’m looking for. Increasing the length of the wire that connects the antenna to the circuit also decreases its capacitance by maybe 1 or 2 pF, however, if you make that wired connection too long you actually end up increasing the capacitance and I think you also introduce other undesired effects.

TL; DR: Long story short, I need to know if there is some practical/workable way to make the capacitance of this copper tube smaller (by just a few picofarads!!). Perhaps one could use a dielectric in some way, inside or outside the tube? Or the addition of another object/material near the antenna? There are probably a good few ways to do this but if anyone had any practical suggestions or ideas I would be very very grateful!
There is an error on diameter as 10 mm is not 3/4 inch, it is 0.4 inches. Looking at old Theremin photos I think 10 mm is correct. Modeling this with EZNEC the antenna capacitance is 6 pF. As a sanity check the ARRL paper linked below says 25 pF/meter which for your 40 cm antenna is 10 pF. Let's try another method to calculate the antenna capacitance. Using the formula C = tp/Zo with Zo = 300 ohms and tp = 1.3 ns we get 4 pF. 300 ohms is the characteristic impedance I assign to wires to rough in signal integrity models. 1.3 ns is the time it takes for a signal to propagate along 40 cm of T-line in free space. In any case you need less capacitance or an oscillator circuit change. Changing the EZNEC modeled antenna from 10 mm to 1 mm diameter reduces the capacitance from 6 pF to 4 pF, or just 2 pF. So, we don't get very far very fast by thinning the antenna.

From this I think your capacitance measurement is off by a bit. Also, adding to the connecting wire length should increase the circuit capacitance rather than decreasing it. If I understand the Theremin circuit correctly (see Theremin link, FIG. 2) decreasing the series inductance (40 mH in the circuit) allows for higher antenna capacitance. Perhaps this is a change you can experiment with.


Handy Formulas
C = tp/Zo, Zo is transmission line characteristic impedance, tp is propagation time

L = Zo x tp

ARRL Short Antennas
https://www.arrl.org/news/view/tuni...explains, electrically,pF per meter of length

Theremin https://www.cs.nmsu.edu/~rth/EMTheremin.pdf
 
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