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Antenna gain question
Assume an incoming rf signal has exactly the same strength in all 3
dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? Ron, W4TQT |
Antenna gain question
"Ron" wrote
Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? __________ Whatever the net field that arrives from the transmit antenna (whether or not the transmit antenna is omnidirectional), a receiving antenna having gain in the direction toward the transmit antenna will perform better than if it is omnidirectional. Part of the improvement is due to its added gain, and part due to interference rejection from co- and adjacent-channel signals arriving from directions where the receiving antenna has less gain than an omni antenna. RF |
Antenna gain question
On Fri, 28 Oct 2005 20:37:07 GMT, Ron wrote:
Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? Your scenario is a little confusing. Here are my thoughts: I we took the case of say, noise that was sourced from all around you (that is not to mean an isotropic transmitter antenna), a directional antenna would receive about the same power as an isotropic antenna, and the difference would be due to antenna losses, ground reflection losses (if relevant). Galactic noise on HF might nearly fit into that scenario (or perhaps more topically, neighbourhood BPL interference), and I would expect that a 8dB yagi would receive similar power to a half wave dipole. Galactic noise is a little lower at the galactic poles, so in sweeping the yagi you may observe a very small directional effect. Further, ground reflection and different antenna + feed losses will introduce small differences. If at the end of that, you are trying to rationalise why a beam is better than a dipole, although the beam does not receive more or less of the "directionless" noise, it does increase the receive power from noise, interference and signal from the main beam direction and reduce receive power from noise and interference from away from the main beam. Does that hang together? Owen -- |
Antenna gain question
Ron wrote: Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? Ron, W4TQT Yes. The amount of signal "captured" from a given direction is exactly proportional to the gain in that direction. "Capture area", "effective aperture" and "gain" are simply different ways of expressing the same thing, as long as perfect efficiency is assumed; if you know any one you know the other two. Roy Lewallen, W7EL |
Antenna gain question
Ron wrote:
Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? Does a vertical Yagi receive more signal than a vertical monopole? -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Maybe I should restate my question. Assume a receiving antenna is in
the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. Ron Ron wrote: Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. Question: would an antenna having gain capture any more signal power than a completely omnidirectional antenna with no gain? Ron, W4TQT |
Antenna gain question
On Sat, 29 Oct 2005 00:21:02 GMT, Ron wrote:
Which receiving antenna would capture more power, an omni or a high gain beam? An isotropic antenna would be the best, as the field described came from the same emitter (such a field would be impossible otherwise). Stepping out of this enigma (that the emitter and detector are different, and the field which could only be generated by an isotropic would then suddenly turn and come back) would answer the omni. 73's Richard Clark, KB7QHC |
Antenna gain question
Ron wrote:
Maybe I should restate my question. Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Are you trying to receive the background radiation left over from the big bang? That's the only source outside of the sphere that I know of that can accomplish your boundary condition. -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Ron wrote:
Maybe I should restate my question. Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. They'll intercept equal amounts, assuming both are lossless. The directional antenna will intercept a larger fraction than the isotropic antenna in the directions it favors, and less in others. The total will be be the same. In reverse, this is equivalent to calculating the average gain of the antennas, which is the same for all lossless antennas. Roy Lewallen, W7EL |
Antenna gain question
No, I'm just trying to understand antenna gain.
