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No antennae radiate all the power fed to them!
Percy Picacity wrote in
: However, this does not change the fact that standing waves do not 'use up' any of the power fed to the aerial Is that like potential vs kinetic energy? After all, a filter could be said to 'store' energy in an eternal oscillation if it had no losses, and nothign drawing output from it. The moment you do, you lose energy, the 'note' fades. Given that if you produce a standing wave in a tank of liquid such that one bulge exists above the rim, the standing wave can be considered a form of storage (potential energy), because that tank will hold more liquid that it would if brim full without the wave. |
No antennae radiate all the power fed to them!
gareth wrote:
"gareth" wrote in message ... Ignoring, for the moment, travelling wave antenna, and restricting discussion to standing wave antennae ... A wave is launched, and radiates SOME of the power, and suffers both I2R losses and dielectric and permeability losses associated with creating and collapsing the near field. Of course, it goes without saying that the wave was already travelling up the feeder and it diffracts along the elements of the antenna, rather than being launched from the feedpoint! Nope; there is an electric field in a feed line (other than wave guide) but no electromagnetic field. As a problem for the student, how big would a wave guide have to be to be able to transfer 7 Mhz? About the only antenaa where a "wave is launched" is a dielectric lens antenna with a wave guide feed. Of course, at the other end of the wave guide is an antenna to which voltage is applied, which causes current flow in the antenna, which causes an electromagnetic field to be created in the wave guide which then flows to the antenna. -- Jim Pennino |
No antennae radiate all the power fed to them!
On Sat, 01 Nov 2014 16:05:53 -0500, Lostgallifreyan
wrote: Jeff Liebermann wrote in : Without a reflection, there can be no standing waves. That's the one bit that comes naturally to my own understanding, such as it is. How far does this parallel with an optical laser cavity? I'd find it easier to understand if someone here who knows both can point out a few essential similarotes and differences. Sigh... topic drift again. The parallel with a longitudinal mode laser cavity is fairly close. http://en.wikipedia.org/wiki/Longitudinal_mode The transmission line is some multiple of 1/2 wavelength long. The signal bounces back and forth between the ends, reinforcing itself with every bounce, until it spews forth from from one end or edge. Obviously, without reflections, there would not be any laser action. Also, in the ringing of a resonant audio filter (or any electronic filter), there seem to be parallels there too. After all you can only have ringing, a note produced, while energy remains in the system. Not quite. If you apply energy to a resonant circuit (electrical or mechanical), that then remove the input, you'll get a damped wave (i.e. exponential decay) output where the rate of decay is determined by the losses in the system. You could build a transmission line oscillator, which would exhibit some rather small damped wave output when turned off, but in most cases, there's no connection with reflected or standing waves because there is usually no transmission line. Might as well be part of the problem. What I do in my spare time. I recorded these in about 1998. Please forgive my screwups, plagerism, lack of coherent style, sloppy fingering, etc: http://802.11junk.com/jeffl/music/ -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
No antennae radiate all the power fed to them!
"Brian Reay" wrote in message
... He is confusing the current and voltage distribution plots for waves. No, there is no confusion on my part. Please explain why you think that, for I fear that there may be confusion on your part. Plus, an RF wave has a magnetic component. Well, i think we all knew that. That can't exist IN the antenna element as it is conductor. Yes, and no, for it is the magnetic componentry in the wire that causes the skin effect. |
No antennae radiate all the power fed to them!
"Brian Reay" wrote in message
... It is nonsense, they can be no wave in the element due to it being a conductor. You seem to be unaware that a travelling wave around a wire is what causes the wave to move along the wire, and not the electrons inside, which only oscillate a very short distance about their mean. He is confusing the I and V plots for waves. There is no confusion on my part. Perhaps you could explain where you think I am confused, for I had not mentioned the separated I and V waveforms. Perhaps you are confused yourself, perhaps, by the current maximum at the centre of a dipole, for it is not a DC maximum but rises and falls in magnitude? |
No antennae radiate all the power fed to them!
On Sat, 1 Nov 2014 21:14:48 +0000, Percy Picacity
wrote: However, this does not change the fact that standing waves do not 'use up' any of the power fed to the aerial (in principle, increased current intensity increases resistive losses, but this loss can be made arbitrarily low by having a lower wire resistance). Standing waves do not in principle use 'power' at all and certainly do not dissipate energy that otherwise would be radiated. They require a signal to be applied to the transmission line but, whether the power is radiated at the other end or the signal merely meets a mismatch, say an open circuit, the standing wave does not affect, or need to use, any of the power that leaves the other end. Indeed they work just as well if no power whatever is used, as in the open circuit case. I'll make it even easier. An RF signal can only do three things: - Radiate (as in an antenna) - Conduct (pass through as in a transmission line) - Dissipate (convert to heat) Real transmission line and antenna systems involve combinations of these three mechanisms. If you run into something that doesn't quite fit into one or more of these mechanisms, it's probably wrong. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
No antennae radiate all the power fed to them!
gareth wrote:
"Brian Reay" wrote in message ... It is nonsense, they can be no wave in the element due to it being a conductor. You seem to be unaware that a travelling wave around a wire is what causes the wave to move along the wire, and not the electrons inside, which only oscillate a very short distance about their mean. You seem to be unaware that current is the total, net movement of all the electrons in a wire, not just a single electron. He is confusing the I and V plots for waves. There is no confusion on my part. Perhaps you could explain where you think I am confused, for I had not mentioned the separated I and V waveforms. Yeah, right. Perhaps you are confused yourself, perhaps, by the current maximum at the centre of a dipole, for it is not a DC maximum but rises and falls in magnitude? Only a very confused individual would babble on about the instantaneous current or voltage. -- Jim Pennino |
No antennae radiate all the power fed to them!
On 2014-11-01 22:02:48 +0000, Jeff Liebermann said:
On Sat, 1 Nov 2014 21:14:48 +0000, Percy Picacity wrote: However, this does not change the fact that standing waves do not 'use up' any of the power fed to the aerial (in principle, increased current intensity increases resistive losses, but this loss can be made arbitrarily low by having a lower wire resistance). Standing waves do not in principle use 'power' at all and certainly do not dissipate energy that otherwise would be radiated. They require a signal to be applied to the transmission line but, whether the power is radiated at the other end or the signal merely meets a mismatch, say an open circuit, the standing wave does not affect, or need to use, any of the power that leaves the other end. Indeed they work just as well if no power whatever is used, as in the open circuit case. I'll make it even easier. An RF signal can only do three things: - Radiate (as in an antenna) - Conduct (pass through as in a transmission line) - Dissipate (convert to heat) Real transmission line and antenna systems involve combinations of these three mechanisms. If you run into something that doesn't quite fit into one or more of these mechanisms, it's probably wrong. If 'conduct' includes the case where the signal goes to the other end of the transmission line but does not go beyond it to any other component, I'll agree with you. In that case, neglecting losses, no power is used (apart from a truly tiny amount transiently as the wave builds up and energy is stored in the first few microseconds) -- Percy Picacity |
No antennae radiate all the power fed to them!
gareth wrote:
"Brian Reay" wrote in message ... He is confusing the current and voltage distribution plots for waves. No, there is no confusion on my part. Please explain why you think that, for I fear that there may be confusion on your part. Plus, an RF wave has a magnetic component. Well, i think we all knew that. That can't exist IN the antenna element as it is conductor. Yes, and no, for it is the magnetic componentry in the wire that causes the skin effect. Magnetic fields can exist in a conductor. Electromagnetic fields can not exist in a conductor. -- Jim Pennino |
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