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Image theory
Image theory is used to justify the idea that a ground plane reflects the
radio wave emitted by a vertical monopole antenna. The radio wave emitted by the vertical induces currents in the metal ground plane. A charge on the vertical induces an opposite charge in the ground plane. If charge on end of vertical is +q, then it induces a charge of -q in the ground plane. The effect is that the charge -q appears to be a distance below the ground, the distance being equal to the distance between ground and charge +q above ground. A distibution of charge is induced in the ground plane by the radiating vertical. With a dipole the electric field lines go from +q to -q. If a ground plane is inserted at the zero or middle point, with charge below ground plane removed, the electric field lines above the ground plane stay the same. The metal ground plane is a pool of electrons that adjust so that the voltage at the surface is zero i.e. tangent E = 0. Four radials form a counterpoise. The counterpoise has currents induced. These induced currents then re-radiate, altering the radiation pattern and inducing currents back in the vertical. The induced currents in vertical then affect antenna impedance. A counterpoise is a metal conductor that has currents induced in it by the radiating element. The currents re-radiate resulting in a field distribution where the countrpoise is a mirror image or opposite version of the radiating element. What surprises me is the claim that the ground plane can mirror a 3D image e.g. a stacked Yagi. What are your views on above? Why do some articles say that the ground plane needs to be connected to the outer braid of coax, while others says this is not necessary? |
Image theory
Hooo boy, big can of worms... Watching this oughtta be f u n...
denny / k8do David wrote: |
Image theory
David wrote:
Four radials form a counterpoise. The counterpoise has currents induced. These induced currents then re-radiate, ... Let's look at two equal radials, 180 degrees apart, horizontal in free space. The radial currents are equal in magnitude and opposite in direction, i.e. differential. Theoretically, this causes considerable cancellation of the radiating fields of those two radials. EZNEC confirms very little horizontal radiation from two symmetrical horizontal radials in free space. The same thing holds true for symmetrical top hats. -- 73, Cecil http://www.w5dxp.com |
Image theory
Hi David,
My advice would be to expand your reference library. One can't expect to accurately describe an elephant after seeing only its tail. ac6xg David wrote: Image theory is used to justify the idea that a ground plane reflects the radio wave emitted by a vertical monopole antenna. The radio wave emitted by the vertical induces currents in the metal ground plane. A charge on the vertical induces an opposite charge in the ground plane. If charge on end of vertical is +q, then it induces a charge of -q in the ground plane. The effect is that the charge -q appears to be a distance below the ground, the distance being equal to the distance between ground and charge +q above ground. A distibution of charge is induced in the ground plane by the radiating vertical. With a dipole the electric field lines go from +q to -q. If a ground plane is inserted at the zero or middle point, with charge below ground plane removed, the electric field lines above the ground plane stay the same. The metal ground plane is a pool of electrons that adjust so that the voltage at the surface is zero i.e. tangent E = 0. Four radials form a counterpoise. The counterpoise has currents induced. These induced currents then re-radiate, altering the radiation pattern and inducing currents back in the vertical. The induced currents in vertical then affect antenna impedance. A counterpoise is a metal conductor that has currents induced in it by the radiating element. The currents re-radiate resulting in a field distribution where the countrpoise is a mirror image or opposite version of the radiating element. What surprises me is the claim that the ground plane can mirror a 3D image e.g. a stacked Yagi. What are your views on above? Why do some articles say that the ground plane needs to be connected to the outer braid of coax, while others says this is not necessary? |
Image theory
On Wed, 13 Dec 2006 13:18:21 -0000, "David" nospam@nospam wrote:
Image theory is used to justify the idea that a ground plane reflects the radio wave emitted by a vertical monopole antenna. Hi Dave, Pretty crummy theory, isn't it? You should consider better sources of information. 73's Richard Clark, KB7QHC |
Image theory
The books also say that if the antenna is vertical to groundplane the iamge
is in phase and reinforces the wave emitted by vertical. If antenna is horizontal and close to ground plane, the image cancels out wave emitted by antenna. Is this what happens in real life? It is known that if the 2 conductors of twin feeder are close together, the fields cancel out and there is very little radiation. Are you saying that electromagnetic theory is wrong? What about fibre optic cable? If ordinary glass strands were used, when the cable was bent, the light would be blocked and not travel any further. |
Image theory
David wrote:
Are you saying that electromagnetic theory is wrong? Without a quote, nobody can tell to whom you are posing that question. My threaded newsreader says you are replying to your original posting. -- 73, Cecil http://www.w5dxp.com |
Image theory
On Wed, 13 Dec 2006 22:44:38 -0000, "David" nospam@nospam wrote:
