The Theremin Pitch Field, The Mystery Deepens . . .

Just so I don’t highjack podmo’s thread (http://www.thereminworld.com/forum.asp?cmd=p&T=3745&F=1) I will make my "last" comment here.

My statement about the hand capacitance factoid is directed toward future theremin developers. If an enthusiast believes hand capacitance is the proper term for control of heterodyne theremin pitch, this is ok . . . because I can also understand, like most people, that the “Beach Boys” used a theremin in “Good Vibrations”.

Thomas your video is wonderful and what you demonstrate should dazzle the curious in what exactly caused the pitch to change as the tiny remote object approached the antenna.

link ( http://www.youtube.com/watch?v=TU3pgLYdjhQ&feature=email)

You could also dangle a small conductive mass from fishing string and get a similar effect.

But if you want to get really daring use the driest wooden broom handle you can find and stick it into the densest area of the pitch field which is closer to the antenna and you will also get a response. If the driest mass can transfer RF energy away from the antenna, imagine what our conductive fingers will do attached to our large body mass.

Another interesting thing about perfect linearity and “Lev Antenna ( http://www.oldtemecula.com/theremin/ult_antenna/index.htm)” LC tuning happens when you are playing theremin on the grass in an open area, then take off your shoes to play barefoot. This widens the pitch field but perfect linearity remains at any pitch field width! You would just readjust the pitch knob to compress the playing field to your taste

Vibrato movement next to the antenna has the same response at 18” from the antenna.

[i]”A perfectly linear theremin pitch field will eventually be the “expected norm” on even the simplest and most affordable theremin designs. It is not magic but an understanding of the basic physics involved.”[/i]

With all future theremins demonstrating consistent linear note repeatability, children will choose this musical instrument and master it from a very young age . . . then all will play together.

The theremin will be influence by children’s toys and books much loved which is going to bring about a universal renewal in the interest of the discoveries of Lev Sergeyevich Termen.

"Let the sun shine forever, let me be forever!"
.

There is no mystery about that. Each electrical conductive mass is less or more a kind of electrical ground, if classically grounded or not. All depends on the electrostatic charge it may take, that means on the number of moveable electrons (elementary physics on high school level). It is though clear that a 120V 60Hz circuit would not see the small helicopter as a real ground, but a 285kHz field with a strength of only a few uV might be completely short-circuited by it.
*sigh*

If the above described effect would not exist, you would always need to connect to a grounding wire in order to make your cell phone receive calls...

Grounding means physically the ability to absorb or to deliver a quantity of electrons. The lesser the needed quantity, the lesser the required mass.

RS, hijack away, brother...I will just read and learn.

Thierry, at the same distance, would things that can store a whole bucketload of electrons (a balloon, wool sock, rayon shirt) have a greater effect on the pitch field?

As I write, I am shuffling my feet on the carpet to see If I can generate a little lightning between myself and the pitch antenna.

Pod

[i]Thierry, at the same distance, would things that can store a whole bucketload of electrons (a balloon, wool sock, rayon shirt) have a greater effect on the pitch field?[/i]

No, because the electrons aren't sufficiently movable since the basic materials are bad conductors. To get them in move you have to bring first lots of energy (a strong electrostatic field or mechanical action like friction). If the electrons were better movable, they couldn't be stored.

[i]As I write, I am shuffling my feet on the carpet to see If I can generate a little lightning between myself and the pitch antenna.[/i]

These little lightnings tend to kill sensitive parts like transistors in your theremin. Lots of Tvox Tour theremins died because of that.

Are you sure that shuffling your feet on the carpet will not produce something similar to old Italian grated hard cheese? :-)))

This reminds me of a cartoon I saw in the 60's.. Astronoughts unpacking equipment on the moon.. Lead labelled "Earth"... "Where do we connect this?"

Thierry - I think you are absolutely correct on all you have said in this thread, and that there is no real 'mystery' regarding the mechanisms of 'linearity'.. Also, as far as I can see, both from theory and experimentation, Capacitive sensing is by far the most important mechanism - and all other possible mechanisms have comparitively trivial importance.

Saying that there is little 'mystery' is, however, completely different to saying that there is little complexity.. The complexity of field interactions is huge - and I think this does present itself as 'mysterious' even to those who believe they understand the processes fully (I do not count myself as one of these people!).. so it probably makes little difference what 'philosophy' one adopts when designing Theremins.. as whatever one chooses will only, at best, get one started.. The rest is down to experimentation - a lot of which will result in incredulity "Why does this work!?" "Why doesnt this work?!" if my expierience is anything to go by!

"Thierry, at the same distance, would things that can store a whole bucketload of electrons (a balloon, wool sock, rayon shirt) have a greater effect on the pitch field?

As I write, I am shuffling my feet on the carpet to see If I can generate a little lightning between myself and the pitch antenna."

Yes this can make an impressive sound ... ONCE! And that reminds me about the story of the couple who did lots of sound effects in a home studio in the late 1950s. The husband was adept at building "sacrificial circuits" ... things that made an interesting sound effect before they self-destructed. It would be interesting to know more about the things he actually wired up!

