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No antennae radiate all the power fed to them!
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No antennae radiate all the power fed to them!
On 11/1/2014 5:23 PM, Lostgallifreyan wrote:
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. What? For a wave to have a "bulge" above the top of the tank means there is a trough well below the top of the tank. The amount of liquid does not change because you make waves in the tank. -- Rick |
No antennae radiate all the power fed to them!
On 01/11/14 21:38, wrote:
As a problem for the student, how big would a wave guide have to be to be able to transfer 7MHz? I'll guess at 34.8488m x 15.7988m by scaling the dimensions for 5.85 to 8.2GHz. (C Band) -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
No antennae radiate all the power fed to them!
wrote in message ... gareth wrote: Ignoring, for the moment, travelling wave antenna, and restricting discussion to standing wave antennae ... An antenna is an antenna. 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. Nope, voltage is applied to an antenna causing currents to be created which in turn cause an electromagnetic field to be created. As antennas are made of real materials they have a resistance and the current through that resistance leads to losses. However, in the real world most antennas have an impedance in the tens of Ohms while the resistance is in milliohms, so normally the losses are trivial compared to the radiation. At first, there is no standing wave, until the wave reaches the point of reflection in the antenna and heads back the way it has come (because not all has been radiated*****) On the way back, it againn suffers the losses described above, as well as radiating a bit more. Pure nonsense. It then reaches the other end and suffers further reflections ad infinitum. Pure nonsense. An interesting conclusion is, therefore, that the I2R losses are repeated, each tiome with a smaller loss, as the wave decrements. A nonsense conclusion based on a nonsense assumption. ***** Without the remnants of non-radiated power, there could NOT be a standing wave! Sigh. ^^^^^^^^^^^ I was going to point out to Gareth that he is describing behavior in an antenna system, not an antenna. But, I'm done now. No more. |
No antennae radiate all the power fed to them!
rickman wrote:
On 11/1/2014 6:20 PM, wrote: 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. Now I'm very confused. How can an EM field not exist in a conductor? Isn't it the E part that creates a gradient which propels the electrons? Actually it is both. As long as the antenna is made of linear material, transmit and receive are reciprocal properties. The only antennas I can think of that use non-linear materials is some microwave antennas that include ferrites. In a perfect conductor, I thought it was the M part that can't exist inside the conductor. That is one of the causes of the loss of superconductivity, penetration by an M field. Or do I have this mixed up? A bit. -- Jim Pennino |
No antennae radiate all the power fed to them!
rickman wrote:
On 11/1/2014 5:31 PM, wrote: rickman wrote: On 11/1/2014 1:03 PM, wrote: gareth wrote: Ignoring, for the moment, travelling wave antenna, and restricting discussion to standing wave antennae ... An antenna is an antenna. Deep thoughts... 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. Nope, voltage is applied to an antenna causing currents to be created which in turn cause an electromagnetic field to be created. As antennas are made of real materials they have a resistance and the current through that resistance leads to losses. I thought there were *real* materials with no resistance. Isn't that what a superconductor is? Well, to be pendatic, there are no real materials with zero resistance that can be used to build antennas. Why can't you build an antenna with a superconductor? As all the current existing superconductors require a bunch of supporting equipment to keep them cold, they can't be used for antennas. Really? What is the problem? There are super conductors at liquid nitrogen temperatures and you can have that sitting in a flask on your desk. Why couldn't that cool an antenna? Once you remove the I*R losses, you don't even have to worry about the radiated power heating the N2. If one were realy determined to do it, one could build the antenna in a non-metalic container of some sort and keep the container filled with LN2. I think you are confusing need with practicality. There is nothing to stop you from making a superconducting antenna. There just isn't a need for it unless you live in Gareth's world. Hmmm... wasn't that a movie? Gareth's World? It is not need versus practicality, it is practicality period. If room temperature superconductors are ever invented... However, those are like a cure for the common cold, practical fusion power, and peace in the Middle East, all just around the corner for the past half century or so. I've never heard anyone say either a cure for the common cold or fusion was "around" the corner. I've never heard anyone say at all that peace is expected in the middle east. You must not be very old then... I believe there are rather cold temperatures in space. A superconducting antenna could be used there with *no* supporting "apparatus". You mean other than the shade screen? You do understand two big problems with space stuff is how to get rid of any generated heat and Solar heating? In any case, why? I^2R losses only become significant in very small antennas and there is all the space you could ask for in space to build an antenna. -- Jim Pennino |
No antennae radiate all the power fed to them!
Frank Turner-Smith G3VKI wrote:
On 01/11/14 21:38, wrote: As a problem for the student, how big would a wave guide have to be to be able to transfer 7MHz? I'll guess at 34.8488m x 15.7988m by scaling the dimensions for 5.85 to 8.2GHz. (C Band) Sounds in the ball park to me. For further reading enjoyment and why there is no EM field inside of RG-8: http://en.wikipedia.org/wiki/Cutoff_frequency -- Jim Pennino |
No antennae radiate all the power fed to them!
Wayne wrote:
snip I was going to point out to Gareth that he is describing behavior in an antenna system, not an antenna. I doubt he will EVER understand the difference. But, I'm done now. No more. It does become tiresome correcting the same nonsense over and over again. -- Jim Pennino |
No antennae radiate all the power fed to them!
On Sat, 01 Nov 2014 18:47:32 -0400, rickman wrote:
I think you are confusing need with practicality. There is nothing to stop you from making a superconducting antenna. There just isn't a need for it unless you live in Gareth's world. Hmmm... wasn't that a movie? Gareth's World? (...) I believe there are rather cold temperatures in space. A superconducting antenna could be used there with *no* supporting "apparatus". You don't need to go to outer space to see cryogenic radios in operation. Superconducting radio frequency http://en.wikipedia.org/wiki/Superconducting_radio_frequency In a past project, I worked with cryogenic duplexers and receiver front ends for cellular service. My part had nothing to do with the superconducting components, but I got to watch them perform. Filters with nearly vertical skirts, sky high filter shape factors, zero loss, near zero noise figu http://www.suptech.com/wireless_overview_n.php http://www.suptech.com/pdf_products/cryogenic_receiver_front_end.pdf http://www.suptech.com/pdf_products/SuperLink_850_G3AB.pdf Where cryogenic front ends worked best are in installation without towers, where the coax cable losses were less, and the cryo unit can be located in a nearby rooftop shelter. These tend to be located in urban jungles, where signals are traditionally weak, and handset density rather high. At the same time, TMA (tower mounted amp) technology appeared, which provided many of the benefits of cryogenic receiver front ends, but without the complexity, power consumption, and cost of the cooling components: http://en.wikipedia.org/wiki/Tower_Mounted_Amplifier https://www.google.com/search?q=tower+mounted+amplifier&tbm=isch http://www.commscope.com/catalog/wireless/2147486004/product.aspx?id=162&sortExp=Name&nrp=100 Also, note that spacecraft all have some form of temperature control where the electronics do NOT operate at cryogenic temperatures: http://en.wikipedia.org/wiki/Spacecraft_thermal_control -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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