# Copper plate as antenna?

Posted: 4/19/2007 12:10:14 PM

From: UK

Joined: 1/6/2007

Could someone give me a measure of how much difference (in Farads) a hand will have with a plate versus antenna. I'm trying to prototype a theremin and am having trouble with the input. Are there any other methods of doing it apart from the plate and the aerial?

Also two very simple qestions, where in the circuit does the aerial go, and does the hand decrease or increase the capacitance as it is brought closer? As you may guess, I'm still not really clued up about this whole thing.
Posted: 4/19/2007 12:10:16 PM

From: UK

Joined: 1/6/2007

Could someone give me a measure of how much difference (in Farads) a hand will have with a plate versus antenna. I'm trying to prototype a theremin and am having trouble with the input. Are there any other methods of doing it apart from the plate and the aerial?

Also two very simple qestions, where in the circuit does the aerial go, and does the hand decrease or increase the capacitance as it is brought closer? As you may guess, I'm still not really clued up about this whole thing.
Posted: 4/19/2007 1:10:28 PM

From: Jax, FL

Joined: 2/14/2005

If I am thinking correctly (and that's a pretty bi "if" sometimes...) The capacitance increases as your hand gets closer to the antenna.

Also, I think it is in the 10 pictofarad range.
Posted: 4/19/2007 1:18:56 PM

From: Kansas City, Mo.

Joined: 8/23/2005

The capacitance increases as you move your hand closer to the antenna.

The antenna is usually connected to the variable oscillator's tank circuit. The percentage change in the resonant frequency produced by the antenna arrangement is small -- thus the need for relatively high frequencies.

For example, if the reference oscillator is tuned to 500khz, to vary it by 3khz (about all that is needed for musical purposes) is a frequency change of around .5 percent. Since a tank circuit is a combination of a coil and capacitor, the antenna simply imparts additional capacitance into the circuit.
Posted: 4/19/2007 2:21:33 PM

From: France

Joined: 4/8/2007

To answer the question about difference between aerials and plates, the capacitance of a capacitor is a function of the area of the intersection of the two sides... Let me explain more..

An aerial can be compared as a line, and the hand as a plate. The mathematical intersection between a line and a plate is still a line (yet the smallest part of the two sides), that's the convenient way to use a metal plate, the useful area is wider than a line..

the general formula is C=(E*S)/e, where E is the dielectric's permittivity (8.8E-12 F/m for air and vacuum), S the intersection area and e the distance between the hand and the plate/aerial
(that's why capacitance decrease when your hand gets closer..)
Posted: 4/19/2007 2:21:43 PM

From: UK

Joined: 1/6/2007

Today I tried a copper plate, but the frequency didn't change at all, unless I actually pressed down on it, when the harder I pressed, the lower the frequency went. It went down by 10Hz! It seems however that my oscillator has too big a capacitor, I thought it was around 4nF not pF. I'll try rectifying that tommorow and post back.

Posted: 4/19/2007 2:24:06 PM

From: France

Joined: 4/8/2007

You mean that the frequency didn't change when you moved your hand above the plate? Only when you touched it?
Posted: 4/20/2007 5:26:05 AM

From: UK

Joined: 1/6/2007

Yeah... I thik that it's because my capacitor is too big, I'm going to build a new oscillator and report back.
Posted: 4/20/2007 6:32:21 AM

From: UK

Joined: 1/6/2007

Well, last time I was shorting the capacitor with my hand, which is why it only worked when I touched it... Oops.

I've rebuilt my oscillator, and the plate does vary the frequency [:-)]. The osc. is set to 30kHz approx, and the plate varies it from around 10k to 50k, starting from a distance of around 15cm. The signal is also very unstable. What do I do to decrease the response of the aerial, but increase the distance and improve the stability of the signal?
Posted: 4/20/2007 7:12:10 AM

From: France

Joined: 4/8/2007

Hum what do you mean by decrease the response of the aerial?

To have a wider proximity range, I think you have to put smaller caps in the osccilators (to set a frequency over 30kHz, like 100kHz or more..), but it's possible that itt increases the unstability..