Ron Cecil Moore wrote: Ron wrote: Maybe I should restate my question. Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Are you trying to receive the background radiation left over from the big bang? That's the only source outside of the sphere that I know of that can accomplish your boundary condition. |
Antenna gain question
I think Cecil was being facetious :-)
Ron wrote: No, I'm just trying to understand antenna gain. Ron Cecil Moore wrote: Ron wrote: Maybe I should restate my question. Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Are you trying to receive the background radiation left over from the big bang? That's the only source outside of the sphere that I know of that can accomplish your boundary condition. |
Antenna gain question
Ron, W4TQT wrote:
"Question: would an antenna having gain capture any more signal than a completely omnidirectional antenna with no gain?" Directive gain depends entirely on the distribution of radiated power. See 1955 Terman page 870. The antenna gain figure is identical for transmitting or receiving. To take full advantage of gain in a line of sight path, the electric vector must be parallel with that received. Cross polarization can cause up to almost 30 dB signal loss. There are directional antennas that receive vertical and horizontal polarizations equally well. Yes. An antenna with gain captures more signal than an antenna with no gain when both are in their best positions and equally distant fron the same signal source. Best regards, Richard Harrison, KB5WZI |
Antenna gain question
Richard Harrison wrote:
Yes. An antenna with gain captures more signal than an antenna with no gain when both are in their best positions and equally distant fron the same signal source. This thread has got me wondering. Which antenna would capture more Big Bang Background Radiation? -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Here's a related question that may help you answer yours:
Do all (assume lossless) antennas with the SAME directionality caputre the same signal power? If the answer to that one is "no," then I think you must first further qualify your question, or it will be like comparing canteloupes and grapes. Cheers, Tom |
Antenna gain question
On 29 Oct 2005 14:18:02 -0700, "K7ITM" wrote:
Do all (assume lossless) antennas with the SAME directionality caputre the same signal power? Is "directionality" equivalent to the well known term Directivity? Owen -- |
Antenna gain question
Roy Lewallen, W7EL wrote:
They'll intercept equal amounts, assuming both are lossless. The directional antenna will intercept a larger fraction than the isotropic antenna in the directions it favors, and less in others. The total will be be the same. In reverse, this is equivalent to calculating the average gain of the antennas, which is the same for all lossless antennas. Roy Lewallen, W7EL Hello to all, my name is Miguel, LU 6ETJ. It is a pleasure to read this group. This topic is really an interesting question... Modestly I would want to point out the following thing: Imagine an inner radiant spherical surface with a finite and uniform density radiant energy. Aim a directional antenna with a directivity of, for example, one stereoradian on any direction. How is it able to such an antenna to receive equal quantity of energy of a smaller portion of the sphere than an antenna that is able to receive the energy taken place by the entirety sphere? 73´s of Miguel Ghezzi (LU 6ETJ) Untranslated text for reference (my written english is a little poor ;( ): Hola a todos, mi nombre es Miguel LU 6ETJ. Es un gusto leer este grupo. Este tópico es realmente una interesante pregunta... Modestamente desearía señalar lo siguiente: Imagine la parte interior de una esfera radiante, con una densidad de energia radiante finita y uniforme. Apunte un antena direccional con una directividad de, por ejemplo, un estéreo radian en cualquier dirección. ¿Cómo puede tal antena recibir igual cantidad de energía de una porción menor de la superficie radiante, que una antena que es capaz de recibir la energía producida por la totalidad de la esfera? 73´s de Miguel Ghezzi (LU 6ETJ) |
Antenna gain question
On 30 Oct 2005 10:41:53 -0800, "lu6etj" wrote:
Imagine an inner radiant spherical surface with a finite and uniform density radiant energy. Aim a directional antenna with a directivity of, for example, one stereoradian on any direction. How is it able to such an antenna to receive equal quantity of energy of a smaller portion of the sphere than an antenna that is able to receive the energy taken place by the entirety sphere? Hi Miguel, You are quite right. Only an isotropic antenna can take all the energy as only an isotropic could have transmitted it. The uniform distribution and the spherical geometry force this solution even though it is a practical impossibility. The real question is, how did that energy get turned around to come back? 73's Richard Clark, KB7QHC |
Antenna gain question
Richard Clark wrote:
The real question is, how did that energy get turned around to come back? A conductive Dyson's sphere? -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Would it be possible that the question went by the following thing?