The books also say If antenna is horizontal and close to ground plane, the image cancels out wave emitted by antenna. Hi David, Your sources are pretty crummy. This information is quite in contradiction with the operation of a halfwave horizontal a quarterwave above earth, isn't it? A simple EZNEC model will reveal this clearly. Is this what happens in real life? Umm, no. It is known that if the 2 conductors of twin feeder are close together, the fields cancel out and there is very little radiation. Not the same thing. Are you saying that electromagnetic theory is wrong? Who is "you?" What about fibre optic cable? If ordinary glass strands were used, when the cable was bent, the light would be blocked and not travel any further. I've worked with a lot of fiber optics, and that statement simply doesn't make much sense. Ordinary and un-ordinary glass (as well as plastic) strands work differently by degree, not differently by theory. 73's Richard Clark, KB7QHC |
Image theory
The image "theory" is a simplification of a problem involving an antenna
over an infinite, perfect, ground plane. Four radials behave nothing like an infinite, perfect ground plane, so the premises upon which the image model is based do not apply. Trying to extend it to such systems as four radials will inevitably lead to seriously mistaken conclusions. The claim about stacked Yagis is just one example. Roy Lewallen, W7EL David wrote: Image theory is used to justify the idea that a ground plane reflects the radio wave emitted by a vertical monopole antenna. The radio wave emitted by the vertical induces currents in the metal ground plane. A charge on the vertical induces an opposite charge in the ground plane. If charge on end of vertical is +q, then it induces a charge of -q in the ground plane. The effect is that the charge -q appears to be a distance below the ground, the distance being equal to the distance between ground and charge +q above ground. A distibution of charge is induced in the ground plane by the radiating vertical. With a dipole the electric field lines go from +q to -q. If a ground plane is inserted at the zero or middle point, with charge below ground plane removed, the electric field lines above the ground plane stay the same. The metal ground plane is a pool of electrons that adjust so that the voltage at the surface is zero i.e. tangent E = 0. Four radials form a counterpoise. The counterpoise has currents induced. These induced currents then re-radiate, altering the radiation pattern and inducing currents back in the vertical. The induced currents in vertical then affect antenna impedance. A counterpoise is a metal conductor that has currents induced in it by the radiating element. The currents re-radiate resulting in a field distribution where the countrpoise is a mirror image or opposite version of the radiating element. What surprises me is the claim that the ground plane can mirror a 3D image e.g. a stacked Yagi. What are your views on above? Why do some articles say that the ground plane needs to be connected to the outer braid of coax, while others says this is not necessary? |
Image theory
"David" nospam@nospam wrote in message
... The books also say that if the antenna is vertical to groundplane the iamge is in phase and reinforces the wave emitted by vertical. If antenna is horizontal and close to ground plane, the image cancels out wave emitted by antenna. Is this what happens in real life? Yes, once you throw in the (sometimes negligible, sometimes significant) complications that real "ground planes" are finite in extent and have non-zero loss. What about fibre optic cable? If ordinary glass strands were used, when the cable was bent, the light would be blocked and not travel any further. Fiber optics aren't antennas; fiber optic "cables" are low loss (ideally lossless) dielectrics clad in another dielectric with a different enough permittivity to create total internal reflection. Using traditional terminology, fiber optics are actually just a particular type of waveguide. |
Image theory
But the academic books on Antenna theory written by Professors of
Electromagnetics all use image theory for vertical monopole antennas including those with elevated radials. Can anyone quote an antenna theory book that does not? |
Image theory
On Fri, 22 Dec 2006 17:44:41 -0000, "David" nospam@nospam wrote:
But the academic books on Antenna theory written by Professors of Electromagnetics all use image theory for vertical monopole antennas including those with elevated radials. Can anyone quote an antenna theory book that does not? Hi David, The objection arises out of your commingling radials in the discussion where the dons never asserted an image theory for them. Can you quote any source that does? (Let's try proving a positive.) 73's Richard Clark, KB7QHC |
Image theory
On Fri, 22 Dec 2006 10:08:03 -0800, Richard Clark
wrote: On Fri, 22 Dec 2006 17:44:41 -0000, "David" nospam@nospam wrote: But the academic books on Antenna theory written by Professors of Electromagnetics all use image theory for vertical monopole antennas including those with elevated radials. Can anyone quote an antenna theory book that does not? Hi David, The objection arises out of your commingling radials in the discussion where the dons never asserted an image theory for them. Can you quote any source that does? (Let's try proving a positive.) Let's just cut to the chase, vis-a-vis radials. These elements serve to balance and match, not to propagate (in the sense of ground reflections). In fact, when all the radiative contributions of a ground plane (radials in a plane) are considered, they are self negating. If we were to consider the aspect of this image theory (reflection of a wave); then those radials would have to consume both a lot of distance out from the feed point, and a lot of real estate. By any standards found in the market place, or in implementation, this is so rare as to be exceedingly exceptional. Commercial AM antenna radial fields DO NOT come close to this either. 73's Richard Clark, KB7QHC |
Image theory
David wrote:
