coil tuning

RST Said: In another experiment I made the observation that when a powerful magnet was brought near an operating theremin pitch oscillator's unshielded ferrite cores its operating frequency would "momentarily" be pulled off center. The soft ferrite seemed to magnetize briefly while after 15 seconds the oscillator returned to it’s original frequency as if oscillation degaussed the soft ferrite core.

What I believe affected the pitch oscillator’s soft ferrite core, which shifted its inductance momentarily, was the external movement of a magnetic field  where a stationary magnet had little affect other than the presence of a conductive mass in proximity.

Fred I do not disagree with your theremin antenna theory, I just don’t understand why you cannot acknowledge that RF energy is also present at the antenna however weak or that current flow occurs in the antenna? This is what separates our logic.  Place a AM radio near the theremin.

"Fred I do not disagree with your theremin antenna theory, I just don’t understand why you cannot acknowledge that RF energy is also present at the antenna however weak or that current flow occurs in the antenna? This is what separates our logic. " - RST

Christopher - I agree with everything you say above! - So we have no disagreement!

I thought that you believed that the capacitive theory of theremin operation was a "factoid" - as in, not a fact, but an erroneous analogy.

"RF energy" is something of a misnomer - it all comes down to where one defines "RF" as .. "Radiated energy" is probably less specific - 50Hz "radiates", even 1Hz "radiates" - It is the primary "transfer" mechanism which changes as the frequency changes - and for the frequencies and distances involved with theremins, I believe, both from theoretical evaluations and experimental data, that capacitive coupling is the only significant mechanism of loading on the equalization inductor and/or oscillator.

The currents "passed" between the antenna and ground can be quite high - 10mA P-P is possible from a series inductor connected to an antenna at resonance - this is not what I would call "weak" by my standards (microamps are "weak" - lol) - The EW simulation shows about 8mA P-P at resonance, but the actual value will depend a lot on the actual charactaristics of the inductors.. Probably be more like 6mA P-P.

But 99.999% of this current is most likely electrostatic (capacitive) coupled - If this was not the case, theremins at the same frequency would interact with each other over much greater distances - and thereminists living in the same area would hear each others performances! LOL!

RF theory (the physics aspects) is not my strong point - And there are a lot of details about EM radiation which I have great difficulty visualizing.. But I can visualize capacitive coupling - and have been able to test these 'visualizations' - and, all I can say is that, if another mechanism is at work, it behaves exactly as I would expect capacitive coupling works!

Fred.

"What I believe affected the pitch oscillator’s soft ferrite core, which shifted its inductance momentarily, was the external movement of a magnetic field  where a stationary magnet had little affect other than the presence of a conductive mass in proximity." - RST

This really doesnt make much sense to me - Moving a magnet close to the coil will induce a current waveform proportional to the movement - but this will be extremely low frequency - the only mechanism I can see is if the oscillator circuitry is extremely sensitive to changes in 'dc' current - The usual mechanism wherebye 'dc' current affects a LC oscillator is by changing the operating point on the inductors BH curve - But if the inductor is sensitive to this degree, I would expect it to be extremely sensitive to the applied magnetic field when the magnet was positioned stationary close to it.

So I dont know! ;-)

Fred.

RS Theremin wrote :"Invisiblejelly, I admire your curiosity and exploration. I also have a simple and adjustable method to enhance the Bass response of the EWS, costs less than a $1. I will keep that to myself for now as my .mp3 sample has been posted at TW several times before."

Invisiblejelly says "Well RS theremin I certainly admire YOUR experimentation and contributions to this fascinating and engaging field of theremin research.Also Fred's work is marvelous and I have learned a lot from you Fred from the short time I have been here ( so no need to apologise for tangential topics in threads I've stated I'm just happy someone has replied and I'm learning stuff).This field of research is really interesting and you get to play music to...and the music is directly affected by the research and tinkering...no other field is quite like it.Talking about fields a helical spiral antenna's radiative field both capacitive and RF is gonna be different from a tube of similar dimentions intuitively I feel it is the relationship between these two fields that affects the linearity of the playing field.And I love the way you, RS theremin, give mysterious hints like the one above it's very encouraging(and amusing) for newbie researchers like me it makes me want  to get stuck into the research to find your simple method costing less than a dollar...I'll find it out sooner or later!...with my laser like research brain and endless capacity for tinkering...the Sophia lady goddess of the theremin always smiles on my efforts because I love her singing.