When we study directional or isotropic receiving antennas, we assume for example: Punctual sources generating spherical wave fronts (convex) or infinitely far away punctual sources creating plane wave fronts for all the practical effects. Under these conditions the receiving antennas are "outer" of the radiant sphere; this way, the effective area of a directional antenna represents a bigger external surface and it intercepts more energy than the corresponding to an isotropic antenna, then everything agrees with what we have learned on the directivity of the antennas, but in this example the conditions are inverted, now we don't have plane or convex fronts, we have concave fronts. The solution under the new conditions is different from the habitual one... I think that the environment of the problem is similar that of the Kirchhoff law of thermal radiation: "a small sphere inside a radiant sphere". I also think that the conditions of this problem could be similar (and therefore taken place artificially) to those of light`s receiver inside a luminous sphere. In this case we proceed as when we study the entropía of an isolated system, in such a system the entropía can diminish, although that is not possible for the whole universe (I suppose this allows me to escape elegantly of Richard's question... ; D 73´s for all, and thank you very much for your very interesting and instructive habitual postings. Miguel Ghezzi (LU 6ETJ) Spanish text for reference (withouts my translation errors). ¿Sería posible que la cuestion pasara por lo siguiente?: Cuando estudiamos antenas receptoras direccionales o isotropicas asumimos por ejemplo: Fuentes puntuales generando frentes de onda esféricos (convexos) o fuentes puntuales infinitamente alejadas que producen frentes de onda planos para todos los efectos practicos. En estas condiciones las antenas receptoras están "fuera" de la esfera radiante; así, el área efectiva de una antena direccional representa una superficie exterior mayor e intercepta más energía que la correspondiente a una antena isotrópica, entonces todo concuerda con lo que hemos aprendido sobre la directividad de las antenas, pero en este ejemplo las condiciones se invierten, ahora no tenemos frentes planos o convexos, sino de frentes cóncavos. La solución en las nuevas condiciones es diferente de la habitual... Pienso que el entorno del problema es parecido al de la ley de Kirchhoff de la radiación térmica: "una pequeña esfera dentro de una esfera radiante". También pienso que las condiciones de este problema pudieran ser similares (y por lo tanto producidas artificialmente), a las de un receptor de luz dentro de una esfera luminosa. En este caso procedemos como cuando estudiamos la entropía de un sistema aislado, en tal sistema la entropía puede disminuir, aunque eso no sea posible para el universo entero (supongo que eso me permite huir elegantemnnte de la pregunta de Richard ;D 73's para todos y muchas gracias por sus interesantes e instructivos escritos habituales. Miguel Ghezzi (LU 6ETJ) |
Antenna gain question
On 30 Oct 2005 15:50:50 -0800, "lu6etj" wrote:
although that is not possible for the whole universe (I suppose this allows me to escape elegantly of Richard's question... ; D 73=B4s for all, and thank you very much for your very interesting and instructive habitual postings. Hi Miguel, Can there be an escape? Ron's question was posed with an impossible proposition. A collapsing sphere of electromagnetic energy? This has so many so many fantastic presumptions built in. I hope this translates well for you (it does not make sense in any language). 73's Richard Clark, KB7QHC |
Antenna gain question
Hello Richard (my middle name is Ricardo = Richard. We are
"Tocayos"...) Richard wrote: Can there be an escape? Ron's question was posed with an impossible proposition. A collapsing sphere of electromagnetic energy? Ron wrote: Assume an incoming rf signal has exactly the same strength in all 3 dimensions i.e., completely omnidirectional. If I have understood well Ron's question... What about a number tending (spreading?, going to? - a limit, as in calculus ) to infinite, of coherent punctual electromagnetic identical sources on the inner surface of a sphere with testing antennas in center of it? (I think it isn't necessary neither coherence or identicals sources. Noise sources fix well in my interpretation of the concept that (I believe) Ron it wanted expose to us). Can it these conditions to be thought? Could it be simulated with an electromagnetic CAD as FEMLAB? Are they agree with Ron question? What do you say Ron? Miguel Ghezzi (LU 6ETJ) PS: I have another physical