But the academic books on Antenna theory written by Professors of Electromagnetics all use image theory for vertical monopole antennas including those with elevated radials. Can anyone quote an antenna theory book that does not? Kraus, _Antennas_ Johnson, _Antenna Engineering Handbook_ Lo and Lee, _Antenna Handbook_ Balanis, _Antenna Theory_ King and Harrison, _Antennas and Waves_ Jordan & Balmain, _Elecromagnetic Waves and Radiating Systems_ King, Mimno, and Wing, _Transmission Lines, Antennas, and Wave Guides_ Brown, Sharpe, Hughes, and Post, _Lines, Waves, and Antennas_ Stutzman & Thiele, _Antenna Theory and Design_ to name a few. I'm sure there are many more, but those are the ones currently on my shelf. Several of these authors use an image to analyze a vertical antenna over a perfectly conducting ground of infinite extent. None use it for any other situation, including elevated radials. And for a very good reason -- it's not valid for other situations. Roy Lewallen, W7EL |
Followon to image theory
Various books claim that a ground plane reflects the radio wave emitted by
the vertical, and then claim that a ground plane is formed or simulated by four elevated radials. I emailed two Professors of antenna theory about this. Reply from Professor Constantine Balanis: "The radials should act more as a ground plane. Four of them are usually the minimum. The more of them, the better the ground plane. The objective of the ground plane is to reflect the energy from the main element; the vertical wire". Reply from Professor Vincent Fusco: "My view would be that the radials form an image plane, the radials themselves do not radiate". The following link on Navy Training Series show the radials as reflecting the radio wave and forming an image antenna: http://www.tpub.com/content/neets/14.../14182_219.htm Quotations from books below. Antenna Theory third edition by C.A. Balanis page 205 "Improvements in the efficiency can be obtained by placing radial wires or metallic disks on the ground. [New paragragh] The analytical procedures that are introduced to examine the ground effects are based on the geometrical optics models of the previous sections. The image (virtual) source is again placed a distance h below the interface to account for the reflection." Antenna Theory third edition by C.A. Balanis page 511 "To reduce the wind resistance, to simplify the design, and to minimise the costs, a ground plane is often simulated, especially at low frequencies, by crossed wires as shown in Figure 9.11(b). Usually only two crossed wires (four radials) are employed. A larger number of radials results in a better simulation of the ground plane." Antenna Theory and Design by Stutzman and Thiele page 198: "A ground plane can take many forms, such as radial wires around a monopole, the roof of a car, or the real earth....The pattern of an antenna over a real earth is different from the pattern when the antenna is operated over a perfect ground plane. Approximate patterns can be obtained by using image theory. The same principles discussed in Sec 2.3.1 for images in perfect ground planes apply, except that the strength of the image in a real ground will be reduced from that of the perfect ground plane case". Antenna Theory and Design by Stutzman and Thiele pages 66 & 67: "The principles of image theory are illustrated in this section with several forms of the monopole antenna. ... Figure 2-12(c) Practical monopole antenna with radial wires to simulate a ground plane". Antennas for all applications third edition by J.D. Kraus page 719: "In Fig 21-9d the solid-sheet ground plane is replaced by 4 radial conductors. Fig 21-9(d) stub antenna with 4 radial conductors to simulate a ground plane". Antenna Engineering Handbook by R.C. Johnson page 110: "The geometrical shape of a sleeve antenna, or a sleeve monopole, is sketched in Fig. 4-22a. If the image of the structure is included, then we have a sleeve dipole as shown in Fig. 4-22b". Figure 4-22a shows a coax cable with the centre wire going upwards forming a vertical element, and the braid being connected to a ground plane. Fig 4-22b appears to show a dipole being formed from a monopole and its image antenna. Foundations of Antenna Theory and Techniqes by Professor Vincent Fusco pages 190 and 191. Claims that image theory i.e. image cancellation applies to the horizontal section of an inverted L antenna. The vertical part of inverted L antenna is a short monopole i.e. less than a quarterwave long. However references against this are below. Antenna Engineering Handbook by R.C. Johnson page 118: "When the ground plane is of finite size, the image theorem does not apply." Professor Douglas Miron in his book Small Antenna Design says that the vertical element is driven against some horizontal elements. The horizontal elements have canceling fields. He says that the radials are an artificial ground called a counterpoise that also radiates. Various books also say about using image theory to analyse corner reflector antennas. RF experts appear to disagree with each other over the vertical monopole with quarterwave radials - Roy Lewallen and Professor Douglas Miron versus Professor Constantine Balanis and Professor Vincent Fusco. |
Followon to image theory
David wrote:
RF experts appear to disagree with each other over the vertical monopole with quarterwave radials - Roy Lewallen and Professor Douglas Miron versus Professor Constantine Balanis and Professor Vincent Fusco. Exactly what is it upon which you think Roy Lewallen and Constantine Balanis disagree? -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
David wrote:
Various books claim that a ground plane reflects the radio wave emitted by the vertical, and then claim that a ground plane is formed or simulated by four elevated radials. [big snip] David, The images in these cases are not real. That's why they are called virtual. However, real or not, these images can provide useful assistance in simple determination of the expected radiation patterns. Like most models, the image model has limitations. Some applications will succumb to image modeling more readily than others. Bottom line: There is no real conflict among the experts. Image modeling is generally quite easy. If the results make sense, be happy. If not, then try more complex methods. I would expect all of the experts to agree that detailed mathematical modeling is required for the best accuracy unless the image model has unrealistic parameters such as infinite extent, perfect conductivity, etc. 73, Gene W4SZ |