Fred said: "The currents "passed" between the antenna and ground can be quite high - 10mA P-P is possible from a series inductor connected to an antenna at resonance"

With a theremin transistor emitter having a ~1 ma current flow 10 ma p-p does seem high in the circuit, that is why I enjoy real world wiring and measurement.

This is on topic and also relates to another thread when someone wanted to connect a keyboard to a theremin. You can do this by controlling the ground current which in turn controls the pitch frequency. Put a (1K Pot) variable resistor inline with the ground lead of a "ground isolated"  theremin and sweep it back and forth.

That should reveal the logic for setting up each key with a variable resistor for each key tuning. The theremin needs to work from a predesignated fixed frequency. Another method is emitter current control, very similar.

It is too bad most never got to see any of Lev Sergeyevich's transistor designs. It seems we have him isolated to the 1930's but he continued his research right up to the 1980's.

Edit: Invisiblejelly if curiosity drives you to madness then email me from my webpage in Profile, I have no secrets.  Wait, I have one and that is Clara's Voice which was a gift to me.

Fred said: "I thought that you (RST) believed that the capacitive theory of theremin operation was a "factoid" - as in, not a fact, but an erroneous analogy."

For ten years RST webpages said:

"Beyond 4 inches (10cm) the belief hand capacitance alone controls
the RF heterodyne theremin pitch is a factoid."

RST's logic develops from the fact I have had perfect linearity from the beginning of my research and the EWS never has, so how can this be? Something must be different in our logic as I have no need for inline antenna inductors. The problem with modeling is how does someone avoid steering towards the results they hope to find from something they do not understand? Is modeling more a science of black magic?

That photo of a simple theremin skeleton on the previous page blows the sound and playability of the EWS away.

Christopher,

Your information about how inserting a dowel into your antenna changes its charactaristics (even if only slightly) could be an important clue..

Unless the dowel was somehow ferrous, or had some  ferrous content, or was otherwise highly conductive, we are left with two "likely" possibilities - moisture or dry dielectric influence.. Both are effectively likely to influence the capacitive coupling of the most distant section of the spring from the players hand -

The effect of the dowel (as far as I can imagine) will be either to reduce the dielectric constant, or to increase it (moisture causing reduction, otherwise I suspect that wood will have a higher dielectric constant than air, so therefore increase it).

The drawing below shows a possible arrangement which may emulate some of the mechanism (and I am working on the hypothesis that the mechanism is geometry not inductance) by which your antenna operates..

This may be a completely stupid idea, but I float it anyway..

This is a 'plan' view of a multi-part antenna (viewed from the top, all antennas are connected to the oscillator)

If we ignore the C antennas first, we have the frontal (main) antenna with diameter "A", and we have two "B" antennas with diameter A/2. The effective capacitive 'plate' area (facing the vertical plane) of A is the same as the combined 'plate' areas of both 'B' antennas.

The distance between the horizontal planes on which the A and B antennas are placed correspond (from an analysis perspective) to the diameter of your spring - the major difference between your spring and this extreme simplification is that the rear antennas are offset on the horizontal plane and therefore any similarity in operation will only apply when the hand is directly (vertically) aligned with the A antenna.. For this reason the B antennas need to be as close on the vertical planes as possible without being obstructed by the A antenna.. Moving these antennas further apart will decrease the directional nature, but move away from equivalence to the spiral geometry of the spring.

 Adding more antennas (C) and perhaps mathematically calculating optimum geometries, one may be able to create a linear antenna array suitable for operation at frequencies different to 900kHz - IF the geometric hypothesis is correct.

