How does a Radio Antenna Work?

[darrink1]

How does a Radio Antenna Work?
I'm trying to get a grasp of the physical mech.

You generate an AC signal on a conductor (i.e. at 30mhz)
The free electrons on the skin of the conductor osc. back/forth at 30mhz.
The acceleration / decleration of the free electrons causes photons to be released from the free electrons (Photons being the E field)?
The acc / decl will only be large enough to cause photons to be released, if the conductor is long enough to accommodate at least 1/4 wavelength?

 

[chroot]

Photons are "released" anytime an electron oscillates, regardless of the length of the antenna. The only difference is the efficiency with which electrical power is converted into EM radiation. A better antenna -- with the correct length for the frequency in use -- is a more efficient radiator.

- Warren

 

[billiards]

I take it you're discussing transmitters, how does a receiving antenna work?

 

[darrink1]

Photons are "released" anytime an electron oscillates, regardless of the length of the antenna. The only difference is the efficiency with which electrical power is converted into EM radiation. A better antenna -- with the correct length for the frequency in use -- is a more efficient radiator.

- Warren

When you say "efficient" do you mean, that one antenna design vs another may generate more photons, or that the photons shoot off in a direction that makes them more usable for the receiver?

 

[chroot]

People don't really think of radio-frequency radiation in terms of discrete particles -- they typically use the wave model instead, as it's much easier to use. An efficient antenna produces a large-amplitude EM wave for a given feed power, and produces little heat. An inefficient antenna produces a small-amplitude EM wave for the same feed power, and converts most of the power into heat.

- Warren

 

[berkeman]

Here's a reasonable intro to antennas:

http://en.wikipedia.org/wiki/Antenna_(radio)

By "efficiency", we mainly are referring to the "radiation efficiency" of the antenna, which is how much power is actually radiated into the far field and beyond, versus what the input power to the antenna is. If you plot the input impedance of an antenna versus frequency, you will see that it has a very reactive impedance at frequencies away from its resonance(s), which means that the power is not being launched off the antenna. At the resonance(s), you get a mostly real input impedance, which means that the power is going away somewhere, and that somewhere is mostly radiation away from the antenna. The main resonance for a "half-wave dipole" antenna is where each of the two dipole elements is 1/4 of a wavelength in free space. The main resonance for a "quarter-wave monopole" is where the single monopole element (usually vertical) is 1/4 of a wavelength, placed above a ground plane. The input impedance of a half-wave dipole antenna at resonance is about 75+j0 Ohms.

It is common to use a Standing Wave Ratio (SWR) meter when tuning up antennas. An SWR of 1.0 means that the antenna is perfectly matched to the feed amp and coax, and the radiation efficiency is 1.0. SWRs of >1.0 mean that there is a mismatch, and you are losing radiation efficiency.

 

[disregardthat]

What decides in a material what frequenzy the light waves emitted after it absorbs a photon?

It doesn't send out the same light frequenzy as it absorbs, and some goes to produce heat. I wonder what in the material that decides it.

 

[berkeman]

Oh, and as far as receiving, the same resonance issues apply. It's just that when receiving, the currents induced on the antenna by the passing EM field cause a terminal voltage at the feedpoint of the antenna, which generates a propagating EM wave down the coax to the receiver's input amp circuit. Of course, these receive signals vary tremendously in amplitude, so the receive circuit will use Automatic Gain Control (AGC) to deal with the large dynamic range, and will generally use low-noise circuit design techniques (especially sharp bandpass filters) to be able to recover faint receive signals.

 

[cesiumfrog]

I take it you're discussing transmitters, how does a receiving antenna work?

Take the same equations, but everywhere you see a "t" write "(-t)".