Followon to image theory
On Sun, 7 Jan 2007 19:17:55 -0000, "David" nospam@nospam wrote:
Various books claim that a ground plane reflects the radio wave emitted by the vertical, and then claim that a ground plane is formed or simulated by four elevated radials. I emailed two Professors of antenna theory about this. Reply from Professor Constantine Balanis: "The radials should act more as a ground plane. Four of them are usually the minimum. The more of them, the better the ground plane. The objective of the ground plane is to reflect the energy from the main element; the vertical wire". Hi David, This is becoming tiresome by half. "Quotes" that do not have a context (what was the question asked?) are more name dropping than an informed discussion. It is also called shopping for testimony. "Does a ground plane reflect?" This is a loaded question much like "Have the Republicans offered a balanced budget?" Each question can be so heavily qualified as to guarantee success and bragging rights. Each can be so heavily doped up on the narcotic of self-indulgence as to offer only the prospects of a somnambulistic nod of affirmation. If one conspired to erect a 2M vertical on the radial field of a former AM station; then, yes, these radials reflect admirably. If one takes the same 2M vertical (as shipped from, say, Comet) and looked at the reflection components of its radials; then, yes, these radials reflects, but with less than pedestrian results. Difference is found in the application. The AM station ground field will produce more 2M reflection contacts to the horizon, the standard radials of a 2M Comet will have indifferent reflection contacts oriented towards satellites. Simple modeling and analysis reveals this in less than an hour's work. More effort in the field confirms the modeling and analysis. Modeling and analysis conform to every expectation offered by all the authors quoted, given they use the same tools and were, perhaps, instrumental in their development. The "Image" theory relates to reflections found in the far field beyond the physical extent of practical radials. To force the standard implementations of radials into supporting this theory (to the exclusion of the greater application), debases the intent of the theory. Actually, to call it a theory is painting the rose and gilding the lily. This tarted up observation is used mainly to soothe the troubled minds of neophytes until they become accustomed to dealing with larger, more involved problems. It is a suitable metaphor; however, metaphors are the weakest form of argument as they often fail early on close examination. 73's Richard Clark, KB7QHC |
Followon to image theory
David wrote:
Various books claim that a ground plane reflects the radio wave emitted by the vertical, and then claim that a ground plane is formed or simulated by four elevated radials. I emailed two Professors of antenna theory about this. Reply from Professor Constantine Balanis: "The radials should act more as a ground plane. Four of them are usually the minimum. The more of them, the better the ground plane. The objective of the ground plane is to reflect the energy from the main element; the vertical wire". Reply from Professor Vincent Fusco: "My view would be that the radials form an image plane, the radials themselves do not radiate". The following link on Navy Training Series show the radials as reflecting the radio wave and forming an image antenna: http://www.tpub.com/content/neets/14.../14182_219.htm Quotations from books below. Antenna Theory third edition by C.A. Balanis page 205 "Improvements in the efficiency can be obtained by placing radial wires or metallic disks on the ground. [New paragragh] The analytical procedures that are introduced to examine the ground effects are based on the geometrical optics models of the previous sections. The image (virtual) source is again placed a distance h below the interface to account for the reflection." Antenna Theory third edition by C.A. Balanis page 511 "To reduce the wind resistance, to simplify the design, and to minimise the costs, a ground plane is often simulated, especially at low frequencies, by crossed wires as shown in Figure 9.11(b). Usually only two crossed wires (four radials) are employed. A larger number of radials results in a better simulation of the ground plane." Antenna Theory and Design by Stutzman and Thiele page 198: "A ground plane can take many forms, such as radial wires around a monopole, the roof of a car, or the real earth....The pattern of an antenna over a real earth is different from the pattern when the antenna is operated over a perfect ground plane. Approximate patterns can be obtained by using image theory. The same principles discussed in Sec 2.3.1 for images in perfect ground planes apply, except that the strength of the image in a real ground will be reduced from that of the perfect ground plane case". Antenna Theory and Design by Stutzman and Thiele pages 66 & 67: "The principles of image theory are illustrated in this section with several forms of the monopole antenna. ... Figure 2-12(c) Practical monopole antenna with radial wires to simulate a ground plane". Antennas for all applications third edition by J.D. Kraus page 719: "In Fig 21-9d the solid-sheet ground plane is replaced by 4 radial conductors. Fig 21-9(d) stub antenna with 4 radial conductors to simulate a ground plane". Antenna Engineering Handbook by R.C. Johnson page 110: "The geometrical shape of a sleeve antenna, or a sleeve monopole, is sketched in Fig. 4-22a. If the image of the structure is included, then we have a sleeve dipole as shown in Fig. 4-22b". Figure 4-22a shows a coax cable with the centre wire going upwards forming a vertical element, and the braid being connected to a ground plane. Fig 4-22b appears to show a dipole being formed from a monopole and its image antenna. Foundations of Antenna Theory and Techniqes by Professor Vincent Fusco pages 190 and 191. Claims