About the most impressive (more depressive, really) "sacrifical sound effect" I've ever created is the short "ZZZZT" sound of something shorting out :(

Don

Regarding radiation resistance ...

Impedance (of any sort) = Z = R +/- jX

For RF, the R has both a pure copper resistance part, R0 (Ohmic resistance) and radiation resistance, Rr. The reactive part (X) can be either capacitive (-jX) or inductive (+jX).

There's an efficiency factor, K, that relates the Ohmic and radiation resistance factors:

K = Rr/(Rr + R0)

RF designers (of transmitters, at least) try to make R0 as small as possible because any energy delivered there just gets wasted as heat.

Rr is set by the antenna design and installation and is defined as V/I at the feed point (where the wire connects to the antenna), minus the losses.

This information was obtained from the book "Secrets of RF Circuit Design", by Joseph J. Carr, TAB books 1991. ISBN 0-8306-8710-0

The reactive part is most certainly all capacitive from the antenna outward. Of course this part is frequency dependent, so the it has a slightly different effect for different notes.

The radiation resistance part is not frequency dependent, so its effect would be constant for all notes.

It would seem that the thereminist can alter either the capacitance or radiation resistance part. Dare I say "stop, you're both right"? :)

Don

The foot shuffling part was actually a lame (!) joke, though in the throws of a toxic mix of testosterone and a can of Foster's Lager, I did think about it the night before. Happily reason prevailed...the Etherwave lives yet.

Thanks for the eletronic lessons in the replies. It is interesting to know the whys and wherefores.

Pod

Don is absolutely right: The pure capacitive theory and the common radiation theory are equivalent, since the capacitive factor is the most important component of the much more complex vector.

Each antenna is intended to be a resonant circuit, be it alone, be it in conjunction with other circuit elements. But they are used, adjusted, calculated and optimized in different manners, following the needs:

- a transmitter antenna shall radiate most of the fed energy and acts (in conjunction with its loading circuitry) as an impedance transformer between the transmitter output stage and the free field.

- a receiver antenna is intended to capture most energy (of the desired frequency) and to feed it into the input stage of the receiver. So it acts ideally as a band pass filter and in conjunction with its circuitry as an impedance transformer between the free field and the receivers input amplifier stage.

While the antenna matching to the free field is in both cases above more or less static and there are buffer and amplifier stages in order to decouple the circuitry from backwards effects of the free field, a theremin antenna is part of an oscillators frequency determining circuit. The intention of the theremin designer is now to get a circuitry which shows a well defined reaction on changes in the free field. These changes may have resistive, capacitive and inductive character.

As long as the oscillator is working with a LC tank circuit, the resistive component is not frequency determining and has not to be taken into account. It has still to be verified if there is an influence of it on relaxation or Wien bridge oscillators, but I suppose that these values >10GOhm will have no important impact in reality.

So let us have a look on the imaginary component, either capacitive or inductive: As long as the oscillators frequency goes down when approaching the antenna it can easily be proven that the capacitive component prevails over the inductive component. The antenna sees multiple capacitances besides the static capacitance which is part of the LC tank circuit:
- a static and not varying capacitance Cc vs. the theremins circuitry (by design)
- a static and only varying capacitance when the theremin is moved Ce vs. the environment
- a slightly variable and much smaller capacitance Cb vs. the players body
- a mostly variable and a little more important capacitance Ch vs. the players hand

Cb and Ch are thus in parallel but in series with the capacitance Cp between the players body and ground. The resulting capacitance is in parallel with Ce and Cc.

So the behavior of the oscillator when Ch (inverse proportional to the distance between hand and antenna) changes depends on the amount of capacity change in relationship to the other capacities and the tank circuit.

One may also raise or lower the impact on the oscillator by adding components in series with the antenna, i.e. a loading coil which makes small capacitance changes at higher frequencies more important (compressed field at bigger hand-antenna distances) and big capacitance changes at lower frequencies less important (stretched field at smaller distances).

This is my actual working model. I'll need some time (months?) to set up the equations and there will not be enough space here for them, but I promise that they will explain among others the "stretched field when playing with wet feet on the green" effect as also the behavior of the "Lev's antenna".

My interest in this is in finding a simple metaphor that non-electricians can grasp, but that doesn't make the electricians cringe too much.

So here's one attempt (ignoring coupling effects near zero beat and assuming linearity)...

Consider a jug mostly full of water. This corresponds to the pitch field. The top of the water corresponds to the lowest note, the bottom of the jug to the highest note. Notes are spaced relative to these two points. So a particular note is, say, two thirds of the way down the water. To play that note requires you to reach into the jug, which causes the water level to rise, so the note is always two thirds of the way down, but where that is exactly depends on the mass of your arm - a thick arm (corresponding to an object that can accept a lot of charge) causes the water to rise more, spreading the notes out, and a thin arm alters the water level less.

?