antenna design

Posted: 2/15/2007 2:48:25 AM

From: melbourne, oz

Joined: 6/10/2005

just wondering if anyone could share their experiences or suggest any texts discussing how antenna design affects the playability of a theremin.

i'm particularly interested in how the antenna design affects the distance over which the circuit will be sensitive to hand movements. for example, are thin antennas better at detecting movement at a greater distance than thicker ones? what about material density, type of metal, cross-sectional shape, or whether it is hollow or solid?

my current project is a digital theremin that outputs 16bit data rather than sound. i'm using a QProx QT300 capacitance-to-digital sensor. so far, i'm only getting readings when my hand is about 10cm away from my metal plate "antenna" or closer.

the project's in very early stages at the moment, but you can see little bit about it here:

Posted: 2/17/2007 4:30:05 PM

From: sunnyvale california

Joined: 1/28/2007

That seems to depend on the tuning, of the circuit, not antenna size at any practical level.

The frequencies used by these things are LOW. ~250kHz and ~450kHz. Half-wave or 1/4-wave antennas for these freqs are HUGE.

That's why my Etherwave standard has those big inductors inside!

Essentially, those inductors make the bitty antennas on this thing electrically equivalent to the kilometers-long antennas needed otherwise.
Posted: 2/18/2007 2:29:45 AM

From: melbourne, oz

Joined: 6/10/2005

thanks lowfer - my knowledge of antennas and resonant circuits is practically zilch, but that's given me some good ideas to explore...

actually, i've had a bit of a lucky breakthrough since starting this thread. i took a set of 2 rabbit-ear (ie, 'V' shaped) television aerials, hooked my 'theremin' up to them, and discovered that i can increase the field of sensitivity dramatically by altering the angle of the V formed by the 2 aerials. (i'm using both aerials as one single theremin antenna)

if the 2 aerials are parallel, both sticking up vertically, the sensitive distance remains only about 100mm. but if i seperate the into a V shape with the tops about 200mm apart, the sensitive range suddenly jumps to around 500mm - ie, perfect for theremin playing.

i've got absolutely no idea why this happens! the only thing i can think of is that it has something to do with the resistance between the two aerials, which i measured as 33 ohms.

can anyone suggest the reason for this??

anyhoo, whatever the physics behind it all, it's really good news for my theremin. since it outputs numeric data, i can take care of linearisation using software and/or programmable microprocessors.

the theremin data stream ranges at the moment from about 3300 far, to 3900 near (integer), with a noise level of about +/- 2 or 3. only just useable, but i really need to work on this to make things smoother, especially if i use linearisation mathematics...

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