that image theory i.e. image cancellation applies to the horizontal section of an inverted L antenna. The vertical part of inverted L antenna is a short monopole i.e. less than a quarterwave long. However references against this are below. Antenna Engineering Handbook by R.C. Johnson page 118: "When the ground plane is of finite size, the image theorem does not apply." Professor Douglas Miron in his book Small Antenna Design says that the vertical element is driven against some horizontal elements. The horizontal elements have canceling fields. He says that the radials are an artificial ground called a counterpoise that also radiates. Various books also say about using image theory to analyse corner reflector antennas. RF experts appear to disagree with each other over the vertical monopole with quarterwave radials - Roy Lewallen and Professor Douglas Miron versus Professor Constantine Balanis and Professor Vincent Fusco. Hi David, What does Aristotle have to say on this subject? Quoting from authorities may have been good enough for the ancients, but it doesn't have much value here. You have to descend one level of abstraction down, and use what you should know about electromagnetic theory in order to understand what's going on here. If you did, I don't think you'd see any serious disagreement between the people you've quoted. 73, Tom Donaly, KA6RUH |
Followon to image theory
The responses from the professors frankly baffle me. If that's what they
said, and if they meant what you think they mean, then yes, I disagree with the professors. Folks will have to decide whether to believe them, or me, or learn more about antenna operation so they can come to their own informed conclusions. I won't comment on the Navy training manual, recalling some of the simplifications made in the equivalent Air Force documents in an effort to make electronics understandable by the target audience. Roy Lewallen, W7EL David wrote: Various books claim that a ground plane reflects the radio wave emitted by the vertical, and then claim that a ground plane is formed or simulated by four elevated radials. I emailed two Professors of antenna theory about this. Reply from Professor Constantine Balanis: "The radials should act more as a ground plane. Four of them are usually the minimum. The more of them, the better the ground plane. The objective of the ground plane is to reflect the energy from the main element; the vertical wire". Reply from Professor Vincent Fusco: "My view would be that the radials form an image plane, the radials themselves do not radiate". . . . |
Followon to image theory
"Roy Lewallen" wrote in message ... The responses from the professors frankly baffle me. If that's what they said, and if they meant what you think they mean, then yes, I disagree with the professors. Folks will have to decide whether to believe them, or me, or learn more about antenna operation so they can come to their own informed conclusions. I won't comment on the Navy training manual, recalling some of the simplifications made in the equivalent Air Force documents in an effort to make electronics understandable by the target audience. Roy Lewallen, W7EL I am also very familiar with the Military's method of training. even the FAA does the same thing. They are trying to train a group of people that just walked in off the street and a test said they had an aptitude to learn electronics. In reality they may have no previous knowledge at all. All of this training must take place in a few months so shortcuts in explaining it may be taken, math used is often limited to an 8th grade level. Believe me military training manuals are not the gospel of electronics but they are a very good place to start without a pre-engineering prerequisit. |
Followon to image theory
Jimmie D wrote:
Believe me military training manuals are not the gospel of electronics but they are a very good place to start without a pre-engineering prerequisit. Does military current still flow from minus to plus? -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
"Cecil Moore" wrote in message t... Jimmie D wrote: Believe me military training manuals are not the gospel of electronics but they are a very good place to start without a pre-engineering prerequisit. Does military current still flow from minus to plus? -- 73, Cecil http://www.w5dxp.com They did in the mid 70s. We have a newbie at work that just got out of the Navy and was showing me his training books, I will ask him.I always considered flow from + to - becasue it make dealing with solidstate easier even though I was taught - to +. Also I always remembered they said it flowed from - to + to answer their irrelavent questions.Last I heard an electic current was a flow of cuurent carriers tha tcould be electrons or holes flowing in opposite direction, to me it is mox nix. Jimmie |
Followon to image theory
When I read many academic books on antenna theory, most of them appear to
say that the radials reflect the radio wave emitted by the vertical. I do not think that I am reading the books out of context. The replies from two professors of antenna theory indicate that they believe that the radials reflect the radio wave and form a mirror image. Looking at the vertical monopole with four radials, I suspect that Douglas Miron is correct in his book. He says that the radials radiate but have horizontal cancelling fields. I would expect the radials to radiate because current is forced through them by the RF generator. The current splits between the radials, thus causing the fields to cancel in the horizontal direction. The fields probably have an effect in the vertical direction, causing a change in radiation pattern and affecting angle of radiation. I am not sure whether the return current through the radials causes any form of mirror image. On a PCB track above a ground plane, the current returns through the ground plane via the path of least impedance which is often directly underneath the PCB track. The PCB track is the positive voltage side. The ground plane current is the 0V side. It is said that the current in the ground plane mirrors that flowing through the PCB track. |
