Solarwind wrote: > I understand that an > electric field is created which propagates at the speed of light. So, > is that propagating field considered the radio wave? Yes. Actually the magnetic field (B field) is envolved too. I talked about voltage only, but a directed propagating wave is actually tightly linked disturbances in both fields. This is what happens once radiation has left its source and is no longer coupled to that source. In other words, it takes both the E and B fields to make a propagating wave. However, don't confuse that with how you can launch such waves. > Or is it the > propagating electromagnetic field created by an accelerating charge? This makes little sense. Moving charges create a circular B field perpendicular to the direction of motion. I don't think that's what you're referring too. There is a special way to create a propagating wave by accellerating a charge (bremsstrahlen), but that's pretty advanced stuff and not likely what you are referring to either. So in short in your context, I think the answer is "radio waves don't have anything to do with accellerating charges". > Also, if there is no closed circuit, how/why are the electrons > accelerating? Even if there is a potential difference of 12 V, there > is no conductor in between for them to move in? Nobody said anything about electrons accellerating. Electrons sloshing around may start a electromagnetic wave going, but they have nothing to do with propagating one. After all, the propagation can happen in vacuum where there are no electrons. > Ok, I can understand how the field is still propagating at the speed > of light outward. But is that propagating field the radio wave? Yes. > A radio transmitter whose job it is to move electric charges > rhythmically up and down its antenna sets the signal that is to be > propagated in motion. > > How does it "move" charges up and down the antenna if there isn't a > closed circuit? That's a good question and a tough one to explain. In fact there is a closed circuit, but only at AC. If you only look at the AC component, and particularly the carrier frequency, the circuit is a lot different than it appears by looking at a dead end wire sticking into the air. For one thing, there are inductances and capacitances that make it a closed circuit at the carrier frequency. But these are not just any inductances and capacitances. They are specially arranged together with the geometry of the wire so that energy rythmically sloshes back and forth in different forms from one end of the wire to the other. The electrons don't leave, but they are kept sloshing. Since this sloshing is at resonance or close to it, the magnitude of the sloshes are generally much higher than the magnitude of the input signal needed to keep it sloshing. Think of a bathtub half full of water. Leaning over the tub from the outside, you can put your hand in the middle and move it back and forth a little bit. If you do this at just the right frequency, you can get some pretty dramatic sloshing going on. It takes very little force to keep the wave heights at the ends of the tub several times the amount you move your hand each cycle. In this case the tub is sortof like a center fed dipole. The transmitter puts a little bit of energy into the middle, which causes a lot of sloshing back and forth. That means the AC voltage at the ends is high, and the AC current in the middle is high. Eventually these voltages and currents get so high that a significant fraction of the energy gets radiated into space each wave. The transmitter has to put that energy back each wave to keep the sloshing, and therefore the transmitting, going. Antennas can be designed so that the transmitter input can be at one end instead of the middle, but the principle is still the same. A little input makes a lot of sloshing, which eventually gets so strong that it causes enough of a disturbance in the local E and B fields such that they start carrying energy away each cycle. That's the energy the transmitter has to add back each cycle to keep things going. The energy that is carried away eventually (within a wavelength or so) organizes itself into a self-propagating wave. It becomes a continuous dance between the E and B fields which propagates at the speed of light. It is this propagating E/B field dance we call electromagnetic radiation. For simplicity we call the lower frequencies of it (a few 10s of KHz to a few GHz) radio waves. At a little higher frequencies we call them microwaves or radar waves, then terahertz radiation, then infrared, then visible light from red to green to blue, then ultraviolet radiation, then X rays, then gamma rays. Despite the different names for different frequency ranges, they are all exactly the same propagating dance between the E and B fields, differing only in how fast they dance back and forth (the frequency). Since the propagation always happens at the speed of light (in a vacuum anyway), frequency and wavelength are just reciprocals of each other. Often you will hear the higher frequency waves identified by wavelength instead of frequency, but keep in mind that either is specifying the same thing. Since the speed of light in vacuum is about 300Mm/s, a 300MHz radio wave and a 1 meter radio wave are the same thing. It's humbling to ponder the vast dynamic range of electromagnetic field frequencies (or wavelengths). WWVB transmits the NIST atomic clock time signal from Boulder at only 60KHz. That's a wavelength of 5Km, or 3.1 miles. 1MHz is in the middle of the AM band, with a wavelength of 300 meters or about .2 miles. Visible light covers roughly a 2:1 range from about 350 to 700 nm, and X rays and gamma rays are still many times smaller than that. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist