Theremin vs flying RC model

Posted: 2/1/2008 12:34:52 AM
Thomas Grillo

From: Jackson Mississippi

Joined: 8/13/2006

I had a nagging curiosity that goes back to a question several folks had as to wheather or not a theremin could be affected by someone or thing that was not connected to the ground. I got an RC indoor flying helicopter, and flew it near the pitch rod. It certainly does affect the theremin, evin though it only has maybe a fraction of an ounce of metal in it. The aircraft had to be within a few inches to affect a change in pitch. My only other nagging question is: I'm still inside the field, evin though I'm in the zero beat zone. So, am "I", or other electrical devices serving as the ground, and the model working as if an extension of my hand?
Posted: 2/1/2008 2:13:41 AM
Thierry

From: Colmar, France

Joined: 12/31/2007

You can imagine the system (pitch antenna - your hand) as a capacitor of around 10pF when your hand is really close to the antenna.

In an ideal world, your hand would be connected directly to ground. But it does not seem to be. Between your hand and ground there is your body reactance of around 5kO. Behind that, one can imagine your body (or each other "mass" which is an electrical conductor) as ground, since it may store much more electrons than needed to charge or discharge this small 10pF capacitor. So does the helicopter. Not so much as an ideal grounded wire in the same position, but important enough to have an effect.

So it is clear that the more, the reactance/resistance in series with the hand-antenna-capacitor is weak and the grounding "mass" behind is important, the more pitch will be affected.

But even the 3 to 5kO of body reactance doesn't really matter, since a 10pF capacitor has a reactance of 61kO at 260kHz (Etherwave) and still 20kO at 800kHz (Paia) which is much more important, so that your body may become almost negligible.

Disclaimer (as usual): Please excuse my bad English, I could have expressed all this in a more elegant manner in French or German. Don't hesitate to ask me if something seems not clear.
Posted: 2/1/2008 3:19:54 PM
Thomas Grillo

From: Jackson Mississippi

Joined: 8/13/2006

Cool. Thanks for the info.
Posted: 2/4/2008 8:58:43 PM
teslatheremin

From: Toledo, Ohio United States of America

Joined: 2/22/2006

Thomas,
I got one of those helicopters for Christmas. What brand did you have? Do you crash it into the ground? I have crashed mine many times; enough to warrant a new set of propellers--- and a new way of thinking about the ground. Send me some pics of your 'copter 'playing' your Theremin.
My email address can be viewed.
Good Luck!

teslatheremin
Posted: 2/5/2008 3:17:20 PM
Thomas Grillo

From: Jackson Mississippi

Joined: 8/13/2006

Hi Tesla, to keep from going too off topic, I'll tell you all about it via your e-mail. ;)
Posted: 2/5/2008 3:26:00 PM
Thomas Grillo

From: Jackson Mississippi

Joined: 8/13/2006

Woops, your e-mail is still hidden. Anyway, it's an Airhogs Apache. Has a man, and tail rotor, and is tough to fly. I can fly it round the pitch areal somewhat, and get in close enough to trip the pluss threashold just before the areal starts eating rotor blades. lol. I'll do a vid someday.
Posted: 2/6/2008 9:00:38 PM
teslatheremin

From: Toledo, Ohio United States of America

Joined: 2/22/2006

Thomas,
My email is not 'Hidden', it should not be 'Hidden', it has never been 'Hidden'.
But, thanks for the discription of your copter and how it is flown near the pitch rod of your Theremin.
I doubt that this thread, going off-topic, will matter much to Theremin World members.
Good Luck!

teslatheremin
Posted: 2/9/2008 12:11:10 PM
Jason

From: Sammamish, Washington

Joined: 2/13/2005

Yeah, we don't show anyone's e-maiol to help prevent spammers from sending you junk mail. It's a "feature" unfortunately.
Posted: 2/13/2008 1:22:27 PM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

I think an answer to why pitch is affected comes from several reasons..

Metal close to the antenna will act to stretch the capacitive field - which will make the antenna more sensitive to 'grounded' objects within the field.

ANY object (whether metal or not) in the field (particularly, but not exclusively, between the player and antenna) will affect the capacitance 'to ground' 'seen' by the antenna through 2 mechanisms.. Change of effective plate area and/or Change of Deilectric..

Its like this.. At any moment there is a potential difference between the antenna and any 'grounded' objects - This PD is changing at the HF oscillator frequency (rapidly).

Think of antenna and ground as 2 plates of a capacitor - The deilectric is the air between these 'plates' -

The capacitance is a function of the area of the plates, distance between the plates, AND THE DEILECTRIC CONSTANT ..

Placing anything with a dissimilar deilectric constant to air between the plates will change the capacitance; Placing a conductive object close to either plate will effectively be adding an 'inner series' capacitive coupler, effectively increasing plate area, and therefore changing capacitance.
Posted: 2/13/2008 7:35:53 PM
teslatheremin

From: Toledo, Ohio United States of America

Joined: 2/22/2006

FredM: thanks, for the explanation!
Jason: thanks for the explanation!
Good Luck!

teslatheremin

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