Followon to image theory
Elevated radials do radiate. However, assuming a symmetrical structure,
the radiation is minimal (assuming there's another normally radiating conductor in the system) because the fields from the wires nearly cancel in all directions. At right angles to the plane of the radials and along the center line, the cancellation is complete and radiation zero -- again assuming a perfectly symmetrical structure. In a typical ground plane vertical system, the contribution to the total field from the radials is minimal, in any direction. Anyone interested in the pattern could model a radial structure by itself, with a source in each radial wire at the common junction. In the absence of loss, the model would radiate the entire amount of power fed to it, so it wouldn't be representative of the amount of contribution to the total field which the radials provide in a conventional ground plane type antenna. But it would show you the shape of the field radiated from the radials. I did this out of curiosity for two radials -- the pattern is a volume of revolution of a cloverleaf 2D pattern. The radiation from buried radials is much less yet due to the lossiness of the ground. There's no point in my commenting further on the concept of a few radials as a "mirror". Roy Lewallen, W7EL David wrote: When I read many academic books on antenna theory, most of them appear to say that the radials reflect the radio wave emitted by the vertical. I do not think that I am reading the books out of context. The replies from two professors of antenna theory indicate that they believe that the radials reflect the radio wave and form a mirror image. Looking at the vertical monopole with four radials, I suspect that Douglas Miron is correct in his book. He says that the radials radiate but have horizontal cancelling fields. I would expect the radials to radiate because current is forced through them by the RF generator. The current splits between the radials, thus causing the fields to cancel in the horizontal direction. The fields probably have an effect in the vertical direction, causing a change in radiation pattern and affecting angle of radiation. I am not sure whether the return current through the radials causes any form of mirror image. On a PCB track above a ground plane, the current returns through the ground plane via the path of least impedance which is often directly underneath the PCB track. The PCB track is the positive voltage side. The ground plane current is the 0V side. It is said that the current in the ground plane mirrors that flowing through the PCB track. |
Followon to image theory
Cecil Moore wrote:
Does military current still flow from minus to plus? I believe electron current still does. But it doesn't really matter. It's just that I really hate having to explain thermionic emission as a process whereby heating the cathode causes charge to be emitted from the anode. :-) 73 de ac6xg |
Followon to image theory
Jim Kelley wrote:
Cecil Moore wrote: Does military current still flow from minus to plus? I believe electron current still does. But it doesn't really matter. It's just that I really hate having to explain thermionic emission as a process whereby heating the cathode causes charge to be emitted from the anode. :-) Just throw in a little relativity. -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
"Roy Lewallen"
Elevated radials do radiate. However, assuming a symmetrical structure, the radiation is minimal (assuming there's another normally radiating conductor in the system) because the fields from the wires nearly cancel in all directions. At right angles to the plane of the radials and along the center line, the cancellation is complete and radiation zero -- again assuming a perfectly symmetrical structure. In a typical ground plane vertical system, the contribution to the total field from the radials is minimal, in any direction. ________________ Here is a link to a NEC-2 study I did about a month ago for a broadcast board, based on this configuration. Roy, I'd be grateful for your comments if you see anything you don't agree with there. http://www.freefilehosting.org/publi...60%20Plots.pdf RF |
Followon to image theory
With a vertical monopole antenna, the field emitted by the radials forms a
near field and interacts with the wave radiated by the vertical element. Is it mainly in the vertical direction that the radial fields interact with the field from vertical element? I would expect the antenna feedpoint impedance to be formed from the length of antenna conductors and the interaction of the fields. If a monopole is simulated in a NEC program above a perfect ground plane, is it possible to see the reflection? I would expect NEC program to show reflection of waves from metallic surfaces e.g reflection from perfect ground plane or parabolic dish. Can a NEC program allow the user to see whether the wave is reflected (as for perfect ground plane) or whether effect is due to wave interaction (as for radials)? Can the user then vary a finite size ground plane to see whether antenna impedance comes from reflection or wave interaction? |
Followon to image theory
David wrote:
With a vertical monopole antenna, the field emitted by the radials forms a near field and interacts with the wave radiated by the vertical element. Is it mainly in the vertical direction that the radial fields interact with the field from vertical element? The fields "interact" wherever they exist. I would expect the antenna feedpoint impedance to be formed from the length of antenna conductors and the interaction of the fields. You should also expect the feedpoint impedance to be formed by the ratio of the voltage to the current at the feedpoint, among other things. If a monopole is simulated in a NEC program above a perfect ground plane, is it possible to see the reflection? Only on Groundhog Day. :-) I would expect NEC program to show reflection of waves from metallic surfaces e.g reflection from perfect ground plane or parabolic dish. Can a NEC program allow the user to see whether the wave is reflected (as for perfect ground plane) or whether effect is due to wave interaction (as for radials)? Can the user then vary a finite size ground plane to see whether antenna impedance comes from reflection or wave interaction? An interactive video game it's not. Perhaps if you were to become a NEC user, you would come to know these things and more, first hand. Jim, AC6XG |