doubt, if you can help me. Can a real DC current radiate electromagnetic energy?. It is not captious or cheat question. I think yes, but I don't want to condition your answers with my hypotesis. Thank you in advance for your answers. ------------------------------------------------------------------------------------------------------------------- Richard escribió: Puede haber una salida?. La pregunta de Ron fue presentada con proposicion imposible. Una esfera de energía electromagnetica colapsandose? Ron escribio: Asuma una señal de rf entrante que tiene exactamente la misma intensidad en las tres dimensioes, por ej, que es completamente omnidireccional. Si he comprendido bien la pregunta de Ron... Que tal un numero tendiendo a infinito de fuentes electromagneticas puntuales identicas, situadas sobre la superficie interior de una esfera y las antenas de prueba en el centro de la misma? (creo que no es necesaria ni coherencia ni fuentes identicas, fuentes de ruido van bien en mi interpretacion del concepto que (creo) que Ron quiso presentarnos. Pueden estas condiciones ser pensadas? Podrian simularse en un CAD de electromagnetismo, tal como FEMLAB? Estan ellas de acuerdo con la pregunta de Ron? Que dices tu Ron? Miguel Ghezzi (LU 6ETJ) PS: Tengo otra duda física si ustedes pueden ayudarme. Puede una CC real irradiar energía electromagnetica? No es una pregunta capciosa ni una broma. Yo creo que si, pero no quiero condicionar sus respuestas con mi hipotesis. Agradezco sus respuestas por adelantado. |
Antenna gain question
Richard Clark wrote: On 30 Oct 2005 15:50:50 -0800, "lu6etj" wrote: although that is not possible for the whole universe (I suppose this allows me to escape elegantly of Richard's question... ; D 73=B4s for all, and thank you very much for your very interesting and instructive habitual postings. Hi Miguel, Can there be an escape? Ron's question was posed with an impossible proposition. A collapsing sphere of electromagnetic energy? This has so many so many fantastic presumptions built in. On all but the smallest of scales, the sky is quite uniform in its luminosity. It can hardly be described as a "collapsing sphere"; not even from the point of view of a geocentric model of the universe. ac6xg |
Antenna gain question
On 31 Oct 2005 09:35:09 -0800, "lu6etj" wrote:
PS: I have another physical doubt, if you can help me. Can a real DC current radiate electromagnetic energy?. It is not captious [capricious] or cheat [trick] question. I think yes, but I don't want to condition your answers with my hypotesis. Hi Miguel, A "real" DC current? Yes. A "real" DC current (at some point in time) starts - and stops. It is at each of these two points that the step change offers radiation. The "time" it takes to go from one level to the other defines that frequency, and its harmonics. A "perfect" DC current has always been on, and will always be on. No change, no radiation. 73's Richard Clark, KB7QHC |
Antenna gain question
On Mon, 31 Oct 2005 10:16:07 -0800, Jim Kelley
wrote: On all but the smallest of scales, the sky is quite uniform in its luminosity. It can hardly be described as a "collapsing sphere"; not even from the point of view of a geocentric model of the universe. Hi Jim, Perhaps not, but "quite uniform" is rather in the eye of the beholder. When I take panagraphic photographs (a broad scale), it is quite evident that the uniformity is not very uniform. Another variable is that polarization is not very uniform either (which, photographically may be saying the same thing). The eye is a wonderful device, but not very precise. 73's Richard Clark, KB7QHC |
Antenna gain question
Richard Clark wrote: On Mon, 31 Oct 2005 10:16:07 -0800, Jim Kelley wrote: On all but the smallest of scales, the sky is quite uniform in its luminosity. It can hardly be described as a "collapsing sphere"; not even from the point of view of a geocentric model of the universe. Hi Jim, Perhaps not, but "quite uniform" is rather in the eye of the beholder. It's quite impossible to behold anywhere near the smallest of scales by eye, Richard. When I take panagraphic photographs (a broad scale), it is quite evident that the uniformity is not very uniform. Another variable is that polarization is not very uniform either (which, photographically may be saying the same thing). The eye is a wonderful device, but not very precise. It's like the internet in that regard, where people, with just the right amounts of terminology and pomposity, can assume the status of expert at just about anything and everything! ;-) ac6xg |
Antenna gain question
Hi, all folks and Dear Richard (Is right this salutation?)
