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One thing I have always struggled to understand about antennas -- Of course I know that an antenna is most efficient at radiating power when its length is some
by supernova87a 7y ago
One thing I have always struggled to understand about antennas --
Of course I know that an antenna is most efficient at radiating power when its length is some fraction/multiple of the emitted wavelength. But I cannot for the life of me intuit how the electrons are being excited and behaving.
If I use the bathtub analogy of sloshing water, it cannot be (I believe) that the electrons are sloshing in bulk up and down the antenna and "accumulating" at one end at the speed of light.
On the other hand, if each electron along the length of the antenna is oscillating in its own relatively stable position, what then does the length matter to the electron at one end versus the other?
Or should I understand it as, energy is being transmitted out of the antenna, like it is in a flute being played, and the electrons are most constructively-interference being reinforced to resonate at the frequency desired (by standing waves in the conductor) if the length of the material matches the wavelength?
This has always been hard to visualize.
- madengr 7y agoNeed to think about charge displacement and fields rather than electrons. The electrons moving in wire (direct current) circuit only move, on average, a few cm/sec. Now the charge displacement moves at the speed of light. Think of a tube stuffed tightly with marbles. Push the marbles at one end, and they move near instantly at the other end. The force transfers instantly, but the marbles may hardly move. Now imagine the marbles connected with stiff, little springs. Push on one end, and the compression wave moves through the charges quickly (the speed depends on the spring constant). It hits the other end, yielding a little more charge accumulation, then bounces back, yielding a little less charge. Do that a 2.5 billion times a second and you have a WiFi antenna. The charges don’t move much all; it’s the charge displacement that moves, which is the E and H field. The key is also that the displacement must have acceleration (harmonic motion) to have a derivative. Accelerating charges radiate; constant velocity charges (direct current) don’t. The jist of it is that all the energy is contained in the fields. That metal rod has charge that is easily displaced. Now as to how an accelerating charge radiates; I can’t remember, but Feynman covers it in his 3 volume lectures.
- supernova87a 7y agoThat is helpful!
- selimthegrim 7y agoAnother nice thing to think about is spherical symmetry, ie can one have it in an antenna. Some thought with Maxwell’s equations will lead you inexorably to Birkhoff’s theorem.
- madengr 7y agoHa ha, way over my head. I had one modern physics course; a little special relativity but mostly quantum behavior. If you mean a perfect, isotopic antenna; I don’t believe it exists. Though I suppose it could, if you consider an antenna pattern to be a wave function. A spherical mass should be emitting black-body radiation with an isotropic pattern. Maybe only noise can be isotropic. If you try to encode information, it can’t.
- userbinator 7y agoPage 296 of this book has a good illustration of the field changes around a transmitting antenna: https://archive.org/details/principlesunderl00unit https://archive.org/details/principlesunderl00unit (It's almost 100 years old but the physics have not changed. If anything, these early books often have far more detailed explanations because they assumed far less about the reader.)
- supernova87a 7y agoWow, amazing to see that book is from 1918 and the basics were all already laid out.
- pfundstein 7y agoDirect link: https://archive.org/details/principlesunderl00unit/page/296 https://archive.org/details/principlesunderl00unit/page/296