# How does an antenna work

1. Dec 9, 2011

### CrimpJiggler

If I'm not mistaken, an antenna transmits radio waves when electrical current runs up and down the antenna. The frequency at which these electrons travel up and down the antenna, in other words the frequency at which they oscillate is proportional to the frequency of electromagnetic waves transmitted by the antenna. Is that correct? If so, how do antennas receive EM waves then. I saw a table listing the effects that the different frequencies of EM radiation has on matter (i.e. UV and visible causes the valence electrons to jump, infrared causes bond vibrations, microwaves cause molecular rotations etc.) and for radio waves it stated that they cause free or mobile electrons to oscillate. So when radio waves hit an antenna, do they cause the electrons to move up and down the antenna? What about microwaves then, when microwaves hit a cell phone antenna, obviously the metal atoms won't vibrate cuz they have no dipole moment so I'm guessing the microwaves cause the free/mobile electrons to oscillate in the same wave radio waves do. Is everything i said there correct?

2. Dec 9, 2011

### Staff: Mentor

Any electrical path, piece of wire, wet rope, gap between rainclouds, etc., when it carries an alternating electric current is surrounded by an alternating electromagnetic field which propagates out to the fartherest reaches of the universe. It is this EM field that carries useful (and often useless ) information.
By the same process, in reverse. Any conductor bathed in an alternating electromagnetic field has induced in it an alternating voltage. The body of your car has current induced in it, but it is not the optimal shape to concentrate any particular frequency.
Yes, back and forth.
The same thing, only the dimensions and best shapes are different and on a much smaller scale.

Light is just like a higher frequency radio wave. So if we could minaturize the domestic television antenna so its dimensions suited the very short wavelengths of sunlight we could convert electromagnetic energy (i.e., light) from the sun directly into electricity. It has been done experimentally, but poses difficulties in scaling up to be a commercial reality. One consideration has to be that white light covers a band of wavelengths, so no individual antenna dimension is optimal to recover all light energies. Naturally, you'd need millions of these nanoscale antennae distributed over each roof tile to be able to collect much useful energy.

The silicon cell is not endangered just yet.

3. Dec 12, 2011

### chrisbaird

The fundamental principle is that electric and magnetic fields exert forces on charged particles (the Lorentz force). An electromagnetic wave, such as a radio wave or microwave, has electric and magnetic fields that exert force on any charged particle they come across. If the charges are bound (like in an insulator), then the electromagnetic waves typically are not strong enough to rip off the charges, but still exert a force that gets the charges oscillating in their local atomic region. This is the basis for dielectric interactions such as reflection and refraction. If the charges are free (like in any conductor), the EM wave hitting it exerts a force on the charges, accelerating them to form a current. Every time you see your reflection in your shiny soup spoon, it is because the light hitting the spoon is has EM fields which accelerate charges, create currents, and these currents re-radiate EM fields back at you.