Followon to image theory
Isn't the whole question of whether it's a reflection as from a conducting
plate, or radiation from the radials based on misunderstanding what a reflection is? The reflection from a conducting plane is the sum effect of the radiation from the currents induced in the plane by the incident field, say from a quarter wave vertical. The reflection forms an image 'behind the mirror' of the source of the incident field, for example a quarter wave vertical 'under the plane'. If these currents are radial then the plane is well approximated by a large number of long radial wires. The high resistance at right angles to the radials doesn't matter because no current is flowing in that direction. The plane is not so well approximated by a small number, say 3 or 4, radials, especially if they are short, say a quarter wavelength. However the approximation may be good enough to allow us to regard the short radials as reflecting the incident field from the quarter wave vertical especially if we just need a rough picture of the situation and a rough estimate of the input impedance.We may need to keep in mind that the reflection is due to radiation from the currents in the radials or we may be able to forget it. If we want the antenna pattern in detail and the input impedance accurately we have to do the maths properly. That's not simple even for a half-wave dipole! Brian LA0DG |
Followon to image theory
David wrote:
With a vertical monopole antenna, the field emitted by the radials forms a near field and interacts with the wave radiated by the vertical element. Is it mainly in the vertical direction that the radial fields interact with the field from vertical element? I would expect the antenna feedpoint impedance to be formed from the length of antenna conductors and the interaction of the fields. If a monopole is simulated in a NEC program above a perfect ground plane, is it possible to see the reflection? I would expect NEC program to show reflection of waves from metallic surfaces e.g reflection from perfect ground plane or parabolic dish. Can a NEC program allow the user to see whether the wave is reflected (as for perfect ground plane) or whether effect is due to wave interaction (as for radials)? Can the user then vary a finite size ground plane to see whether antenna impedance comes from reflection or wave interaction? What do you mean when you say that waves interact with each other? Explain what you mean by the word "reflection." 73, Tom Donaly, KA6RUH |
Followon to image theory
Brian Anthony Farrelly wrote:
Isn't the whole question of whether it's a reflection as from a conducting plate, or radiation from the radials based on misunderstanding what a reflection is? There is no contradiction in quantum electrodynamics. A photon encounters an electron in a radial and is absorbed. Later, that same electron emits a photon. The original photon energy could have come from anywhere. If it came from the vertical element in the first place, it is a "reflection" but that is irrelevant. If it came from that same radial in the first place, it is not a "reflection" but that is also irrelevant. If it bounced off a flagpole before it was absorbed by the electron, it is a "reflection" but no rational person cares. The photon cloud surrounding a radial contains photons that came from that radial and from everywhere else. That some photons must be put in a basket labeled "reflected" and some put in a basket labeled "not reflected" is simply nonsense. -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
Cecil Moore wrote:
That some photons must be put in a basket labeled "reflected" and some put in a basket labeled "not reflected" is simply nonsense. The very concept of arguing whether a photon was reflected or not seems to violate the uncertainty principle. There is only a probability that it was reflected. -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
On Wed, 17 Jan 2007 20:22:27 -0000, "David" nospam@nospam wrote:
With a vertical monopole antenna, the field emitted by the radials forms a near field and interacts with the wave radiated by the vertical element. Is it mainly in the vertical direction that the radial fields interact with the field from vertical element? Hi David, No. I would expect the antenna feedpoint impedance to be formed from the length of antenna conductors and the interaction of the fields. Sure. There is nothing you've offered to suggest otherwise. If a monopole is simulated in a NEC program above a perfect ground plane, is it possible to see the reflection? Yes. Even above an imperfect ground. Plane has nothing to do with it except for the matter of geometry - radials even less so. I would expect NEC program to show reflection of waves from metallic surfaces e.g reflection from perfect ground plane or parabolic dish. You should then expect an NEC program to show that reflection in relation to wavelength. They do this to as near to what is observed as to show no difference, naturally. Can a NEC program allow the user to see whether the wave is reflected (as for perfect ground plane) or whether effect is due to wave interaction (as for radials)? NEC is not responsible for insight or interpretation. That only comes with experience. Can the user then vary a finite size ground plane to see whether antenna impedance comes from reflection or wave interaction? The user might, but the proof of an argument is through a chain of evidence. A simple test of "reflection from radials" in a real implementation (with ground): 1. Change the angle of the radials by 45°, does the lobe angle change by 45°? Explain why reflection does not follow convention. 2. As suggested by Roy, add a top hat with a structure that identical to the radials, does the lobe angle point into the ground? Explain why reflection does not exist in this case. 3. Alternative: invert the radials/vertical, it is now upside down, does the lobe angle invert with it? Explain why radials are special only in the conventional case. You might want to repeat these in real space, but then you have removed the reflector, ground. Without it, what would you offer as a proof of reflection that can be verified through any of the three simple tests above? Explain why those tests demonstrate reflection in free space when those tests above do not. 73's Richard Clark, KB7QHC |