The "geometric hypothesis" may be complete BS. Sorry, but it kind of bends my brain at the moment - The idea is that, as the hand approaches the A antenna, there will be (assuming only A and B antenna) the capacitive coupling from hand to A, and lesser coupling from hand to B..

If "vertical" distance between A and B were 5cm, and the hand was 35cm from the A antenna, the total coupling would be the capacitance of the hand to the A antennas diameter for a distance of 35cm, + the coupling of the hand to the A antennas diameter (B+B=A) for a distance of 40cm, This ratio will change as the hand is moved on the vertical axis, and I think possibly implement a linearizing function which could possibly be improved with careful (computed) antenna placements.

The spiral, however, is the most elegant way of achieving the above - as it has no directional component - I would like to experiment with the antenna "array" idea just to verify the hypothesis - If this hypothesis turns out to be valid, then it should be a lot easier to work out how the spring (spiral) operates, and compute spiral dimensions for optimum linearity for all frequency / capacitance requirements.. It may be that an oval or other shaped spiral antenna would be optimum.

Fred.

I just want to re-state that the above is an EXTREMELY CRUDE "replication" of the geometry of the spiral antenna - only the spiral antenna's front face and back 'face' are 'replicated' - and even these are extremely crude replications not taking into account the curvature of the faces.

"Beyond 4 inches (10cm) the belief hand capacitance alone controls
the RF heterodyne theremin pitch is a factoid." RST

Christopher,

"Alone" is an ambiguous word - The influence of anything other than capacitance over the full range of 4' /1M will less than 1Hz.. So I dont call the belief a "factoid". But lets leave it at that! I may be wrong! ;-)

Fred.

Fred that was a wonderful explanation of my spiral antenna application. I can honestly say that I do not know exactly why it works but have posted my theory.

Another interesting characteristic is that it works or it does not.

The phenomenon is like flipping a switch, there is no..

"it seems like the linearity is getting better".

Why does it need to be understood? My goal is to allow other theremin builders out there (all three of you) to be aware that perfect linearity of the pitch field exists using a simple principle and it is a frequency dependent behavior. The spring coil I use is well documented as well as the 900 kHz pitch oscillator frequency.

What advantage is their in having a pitch oscillator frequency down below 300 kHz, is it about the big coil attraction I call Tesla envy? My very first theremin build did throw a 1/4" spark, a real finger zapper! My designs are completely original as I never looked at what others were building when I began.

My transistor theremin design transmits a theremin tone to an AM radio about 2 meters away while my tube/valve design transmits a tone up to 10 meters.

I saw this on the website eHow:

"The theremin picks up magnetic fields of resistance from your hands as you move them near the antennas and converts the signal into an audible pitch."

Now that's a bazaar explanation!

"The theremin picks up magnetic fields of resistance from your hands as you move them near the antennas and converts the signal into an audible pitch."

A veil has been lifted from my eyes! :)

RS Theremin said"Why does it need to be understood? My goal is to allow other theremin builders out there (all three of you) to be aware that perfect linearity of the pitch field exists using a simple principle and it is a frequency dependent behavior. The spring coil I use is well documented as well as the 900 kHz pitch oscillator frequency."

Invisiblejelly says;"I wonder if you had a tube of similar dimensions as the spring antenna but drilled holes in it to change the capacitance.Stretching a spring would change the capacitance as well as the small inductance .If you had a spring of 0.5 inch dia with 200 turns and stretched it 20 inches to 24 inches the inductance changes from about 12 uH to 10uH..and is even smaller for a smaller stretch.But a smaller stretch would probably change the capacitance more than it would the inductance.Perhaps start by drilling holes in a helical spiral in the tube and if this did nothing much those holes could be used to thread some plastic insulated hookup wire through to be the coil..er this setup might be easier than having springs under tension with the mechanical problems of adjustment etc...the capacitance would be larger for this setup so the coil part would not have to have as many turns...I dunno it would be a funny looking antenna...I do like the elegance of the spring though...