Inductors as antennas

[wael]

Hi,

Why inductors aren't used as antennas. I know that there are some exceptions, but is there a specific reason ?

Regards,
Wael

[hagster]

All wires have inductance. For example a monopole antenna is bassically an inductor. It has a resonance where the capacitance between it and the ground plane cancels out the inductance. At that point you are basically left with the radiation resistance which which hopefully matches your source if you want maximum power transfer.

You can use spiral wound antenna elements to increase the inductance and shorten the length required to hit this resonance. However when you make antennas shorter you have less aperture to capture(or deliver) em fields so they are less efficient.

At the extreme end you can have loop antennas or ferrite rod antennas that are made to resonate using a discrete capacitors. Again the efficiency is reduced by the small relative size(compared to the wavelength).

[T3sl4co1l]

A pure inductance only connects electrical current to magnetic field in space.

An antenna must connect electrical voltage and current, to electric and magnetic fields in space.

As real, physical inductors have nonzero size, they do exhibit some electric field.  It's just not the focus of the component, and therefore they typically make terrible antennas.

There is a tradeoff between antenna size, bandwidth and directionality.  Basically, you can take a suitable inductor, tune it to a particular frequency, and have it resonate with a humongous voltage.  (The excessive voltage is necessary to couple effectively with the radiating electric field.)  This gives an electrically small mono/dipole antenna, with extremely narrow bandwidth, and acceptable efficiency.

Remember that you can always make an antenna worse than ideal (ideal, given its size, bandwidth and directionality, that is), but you can never make one better.  For example, compare an air-core solenoid winding (like a Tesla coil secondary) to a shielded, ferrite cored inductor: the former can radiate effectively if made to resonate with low losses and high Q factor; the former will never achieve more than pitiful efficiency.

An identical argument applies to capacitors, of course; in that case, a practical example is the patch antenna, where a small parallel-plate capacitor is made to resonate at a certain frequency.  The bandwidth is small, but that might happen to be okay (for example, a Bluetooth antenna needs hardly 1% bandwidth, so can be smaller than a fingernail).

Tim

[Yansi]

I think ferrite rods make a pretty decent antennas, with a nice directional pattern having two very sharp minima at the direction of the longitudal axis. 

[TheUnnamedNewbie]

'Inductors' are used as antennas all the time. They are called "Loop antennas". Ever seen those square antennas used on some AM radio systems? Those are often something along the lines of a 10-turn loop - ie, inductor.

To expand on what T3sl4co1l was saying,

(...)
A pure inductance only connects electrical current to magnetic field in space.

An antenna must connect electrical voltage and current, to electric and magnetic fields in space.
(...)

Antennas are devices that work with electromagnetic fields. Thinking purely in terms of inductors and capacitors doesn't really work anymore. However! We do like to have antennas that 'mostly' work in the electric or in the magnetic domain. Often, we also have parallels between the two. For example, the classic dipole is an "electrical" antenna, where we look at the electric field output by the standing wave on the dipole antenna. The current follows from this and so does the magnetic field - but you mostly consider the electrical behaviour of the rod. The 'magnetic' counterpart of this dipole is the elementary magnetic dipole, which is just a single turn loop.

Also, quick clarification: In a patch antenna, unlike what many think, the resonance does not occur between the top and bottom place but across the top plate (ie, a standing wave from one side to the other).