Odd question: Pitch antenna design ...

"They" hide probably near Roswell, fly around in UFOs and kidnap people in the night to expose them to electromagnetic fields with a frequency around 800kHz. 

Just answerin'

"..expose them to electromagnetic fields.." - Thierry

Man, you arent keeping up to date, Thierry - "electromagnetic fields" is so last decade! - We all know its "dark energy at 800kHz" which powers their machines and to which they expose their victims!

Have a look at the works of the great Dr Thomas Bearden (and ignore the fact that his doctorate was bought on-line.. certainly his Phd is phoney - he was forced to admit this, but still uses it - dont know about his Msc) and you will see how the greatest minds on this planet are working together to develop free energy devices to solve our energy crisis, and scalar weapons to defend us from the reptile invasion being planned by the (thinly) disguised reptiles running our planet..

France is at the forefront of these efforts to save us from the scaleys, having a research centre which was visited by an ex UN secretary general, and the great disciple of Dr Bearden, J.L.Naudin running the show..

Time is running out - LOL !  ;-)

All rather sad,  because these (IMO) fraudsters give research into alternative energy sources a bad name, and when a genuine discovery is made (as I believe has happened) they concoct theories which are obvious BS and which put off any serious scientist from even looking at the discovery..

ooops - going off topic again .. Sorry Amey!  ;-)

Fred.

Technical mumbo-jumbo warning:

NERD RATING 33.3^. (Max value = 42) . 0 to 5 = likely to be comprehensible to most people. 5 to 10 = likely to be comprehensible to most educated people. 10 to 20 = likely to be comprehensible to most educated technical people. 20 to 35 = likely to be comprehensible to most educated technical people, but of doubtful interest to those who are not nerds. 35 to 42 = Truly of interest only to nerds - may be in the domain of metaphysics!

"Oscillator stability?   (Edit: IMHO the simplest answer would be the influence from the closes grounded object)" - Christopher

Chris,

First, in what follows, I will be using my capacitance hypothesis - this is not intended to provoke extension of our "argument" it is simply because I see no other explanation or usable analogy other than this - please feel free to substitute any mechanism which suits your taste!

What is a "Null point" ? When talking in general electronics, IMO it refers to a point of balance - For example, if measuring impedence with a bridge, it is the point at which the impedences in the bridge are balanced.

To me, when talking about theremins, it is the point where the two oscillator frequencies are "balanced" - as in, they are at the same frequency.

The VFO's frequency is determined primarily by the capacitance/s and inductance/s in its tank and (if present) its antenna resonant circuit/s, or by its RC timing components in the case of an RC oscillator. In all these, there is one C which is has a variable component, its value determined by the proximity and area of all capacitive elements which can form part of the resonant or timing circuit - as in, all grounded elements in the "antennas" "field".

In most theremins, there is also a reference oscillator, and for sake of simplifying this posting it will be assumed that the frequency of this oscillator is adjustable to set the null point.

Unreality:

If we imagine the impossible - absolute stability of both the VFO and reference oscillators, absolutely constant background capacitance (as in, nothing in the universe is moving!)  and that the VFO is tuned to some frequency (say 500kHz) with the variable component of its frequency determining capacitance set to say 1pF, then the following can be deduced:

Regardless of what grounded objects are in the vicinity of the antenna, the Ref Osc can be tuned to set a null point (point at which its frequency is the same as the VFO frequency) at ANY (unobscured)  distance from the antenna that one introduces another grounding capacitive object.

Depending on the distance and charactaristics of the introduced "grounding object" movement of this object reletive to the antenna will cause a proportional change in the VFO frequency and therefore a difference frequency to the Ref osc frequency can be derived - For great distances and/or small grounding objects, this difference frequency will be extremely small (could be one cycle per millenia or longer if one was using silly distances or areas !)

(it should also probably be mentioned that, at extremely long distances which get close to or greater than the wavelength of the VFO frequency, other effects may well have an influence - these MAY limit the null point placement to less than 1km from the antenna, but would certainly have no effect under 100m from the antenna - I could look into the physics and check this, but I cant be bothered!)

My reply "Oscillator Stability" was in reply to the question:

"what determines the maximum distance this edge of the Null point can be moved away from the pitch antenna?"

It was not a helpful reply, but I believe it was probably the only accurate reply possible to simply answer the question. What follows is hopefuly more helpful.

Back to reality:

The practical factors influencing the distance one can set the null point and obtain usable frequency change (as in, frequency change we can hear) comes down to: (1) the sensitivity of the VFO to changes in the capacitance of its antenna / antenna circuit, (2) the actual capacitive change "seen" at the antenna.

With LC oscillators (which do not have a secondary antenna tuning circuit), sensitivity is a function of the tank inductance and capacitances across this, and is well detailed by Fred Nachbaur here : On Theremin Sensitivity The capacitive sensitivity of the antenna is a function of its length and diameter - increase in either (or both) will increase the sensitivity.

The addition of a tuned antenna circuit (series inductor and correctly sized antenna) in combination with a correctly selected oscillator tank inductance and capacitance provides the best sensitivity and linearity possible for conventional theremins (Chris - I am only talking here about what I know and what has been verified - I am not excluding the possibility that you have a better method!) as the tuned antenna effectively multiplies the sensitivity, allowing a less sensitive oscillator to become more sensitive and have a more linear response. For full details see Thierry's astoundingly brilliant exposition of this matter here: http://thereminworld.com/Forums/T/28530/antenna-tuning?Page=3

In all the above, the positioning of the null point is determined by the tuning of the reference oscillator, and the practical limiting factor is down to the distance at which changes in the VFO frequency over some usable distance (say 20cm) are too small to be heard.

In the case where there is a series antenna inductor, the possible null position is further constrained if one wishes to optimise the linearity - this constraint is set by the antenna capacitance and inductance tuning, which can theoretically be tuned for any reasonable distance, but beyond about 70cm it becomes extremely difficult to get adequate change in difference frequency without increasing the tank sensitivity and therebye decreasing the linearity.

Fred.

"Technical mumbo-jumbo warning: What follows is unlikely to be of any interest to anyone except extreme nerds.  ~Fred."

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I red it and i didn't understand anything... So this statement is true. i'll stick at playing the theremins you cool Nerdy guys make!

"I red it and i didn't understand anything... So this statement is true. i'll stick at playing the theremins you cool Nerdy guys make!" - Amey

LOL! I did warn you! ;-)

Even for me, that posting was, at times, at my extreme of nerdism!

From now on, I will place a nerd factor (scale 1 to 42) on such postings - that posting was a 33.33333 (recurring) !

Fred.