Thanks for your patience to me... Imagine a simple DC generator in steady state, the closing circuit (sure, with a serie resistance, no short), forms a physical loop, the current travel accross it in a uniform circular movement, therefore charges have a centripet acceleration = charges accelerated = electromagnétic radiation. Electron orbiting nucleus problem - quantum theory solution. Am I in the correct way?. DC current produce electromagnetic radiation (on solenoid more, of course)? I never read something in such a sense (except in atomic theory, of course), but I find reasonable to suppose it. Thanks in advance Miguel Ghezzi (LU 6ETJ) ------------------------------------------------------------------------------------------------------------ Hola a todos y estimado Richard. (es correcto saludar asi?) Gracias por su paciencia. Imagine un simple generador de CC en regimen estacionario, el circuito que lo cierra (por supuesto con una resitenci, no en corto) forma un lazo físico. La corriente viaja a traves de el en un movimiento circular uniforme, por lo tanto las cargas poseen una aceleracion centripeta = cargas aceleradas = radiacion electromagnetica... El problema del electron orbitando el nucleo - solucion de la teoria cuantica. Estoy en lo correcto? La corriente continua produce radiacion electromagnetica (mas en un solenoide, por supuesto)? Nunca lei nada en tal sentido (excepto en la teoria atomica, por supuesto) pero me parece razonable suponerlo Miguel Ghezzi (LU 6ETJ) |
Antenna gain question
Question (repeated here for convenience):
-------------------------------------------------------------------- Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. --------------------------------------------------------------------- First, thanks for all the comments. They have helped me better understand the answer. I am leaning toward the belief that the omni (isotropic) antenna would capture more power and, as odd as it may seem, would have more gain than a high gain beam (or any other directional antenna for that matter). Here is my thinking: This is a very unusual RF field. Usually the field is assumed to be planar with coherent rays - then antennas behave as expected. But this field originates uniformly from all points on the surface of a sphere. It does not spread but converges at the focal point of the sphere. An isotropic antenna placed at the focal point would collect all of the rays whereas a directional antenna at would not. Therefore, in this particular situation, the isotropic would have higher gain and capture more power than any directional antenna. Please correct me if I am wrong. Ron, W4TQT |
Antenna gain question
On 31 Oct 2005 12:07:29 -0800, "lu6etj" wrote:
Hi, all folks and Dear Richard (Is right this salutation?) Hi Miguel, It is fine. Imagine a simple DC generator in steady state, the closing circuit (sure, with a serie resistance, no short), forms a physical loop, the current travel accross it in a uniform circular movement, therefore charges have a centripet acceleration =3D charges accelerated =3D electromagn=E9tic radiation. You are using acceleration in its usual sense. Unfortunately, it is based on a poor description for radiation. It is a poor description in English, or any language. Electron orbiting nucleus problem - quantum theory solution. Circular motion is always acceleration, and orbital electrons are always in circular motion. They are not always radiating. This one observation is enough to invalidate the general description of accelerating electrons causing radiation (it takes more than that). An orbital electron only radiates when it changes orbital levels to a LOWER orbit. Read about deBroglie waves. When an Hydrogen electron in the 3rd orbital falls (acceleration) to the 2nd orbital, it radiates a photon with a wavelength of 653 nM. You see this every night with Neon signs. Am I in the correct way?. DC current produce electromagnetic radiation (on solenoid more, of course)? No. I never read something in such a sense (except in atomic theory, of course), but I find reasonable to suppose it. Reasonable, as I have described above, but not logical. 73's Richard Clark, KB7QHC |
Antenna gain question
Question (repeated here for convenience):
-------------------------------------------------------------------- Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. --------------------------------------------------------------------- First, thanks for all the comments. They have helped me better understand the answer. I am leaning toward the belief that the omni (isotropic) antenna would capture more power and, as odd as it may seem, would have more gain than a high gain beam (or any other directional antenna for that matter). Here is my thinking: This is a very unusual RF field. Usually the field is assumed to be planar with coherent rays - then antennas behave as expected. But this field originates uniformly from all points on the surface of a sphere. It does not spread but converges at the focal point of the sphere. An isotropic antenna placed at the focal point would collect all of the rays whereas a directional antenna at would not. Therefore, in this particular situation, the isotropic would have higher gain and capture more power than any directional antenna. Please correct me if I am wrong. Well, for one thing, your model assumes something which does not and cannot exist. It assumes the existence of an actual isotropic antenna. Such cannot actually be constructed - there's no way to get a truly omnidirectional radiation pattern without violating Maxwell's equations. I suspect that you'll find the same problem existing, in the reverse direction, if you try to construct the sort of RF field you're talking about. If you try to specify the E-plane and H-plane field components for a uniform, arriving-from-all-points-of-a-sphere field, I believe that you'll find that you can't achieve your goal: there will always be "seams" (abrupt discontinuities or cancellations) in the field components in some directions. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Antenna gain question
