About the aerial working

Hello guys!
We are French students in sciences and we are working on a project on the theremin.

We would like to build our own theremin based on the Wein-Bridge oscillator (http://upload.wikimedia.org/wikipedia/commons/6/63/Wien_bridge_oscillator.png) (with an aerial placed between C1 and R1) but we have difficulty finding informations on the effect of the aerial over the value of the capacitance.

Can you please give us some informations about the working of the aerial and the physical principles which are brought in play ?

The "antenna" is one 'plate' of a capacitor, the other (movable) 'plate' is the player, who is coupled to 'ground' via a large (100pF or so) capacitor.


Max Baars Article (http://www.thereminworld.com/silicon_chip_theremin_modifications.html) gives some useful data on antenna length / thickness for operating at 455kHz.. From this you can calculate a rough capacitance for an 'open' antenna based on a 455kHz build as with the EPE Theremin, and get values for other frequencies.

In reality, particularly if working with oscillators which are not the standard LC or RC types, you are not likely to get antenna capacitance 'right' by calculation..

Antennas capacitance (and other capacitances affecting this) are unlikely to ever be below about 12pF, and a thick (>15mm diam)/ long (>50cm) antenna can have a base capacitance as high as 25pF.. The player can add as much as 10pF if hand / body are extremely close to the antenna, but this drops away quickly, and one usually has a capacitance change of no more than a couple of pF over the playing zone. Down at the bass (furthest from the antenna) end, the change required is only a few femto Farads.

Hope this helps,

Fred.

ps.. one 'problem' with your proposed oscillator is that there are two frequency determining capacitances - You can only connect to, and vary the capacitance of C2 (connect antenna to junction of C1/C2) which places the antenna capacitance across C2. This scheme can work, but is unbalanced and far from optimum.

Thank you very much for your answer.
But i think you misunderstood the schema, the antenna is placed right after C1, other values are fixed. We want to study the capacitance modifications caused by the aerial by looking at the output signal and measuring frequency variations, so we chose the easiest oscillator.

Please, could you be a little more precise :
in what extent can we considere the [antenna + capacitor] as 2 capacitors connected in parallel (in order to add the capacitances?) cause we are not realy familiar with ground capacitors in the middle of a circuit.

Thx again.

If one connects an 'antenna' to the junction at C1+R1, you can draw this antenna as a capacitor between this junction and ground.

The value of this 'antenna capacitance' will be something in the order of 12pF +/- 5pF (see last posting for link to chart).

So its quite simple - its just another capacitor in the circuit - the capacitance of this will vary as a function of proximity of the antenna to any grounded objects - you can simulate it easily.

As for the actual form of this capacitance..

1.) You do not need to worry about it - a single capacitance between C1+R1 and ground is an adequate equivalent circuit.

2.) If you want better understanding of the 'reality' then you need to think about the human body as an electrical conductor, this conductor being (assuming galvanic isolation) equivalent to a 'floating' plate of a capacitor.. There will be a capacitive coupling from the body to grounded objects (usually between 50pF and 200pF) and any other sources of electrostatic coupling (power cables etc)..

3.) The body will also couple capacitively to the antenna. This coupling is determined by 'plate' area, and distance (body and hand || antenna area, and distance of each component thereof) This coupling only usually constitutes a few pF.

4.) at its most simplistic - You have the body capacitance to ground -see 2 above- in series with body to antenna capacitance - see 3 above- and the above capacitance will be in parrallel with any capacitance directly from the antenna to grounded objects.

So:

Antenna capacitance (to ground) = [direct antenna to ground capacitance]+ 1/((1/[Body capacitance to ground])+(1/[body capacitance to antenna]))

You say [i]"We want to study the capacitance modifications caused by the aerial by looking at the output signal and measuring frequency variations, so we chose the easiest oscillator."[/i]

No, actually - You didnt! the 'easiest' are probably here http://asmir.theremin.ru/tsensors_sch.htm - or you could have used a TS555C timer with R (say 330k) connected from Q to Th and TR inputs, and antenna for C, also connected to Th and tr inputs - or you could just about have picked any other oscillator and it would have been 'easier' for this application!

TS555C Spreadsheet (http://www.element-14.com/community/docs/DOC-16920/l/555-oscillatorsxls) - the "Feedback 555 Osc" tab has a schematic.. Make C1 = 10pF, R1 = 330k, Control voltage = 0 (will force this voltage to its unconnected potential) - the antenna is connected to th/tr pins (junction of C1 and R1) .. You can play with the values.. and can change C1 (reduce for greater antenna sensitivity - remember, the antenna capascitance is added to C1, so if it is 12pF, you do not even need C1).. I think this is possibly the easiest circuit! - But do decouple the TS555C with 100n across its power rails, and keep the frequency below 1MHz.



Fred.