"Really you think that a 120 mW oscillator with a perfectly tuned loaded-antenna, emits the same as a 2 mW oscillator with a non tuned square plate? " livio
You cannot "tune" the "antennas", regardless of their form, plate or rod or whatever! - I realise that this statement is an error - not an error when it comes to theremins with 1/4 wavelengths of between say 25 meters and 400 meters, but wrong as a general statement.
The first fundamental misconception comes from the idea of "antennas" - These are NOT "Antennas" in the radio sense of the word - they are a plate of a capacitive sensor. I believe (may be wrong, cannot be bothered to check the patent) that Lev Theremin specified what we call "antennas" as capacitive plates, and I am fairly sure his patent references electrical and/or capacitive fields, not "radio" or "EM" fields.
Yes - there are some radiated EM fields - if this was not the case then AM radios would not be able to "pick up" signals from theremins.. But these fields are TINY, as one can determine by the fact that the reciever must be really close to the theremin to pick up signals - Also, I suspect that most of the EM radiation is not being radiated from the antenna but from the inductors - but as I said, radio isnt my forte.
"Tuning" antennas is (as I understand it) a process of matching the antenna dimentions to the wavelength of the signal one wants to radiate or recieve.. Something which is NOT done on theremins (for obvious reasons).
With theremins the only tuning which occurs is to match the capacitance and inductance such that one gets the required change in frequency as the capacitance "seen" by the capacitance sensor (plate) (which is wrongly called an antenna) changes when the player moves their hand, and that this translates to a usable frequency shift on the oscillator - The only other tuning is completely unrelated to the antenna, and this is the setting of the local (reference) oscillator.
The above can be done with either parallel configurations, where the plate acts as a capacitor across the tank inductor (often in parallel with a small fixed capacitance), or with series LC (as Dewster is doing) or with the standard scheme invented by Lev Theremin, where there is a parallel tank having a small inductor and large capacitance, connected to a series large inductor connected to the antenna capacitance..
But IN ALL cases, the tuning is a combination of inductor/s capacitor/s and antenna dimentions - One can change any of these, reduce or increase antenna dimentions (resulting in a change of capacitance presented by the antenna, and a change in sensitivity to players movements) and make proportional changes to inductors and/or parallel / series capacitances and "tune" for the operation one wants...
Tuning is about tuning the RESONANT CIRCUIT of which the capacitive sensing "plate" is a part, it is NOT about tuning this "plate" or "antenna", or tuning to "match" this plate in any way other than to produce the wanted oscillation frequency for the capacitance "seen" by this plate.
When it comes to linearization, there are good reasons why the rod "antenna" is superior to a flat plate "antenna" - The plate tends to increase capacitance more rapidly as a hand approaches it, giving far worse linearity at the treble end, the rod tends to spread the capacitive sensing to the arm, giving less dramatic increase in capacitance as the hand approaches the antenna..
If one is doing linearization through computation (either analogue or digital) then the above may be of little concern, but for classic analogue heterodyning instruments its of major importance.
The simple truth is that Lev got it right! Everything he did, rod antenna, loop antenna, heterodyning, hand away from loop = louder, were all the optimum configurations - His oscillator design was IMO the best for the application, and it is only now all these decades later that we are perhaps getting close to bettering his instrument.. (The only thing Lev may have got wrong was, IMO, that the theremin whilst a brilliant bit of science, is possibly one of the stupidest musical instrument ever concieved from a perspective of user interface and playability!)
But IMO we will only do this if we understand the science Lev was saturated in - Oh, we may now be able to linearize as well as he did without using double resonators - but if we get mislead by flawed terminology (like the word "antenna") and start re-engineering the theremin with flawed ideas about electromagnetic radiation, we will go nowhere.
And thats it - my last word on this subject.. Really! ;-)
Fred.
1) So large coils are not tunable and prevents easy interchanging antennas with very different capacitance.
Changing antennas isnt a common requirement - thereminists are quite happy with one antenna AFAIK ;-)
It is (IMO) true that the lack of ability to tune the series (equalizing) network limits precision - but one can tune the tank for optimum linearity and then adjust the local oscillator - On some of my prototypes I did make the series inductance electrically tunable (rather than the simpler common method of adjusting the local oscillator) - but realised that the added cost and complexity wasnt worth the effort.
2) A coil of such great value type does not exist "for oscillators" but only "RF-blocking" and the impedance will change very much with temperature
You dont understand the function of the antenna inductance at all! You dont understand that with care, all the problems you speak of above dont apply! Go look at Lev Theremins patent, look at the postings here about how the series LC works, because you really dont have a clue! - "RF-Blocking" LOL ;-) Look at the large (30mH for example) inductance, calculate the series resonant frequency with antenna capacitance, and get some understanding from the many postings here which explain whats going on before you start comparing the operation to "RF Blocking" !!!
Apart from which, the inductors DO EXIST! You can buy suitable Hammond or Bourns inductors from many suppliers, and you can wind an air coil.
These inductors are fitted in many theremins - If you are looking for a single 30mH inductor with suitable low capacitance / high SRF, you may not find one - but you can certainly get suitable 10mH inductors you can wire in series - these are not "blocking" inductors - they are pi wound and segmented specifically for HF operation.