Ron wrote: Question (repeated here for convenience): -------------------------------------------------------------------- Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. --------------------------------------------------------------------- First, thanks for all the comments. They have helped me better understand the answer. I am leaning toward the belief that the omni (isotropic) antenna would capture more power and, as odd as it may seem, would have more gain than a high gain beam (or any other directional antenna for that matter). Here is my thinking: This is a very unusual RF field. Usually the field is assumed to be planar with coherent rays - then antennas behave as expected. But this field originates uniformly from all points on the surface of a sphere. Uniformly inward, outward, or both? It does not spread but converges at the focal point of the sphere. By focal point of the sphere do you mean the center of the sphere? How big of a sphere are we talking about, and where is the antenna in relation to the sphere? An isotropic antenna placed at the focal point would collect all of the rays whereas a directional antenna at would not. Probably. Therefore, in this particular situation, the isotropic would have higher gain and capture more power than any directional antenna. Not according to the accepted use of the term 'gain' in connection with antennas. Please correct me if I am wrong. Ron, W4TQT In the instance you describe, the antenna with gain will pick up less signal than an antenna without gain. The gain antenna will be able to sense signal arriving from only a fraction of the sphere, whereas the isotropic antenna responds to signals arriving from the entire 4-pi sphere. Therefore, the antenna with less gain produces the greater signal level. But this should often be the case when a directional antenna is pointed away from most of the signal. The omni, on the other hand, is 'pointed toward' this particular signal in all directions. Out of curiosity, what kind of signal source are you interested in? ac6xg |
Antenna gain question
This was only a mental exercise to help me visualize the concept of
gain. No resemblance to a real antenna or RF field was intended. Thinking about it has helped me understand what antenna gain is (assuming my conclusions are correct). And that's all it was supposed to do. I hope it has helped someone else to do the same. Ron Jim Kelley wrote: Ron wrote: Question (repeated here for convenience): -------------------------------------------------------------------- Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. --------------------------------------------------------------------- First, thanks for all the comments. They have helped me better understand the answer. I am leaning toward the belief that the omni (isotropic) antenna would capture more power and, as odd as it may seem, would have more gain than a high gain beam (or any other directional antenna for that matter). Here is my thinking: This is a very unusual RF field. Usually the field is assumed to be planar with coherent rays - then antennas behave as expected. But this field originates uniformly from all points on the surface of a sphere. Uniformly inward, outward, or both? It does not spread but converges at the focal point of the sphere. By focal point of the sphere do you mean the center of the sphere? How big of a sphere are we talking about, and where is the antenna in relation to the sphere? An isotropic antenna placed at the focal point would collect all of the rays whereas a directional antenna at would not. Probably. Therefore, in this particular situation, the isotropic would have higher gain and capture more power than any directional antenna. Not according to the accepted use of the term 'gain' in connection with antennas. Please correct me if I am wrong. Ron, W4TQT In the instance you describe, the antenna with gain will pick up less signal than an antenna without gain. The gain antenna will be able to sense signal arriving from only a fraction of the sphere, whereas the isotropic antenna responds to signals arriving from the entire 4-pi sphere. Therefore, the antenna with less gain produces the greater signal level. But this should often be the case when a directional antenna is pointed away from most of the signal. The omni, on the other hand, is 'pointed toward' this particular signal in all directions. Out of curiosity, what kind of signal source are you interested in? ac6xg |
Antenna gain question
Ron wrote:
Please correct me if I am wrong. If the moon were made out of green cheese, then a cow could jump over it. That is a true statement, by definition. -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Richard Clark wrote:
An orbital electron only radiates when it changes orbital levels to a LOWER orbit. His question was about a conductive loop RF antenna. RF radiation from an antenna comes from free electrons. Their associated photon energy levels are correlated to the frequency of the excitation energy, not to changes in atomic orbits. -- 73, Cecil http://www.qsl.net/w5dxp |
