Fuzzy about antennas/in air capacitance

[ExpAnonColin]

How exactly does the conductor waving in the air create capacitance?  How are you supposed to wire the antenna up in a circuit, since there's only one lead?  Is there something about copper having the ability to sense the bodies capacitance that causes it to be used in products where the closeness of the body is what the antenna is being used for?  Anyone have any good links?  Google isn't helping.

-Colin

[R.G.]

How exactly does the conductor waving in the air create capacitance?

Capacitance is the storage of energy in an electrostatic field. If I have one metal object, I can pump electrons into it until the metal body builds up a charge. The electrons came from the rest of the planet in general, so there is a measurable electrostatic field from the metal object to everything else. So I can charge up a metal object with even DC. The other plate of the "capacitor" is the rest of the world. Capacitors made like this are of very small values and relatively poor quality compared to ones that are made with two specific plates close together, but the capacitance is measurable.

In fact, any two conductive things have capacitance (i.e. ability to store energy in an electrostatic field) between them. The capacitance decreases as the square of the distance, so it's quite small. Notice that the conductive objects don't have to be all that conductive, since there is no real current flow. A human body and an equal-sized cast iron replica have much the same capacitance to some antenna. There are capacitors literally everywhere. If you build circuits that can sense either the change of electrostatic field or the change of capacitance as objects get nearer or further away, you can detect this.

How are you supposed to wire the antenna up in a circuit, since there's only one lead?

It's a monode - that is, only one terminal. The other terminal is - everything else.

Is there something about copper having the ability to sense the bodies capacitance that causes it to be used in products where the closeness of the body is what the antenna is being used for?

No. Any conductor works as well as long as its conductive enough to not show voltage drops at the relative impedances involved in the circuits.

I'm guessing you're worried about proximity/capacitance antennas, not RF antennas. RF antennas work because the impedance of free space gets near the impedance of the antenna at high frequencies and the energy leaks out into the ether. That's not how low frequency capacitance antennas are, though.

[petemoore]

When my finger and hand get near them.
 They don't buzz much, generally, there's a big flourescent light 3' above my bench, that doesn't seem to cause a whole lot of noise problems.
 Just when I get my hand near a gain circuit, it's like it's picking up a radio signal, just background hiss, noise ,hum...the closer I get to touching it, like it's objecting to having it's input cap changed while the amps on...[shy little circuit].

[ExpAnonColin]

I'm guessing you're worried about proximity/capacitance antennas, not RF antennas.

That's right, just proximity antennas.  Thanks for the insight, RG.  But if the capacitance is between you and the antenna, how do you hook yourself up to the circuit-I guess my question in, since you normally would hook a capacitor between something-that is, break the circuit and put the capacitor in place-how does putting a single conductor in a circuit work?

-Colin

[Joe Davisson]

You put a metal plate on the bottom of a plastic box, and attach it to the negative battery terminal (ground). Then an antenna is attached on top. Each acts as a capacitor plate, and if your body comes near the capacitance is affected slightly.

So there isn't really a single conductor, there's some "hidden" plate somewhere giving a point of reference.

-Joe

[ExpAnonColin]

You put a metal plate on the bottom of a plastic box, and attach it to the negative battery terminal (ground). Then an antenna is attached on top. Each acts as a capacitor plate, and if your body comes near the capacitance is affected slightly.

So there isn't really a single conductor, there's some "hidden" plate somewhere giving a point of reference.

-Joe

Thanks joe, it makes perfect sense now!

(goes and designs theremin devices)

-Colin