Followon to image theory
Cecil Moore wrote:
There is no contradiction in quantum electrodynamics. There's certainly a paradox. A photon encounters an electron in a radial and is absorbed. We know that electrons are inspired to move in a particular fashion when they are irradiated. But how does a photon tell the electron which way it should move? Kinematics would seem to have little to do with this phenomenon. Later, that same electron emits a photon. A photon which, if the direction in which it is emitted is unknown, can be made to create a diffraction pattern - implying that a wave was emitted rather than a particle. Putting us right back where we started. The photon cloud surrounding a radial contains photons that came from that radial and from everywhere else. But what is it about this "cloud" that is actually cloud-like? Isn't it really more like a wave of photons? :-) That some photons must be put in a basket labeled "reflected" and some put in a basket labeled "not reflected" is simply nonsense. I think Feynman uses almost that exact analogy, with probabilities assigned to each, in the first few pages of QED. 73, ac6xg |
Followon to image theory
Jim Kelley wrote:
But how does a photon tell the electron which way it should move? Energy and momentum must be conserved. A photon which, if the direction in which it is emitted is unknown, can be made to create a diffraction pattern - implying that a wave was emitted rather than a particle. Putting us right back where we started. Allow me to paraphrase Feynman: Only particles exist. But what is it about this "cloud" that is actually cloud-like? Isn't it really more like a wave of photons? :-) If you prefer liquid analogies to gaseous analogies, feel free. I think Feynman uses almost that exact analogy, with probabilities assigned to each, in the first few pages of QED. You missed the point. That an individual photon must be put in a basket labeled "reflected" and some other individual photon must be put in a basket labeled "not reflected" is simply nonsense. Talking about exactly where an individual photon goes is nonsense. -- 73, Cecil, http://www.qsl.net/w5dxp |
Followon to image theory
On Thu, 18 Jan 2007 12:51:40 -0800, Jim Kelley
wrote: A photon encounters an electron in a radial and is absorbed. We know that electrons are inspired to move in a particular fashion when they are irradiated. But how does a photon tell the electron which way it should move? Kinematics would seem to have little to do with this phenomenon. And photon/electron interaction is not so indiscriminate (it doesn't always interact in the first place). Later, that same electron emits a photon. A photon which, if the direction in which it is emitted is unknown, can be made to create a diffraction pattern - implying that a wave was emitted rather than a particle. Putting us right back where we started. An electron does not always give rise to just one photon, or any photon, or a photon of the same energy. The photon cloud surrounding a radial contains photons that came from that radial and from everywhere else. But what is it about this "cloud" that is actually cloud-like? Isn't it really more like a wave of photons? :-) Cloud indeed. This is like a sack of magic beans in exchange for a cow. Is this ill-discussion of photons the legacy of image theory? It is like preening in front of a conjugate mirror. 73's Richard Clark, KB7QHC |
Followon to image theory
Richard Clark wrote:
An electron does not always give rise to just one photon, or any photon, or a photon of the same energy. This is true. But in an amateur transmitter driven antenna system, the great majority of radiated photons are coherent with the transmitting frequency. Coherent photons are required to have the same energy level. The photons coherent with the transmitting frequency are first order effects. Assuming one is not close to another transmitter, the photons not coherent with the transmitting signal are Nth order effects. -- 73, Cecil http://www.w5dxp.com |
Followon to image theory
Cecil Moore wrote:
Jim Kelley wrote: But how does a photon tell the electron which way it should move? Energy and momentum must be conserved. Do you really want to go there? :-) According to QED, you first need to find some negative energy photons. Then you need to get them to propagate backward in time and subsequently interact with electrons. Then, the electrons can then move forward in time and emit positive energy photons while conserving momentum. A photon which, if the direction in which it is emitted is unknown, can be made to create a diffraction pattern - implying that a wave was emitted rather than a particle. Putting us right back where we started. Allow me to paraphrase Feynman: Only particles exist. Dear Merrium Webster, On behalf of a friend of mine, please consider adding this definition to your dictionary: paraphrase - to restate text in different form such that it conveys unintended meaning But what is it about this "cloud" that is actually cloud-like? Isn't it really more like a wave of photons? :-) If you prefer liquid analogies to gaseous analogies, feel free. A wave of photons. It's a physics joke, Cecil. Laugh already. 73, ac6xg |
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