Antenna gain question
Hi Richard
Thank you for your answer. You are quite right Richard, this is the great postulate of Bohr, but from: physics.indiana.edu/~sg/p641/chap2.ps I have extracted this paragraph (rememember that I did study from books in spanish, so I had to search the web to find a suitable english references.) "...Some physicists of the time did not at first believe in Rutherford's model of the atom because, classically, such a model is unstable. The argument is as follows: Electrons in orbits around a nucleus undergo acceleration. But accelerating charged particles emit electromagnetic radiation, losing energy. Therefore, electrons orbiting a nucleus should lose energy and spiral into nucleus. Then in 1913 Bohr solved the problem by simply postulating that electrons move around the nucleus in certain stationary orbits without emitting radiation. According to Bohr¡s model electrons emit radiation as photons only when making a transition from one stationary orbit with a higher energy to another stationary orbit with a lower energy." Bohr's postulate would not invalidate the postulate of the classic electromagnetism that a accelerated charge irradiates energy in classical conditions. Their postulate refers to an electron in an atomic orbital, a typical quantum situation, the electrons in a circular wire would be being part of a classic macroscopic system, (I think). In such a case, would not emit energy, according to that pointed out in the cited Indiana's university text ? I can think a more elaborated example of Direct Current to avoid derived issues of the electronic movement in a complicated crystalline structu An electron (or current of electrons) inside a magnetic field in the vacuum, just as in a cyclotron... Would it be rightfully an Direct Current?. Does it fulfill the postulates of the classic physics? Does it irradiate electromagnetic energy? For these reasons, I think that DC irradiates electromagnetic energy (a very, very, very little quantity, of course. This ponderings rises in my mind from a friend's question about an eternal current in a superconductor ring. What do you think about? Thank you very much for your corrections to my translations... Miguel Ghezzi (LU 6ETJ) --------------------------------------------------------------------------------------------------------- Hola Richard. Gracias por tu respuesta. Tienes razon Richard, ese es el gran postulado de Bohr, pero de: physics.indiana.edu/~sg/p641/chap2.ps Extraje este parrafo (recuerda que yo estudie de libros en espaniol de manera que tuve que buscar en la web para encontrar referencias adecuadas en ingles). [Original english text not translated...] El postulado de Bohr no invalidaría los postulados del electromagnetismo clasico que una carga acelereada irradia energia en condiciones clasicas. su postulado se refiere a un electron en un orbital atomico, una típica situacion cuantica. Los electrones en un conductor circular estarían formando parte de un sistema macroscopico clasico, creo. En tal caso, no emitirian energia de acuerdo a lo senialado en el texto de la Universidad de Indiana citado? Puedo pensar en un ejemplo mas elaborado de corriente continua para evitar las cuestiones derivadas del movimiento electronico en una complicada estructura cristalina: Un electron (o corriente de electrones) dentro de un campo magnetico en el vacio, como en un ciclotron. Sería legitimamente una corriente conínua? Cumple con los postulados de la fisica clasica? Irradia energia electromagnetica? Por estas razones creo que una CC irradia energia electromagnetica (una muy, muy, muy pequenia cantidad, por supuesto). Estas cavilaciones surgen en mi mente a partir de la pregunta de un amigo acerca de una corriente eterna en un anillo superconductor. Que piensas? Miguel Ghezzi (LU 6ETJ) |
Antenna gain question
On 31 Oct 2005 19:02:35 -0800, "lu6etj" wrote:
What do you think about [it]? Thank you very much for your corrections to my translations... Hi Miguel, You do very well with English. If you wish to be very, very, very specific, then yes DC will radiate. Let's take something a little more practical than a cyclotron - the monitor you are looking at to read this. The CRT is accelerating an electron to strike a phosphor to illuminate a pixel. We will neglect that light radiation as not being part of the discussion. That electron will be accelerated by a field of some 20KV. What is its frequency? 34.6 exaHertz the wavelength of roughly 1/10 the distance of an electron orbit around a Hydrogen atom. Now, for something completely different. Does a car driving 40 KM/Hour down the road radiate? If you wish to be very, very, very specific, then yes the car will radiate by the same principle at 5 zetta-yottaHertz 73's Richard Clark, KB7QHC |
Antenna gain question
One more thing: Before thinking about all this, I always thought that
since a high gain antenna has a narrower beam than a lower gain antenna, the high gain antenna "sees" a smaller part of the incoming field. I now believe this is wrong. The higher gain antenna sees a larger field area. But as the antenna is rotated the sum of all the rays decreases faster than if there were fewer of them. This is probably due to the rays from the outer edge of the field causing a faster decrease in the coherent summation of all rays than the closer in rays. Of course, as the rotation is continued, many (but not as many) of the rays add coherently again, giving rise to the side lobes. Ron, W4TQT Ron wrote: This was only a mental exercise to help me visualize the concept of gain. No resemblance to a real antenna or RF field was intended. Thinking about it has helped me understand what antenna gain is (assuming my conclusions are correct). And that's all it was supposed to do. I hope it has helped someone else to do the same. Ron |