3) A large coil add much parasitic capacitance
if you use a coil designed for a SMPS running at 40kHz, you may well get "much parasitic capacitance" - but when you use coils with high SRF and low capacitance, you dont get that problem - And if you use well constructed air coils you certainly dont get that problem!
4) A large coil suffer from dimensional changes that cause mechanical stresses and occasional noises
Really? First time ive ever heard of that! - I would imagine that after 80+ years all we would hear from RCA's with their massive ancient coils would be these "noises" if that was the case! - then theres the Moog 91 and the Ether-vox and tube theremins and Wavefront modified EW's all of which have large air coils and ive never heard of "mechanical stress" or "occasional noises" problems, so please let us know where you came across these problems!
"Our CapSensors can work very near one to each other instead EW can not (not tested personally but many people says this) for me this is due to the tuning inductor. If the tuned inductor is not the reason, please explain you to me another reason for this."
I dont know. It may be that the series inductors radiate magnetic fields, or it may be a multitude of things.. The test would be to have a grounded sheild between the theremins - if they still interact, then its magnetic or electromagnetic coupling.. then move one theremins orientation, if this changes the coupling, its likely to be magnetic (radiated from the inductors) but if its EM radiated from the antenna, then coupling wont be affected by orientation of the coils with respect to each other..
It may also have something to do with the way data is extracted from your theremin - It may be that the integration and low resolution of your theremin simply cannot resolve the interactions which are occurring in the way that an analogue heterodyning theremin which outputs all nuances of the interactions (as audio) can.
ADDED ->
"But this requires a complete redesign, if you simply remove the four 10mH coils from a EW, its oscillator become so deaf, that it will be impossible to play it."
Been over this before! Yes - You would need a complete redesign! Thats because the EW oscillator was CORRECTLY designed for use with an equalizing inductor! .. It works like this - there is a small tank (oscillator) inductor in parallel with a LARGE tank capacitor - completely unsuitable and "deaf" if connected directly to the antenna, because the large capacitance utterly swamps the antenna capacitance.. This oscillator is not designed to operate with a varying capacitance - it is designed to operate with a varying INDUCTANCE across its small tank inductance!
Thats what the antenna inductance in combination with the antenna capacitance does! It is effectively series connected across the tank, and operates in its (nonlinear) inductive region - Effectively, changes in antenna capacitance are converted to a "virtual" change in the inductance across the small tank inductance, and it is this change in INDUCTANCE which adjusts the oscillator frequency.
A redesign would involve increacing the tank inductance to probably about 1mH, and decreacing the tank capacitance probably to between 100 and 200pF, one could then connect directly to the antenna without the series inductor, and the oscillator frequency would be directly controlled by the antenna capacitance, and because the tank capacitance was low, antenna sensitivity would be high ..
But doing that you would lose the increased amplitude on the antenna (go from perhaps 180V P-P down to perhaps 20V P-P) and lose the linearizing effect provided by the non-linear transfer function of the series LC combined with the parallel tank.
The EW is designed in much the same way as Lev Theremins design, take away the series inductor and one lowers sensitivity hugely - but one doesnt do that unless one doesnt understand how they work..
Just as adding a series inductor to a design like the Silicon Chip / Jaycar theremins (which have large tank inductor and small tank capacitor, and are "designed" for direct antenna connection) or your theremino design, is completely pointless and probably detrimental to their operation.
This whole issue is one where hobbyists are exposed! And, alas, some of these "designs" find their way even ito somewhat reputable magazines - examples of such bodges are found all over the place - the "strange apperatus" theremin, and the EPE 2008 theremin (a Jaycar/SC theremin with a series inductor tacked on) are examples of such folly.
Its EASY to make this mistake - I never grasped the beautifull and brilliant idea of Lev's for a long time - and was fuc*ing about with tacking series inductors onto anything that oscillated, and wondering why I was getting strange results ;-).. But when I saw it, when I looked at the LC curves and it clicked that the series resonant section was behaving like a capacitance controlled variable inductor, and I saw that the non-linearity of this transfer function compensated for the inverse-square antenna capacitance - distance relationship to produce the required exponential relationship - well, I think that was the moment I recognised Lev's genius, and probably was the moment I saw how it all fitted together - I understood!
Others get understanding through different means - If I had comprehended the mathematics, I may have understood through that mechanism.. But the "virtual inductance" visualisation is the one that suits my "style" best - The frustration is that its real difficult to convey such a simple idea to others - lots of discussions have gone on the lines that series antenna inductance is "pointless" is a "waste of wire" or, as you have presented "a RF Blocking" component -
Think about it! Why would designers like Lev and Bob Moog add an expensive component just for the fun of it? Both were trying to minimise their costs, and if these inductors werent needed they would never have been fitted! - Less competent modern designers fit series antenna inductors to completely unsuitable oscillators, and this misleads people - but go back to competent designs, and the reason for this topology becomes clear!
Ok, the above was my last word! ;-) .. actually, the last quite a few posts shouldnt really be on this thread IMO, a new thread should be started and if possible some of these posts moved there - this is becoming a real hijack of Dewsters "digital theremin" topic!
