Is wireless technology only possible due to inductance?

Question

I asked a friend how capacitative coupling works in glass-mount antennas.
He said it's like when installing a sound system; where you want to keep the power & audio cables separated, to avoid electrical noise/interference.
I guess he was implying the plastic insulation behaves like a capacitor, effectively coupling two parallel circuits.

I'd originally assumed the noise was caused by the electromagnetic field radiating beyond the borders of the insulation, as opposed to via direct physical contact. But I thought to myself, "The cables aren't coiled. The current's direct. There's no inductance here." ^† And then I wondered if it even actually works like that. I always thought electricity implies electromagnetism, regardless; that anywhere an electric current or potential occurs, there's an inherent field of electromagnetic radiation, no matter how small.

So which is true? Is an inductor required to induce an electromagnetic field (ie. an electric current oscillating about a coil), or does a limited field already exist simply by virtue of the presence of electricity?

And by extension: Is all electronic/electrical wireless transmission technology (i.e. power and communications) made possible by virtue of induction? ^‡

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^{† A few things: a) We were referring to an automotive sound system with a 12V+ DC power supply. b) The speakers have coils. c) The audio signal could be considered AC.}

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^{‡ a) Sure; speech, music, sign language, smoke signals, etc. These are all technically wireless communication methods. b) Yes; you could perform photosynthesis or cellular-respiration, rub your hands together, or light a match with a laser-pointer and say it's wireless transmission of energy. c) But it's not a trick question, let's stick to man-made devices, and concepts like electricity and RF.}

Answer 1

The cables aren't coiled. The current's direct. There's no inductance here.

That's not true, there's always some inductance, one millimeter of conductor has roughly 1nH of inductance. You cannot avoid it. The conductor can be a wire, PCB track, metal track on an IC, anything that can conduct electricity. There is always inductance.

The question is: when is that inductance significant? That depends on the frequency (rate of change) of the signal through the wire.

Electricity by itself doesn't mean there is an electric field and/or a magnetic field.

An Electric field exists as a result of applying a voltage between separated conductors, for example 2 plates. Like in a capacitor. The current can be zero. The voltage cannot be zero otherwise the Electric field would be zero as well.

A Magnetic field exists as a result of current flowing through an inductor, for example a current flowing through a wire. The voltage can be zero (but you'd need a super conducting wire for that) but the current cannot.

With an antenna you generally want to receive and/or transmit information, then a static field (one that doesn't change over time) is useless, you want to change the field(s) to transfer your information.

To change the field in a capacitor or around a wire, the voltage and/or current must be changed (modulated) that results in current flowing through a capacitor (to change the voltage, you charge/discharge the capacitor). Similarly for a wire or inductor the current must be changed and that can be done by changing the voltage.

So capacitance and inductance have a similar relation to each other as electric fields and magnetic fields do. There is no one OR the other all are related and cannot be considered on their own.

Wireless communications use Electro Magnetic Waves which have an electrical and a magnetic component. Most Antennas are electrical antennas meaning they mainly work with the E (electric) part of the field, example: the cellular antennas in your phone.

There are also "loop" antennas which mainly work with the M (magnetic) part of the field, example: wireless charging (of phones) and NFC (ID cards etc).

It depends on the application what is more convenient.

Answer 2

Induction and capacitances are just specific versions of the intimate relationship between electric fields and magnetic fields. They are described by the 4 Maxwell equations.

In their local form:

$$\begin{align} \vec\nabla\cdot\vec{E}&=\frac{\rho}{\epsilon}\\ \vec\nabla\cdot\vec{B}&=0\\ \vec\nabla\times\vec{E}&=-\frac{\partial \vec{B}}{\partial t}\\ \vec\nabla\times\vec{B}&=\mu\vec{J}+\mu\epsilon\frac{\partial \vec{E}}{\partial t} \end{align}$$

In words these equations can be understood as:

1. Charges create an electric field proportional to the charge density/number of charges (\$\rho\$). \$\vec\nabla\cdot...\$ is a fancy notation for something called "divergence", ie. more electric field lines are created in a point with a positive divergence.
2. Magnetic field lines can not "originate" or "end" somewhere (unlike an electric field), they always loop around.
3. A changing magnetic field will produce a curly electric field. \$\vec\nabla\times...\$ is a fancy notation for something called the "rotation", it is a measure of how the electric field lines curve.
4. A changing electric field or moving charges (a current) will produce a curly magnetic field or vice versa.

Rather than saying wireless transmission is because of some inductance or capacitance, it is more accurate to say that Maxwell's laws are causing it. Inductors and capacitors are just special cases.

However, a lot of situations are sufficiently close to one of those special cases for argument's sake. Some examples:

• Any current will always flow in a closed circuit, so that closed circuit can be considered a loop that will amplify the magnetic field through it (from equation 4). If that loop is large, and if the current is quickly changing (eg. small fast surges of current through the power supply in a digital circuit), then the magnetic field can become significant and we can think about this circuit as an inductor!
• Let us assume there are two traces close to each other. If one of them feels an increase in potential (ie. by pulling out electrons), then electrons in the nearby trace will start to get attracted to this more positive potential causing a current. This structure will therefore behave much like a capacitor!

I should also mention that in both cases there are both electric fields and magnetic fields at play, but one is more dominant than the other. This in turn depends on the physical location/orientation/etc.