Antenna gain question
On Tue, 01 Nov 2005 16:09:22 GMT, Ron wrote:
I now believe this is wrong. Hi Ron, Such is the problem of your scenario if it lead you here. 73's Richard Clark, KB7QHC |
Antenna gain question
Hi Richard
Tanks for your last answer. Hi hi, well, my friend was very, very very insistent also with his superconducting and eternal current holder "golden ring"... I will try learn more about [it] ;) (I have been hard working for thirty years, and till now, I haven't any time to meditate in all these beautiful things). I am very pleased to know you, Richard (my other ham friends it isn't "very interested" in this type of "exotic questions"... :) I can see this usenet group always talk about very interesting topics, with great knowledge and good "ham spirit". 73's Miguel Ghezzi (LU 6ETJ) |
Antenna gain question
Ron wrote:
This was only a mental exercise to help me visualize the concept of gain. No resemblance to a real antenna or RF field was intended. Thinking about it has helped me understand what antenna gain is (assuming my conclusions are correct). And that's all it was supposed to do. I hope it has helped someone else to do the same. Ron I think it was a good exercise, Ron - not unlike the kind seen in a good text book. My response at the bottom presumed some things about the nature of the sphere that were somewhat unclear in your message. I hope I presumed correctly. 73, ac6xg Jim Kelley wrote: Ron wrote: Question (repeated here for convenience): -------------------------------------------------------------------- Assume a receiving antenna is in the center of a sphere and the received signal is coming in equal amounts from all points on the surface of the sphere. Which receiving antenna would capture more power, an omni or a high gain beam? There are no noise and no losses. --------------------------------------------------------------------- First, thanks for all the comments. They have helped me better understand the answer. I am leaning toward the belief that the omni (isotropic) antenna would capture more power and, as odd as it may seem, would have more gain than a high gain beam (or any other directional antenna for that matter). Here is my thinking: This is a very unusual RF field. Usually the field is assumed to be planar with coherent rays - then antennas behave as expected. But this field originates uniformly from all points on the surface of a sphere. Uniformly inward, outward, or both? It does not spread but converges at the focal point of the sphere. By focal point of the sphere do you mean the center of the sphere? How big of a sphere are we talking about, and where is the antenna in relation to the sphere? An isotropic antenna placed at the focal point would collect all of the rays whereas a directional antenna at would not. Probably. Therefore, in this particular situation, the isotropic would have higher gain and capture more power than any directional antenna. Not according to the accepted use of the term 'gain' in connection with antennas. Please correct me if I am wrong. Ron, W4TQT In the instance you describe, the antenna with gain will pick up less signal than an antenna without gain. The gain antenna will be able to sense signal arriving from only a fraction of the sphere, whereas the isotropic antenna responds to signals arriving from the entire 4-pi sphere. Therefore, the antenna with less gain produces the greater signal level. But this should often be the case when a directional antenna is pointed away from most of the signal. The omni, on the other hand, is 'pointed toward' this particular signal in all directions. Out of curiosity, what kind of signal source are you interested in? ac6xg |
Antenna gain question
Ron wrote:
One more thing: Before thinking about all this, I always thought that since a high gain antenna has a narrower beam than a lower gain antenna, the high gain antenna "sees" a smaller part of the incoming field. I now believe this is wrong. The higher gain antenna sees a larger field area. Hopefully no one else was led to that belief by the exercise. But as the antenna is rotated the sum of all the rays decreases faster than if there were fewer of them. This is probably due to the rays from the outer edge of the field causing a faster decrease in the coherent summation of all rays than the closer in rays. Of course, as the rotation is continued, many (but not as many) of the rays add coherently again, giving rise to the side lobes. Such a claim might be remotely plausible were it not for the fact that rotating a directional antenna does not "coherently sum all the rays". That's where the argument completely falls to the ground, as Monty Python might say. ac6xg Ron, W4TQT Ron wrote: This was only a mental exercise to help me visualize the concept of gain. No resemblance to a real antenna or RF field was intended. Thinking about it has helped me understand what antenna gain is (assuming my conclusions are correct). And that's all it was supposed to do. I hope it has helped someone else to do the same. Ron |
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