Home |
Search |
Today's Posts |
#11
![]() |
|||
|
|||
![]()
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? |
#12
![]() |
|||
|
|||
![]()
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 |
#13
![]() |
|||
|
|||
![]()
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 |
#14
![]() |
|||
|
|||
![]()
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 |
#15
![]() |
|||
|
|||
![]()
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. |
#16
![]() |
|||
|
|||
![]()
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 |
#17
![]() |
|||
|
|||
![]()
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 |
#18
![]() |
|||
|
|||
![]()
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 |
#19
![]() |
|||
|
|||
![]()
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 |
#20
![]() |
|||
|
|||
![]()
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". . . . |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
balun and image | Shortwave | |||
A "single conversion" question | Shortwave | |||
And Incase Lennie Doubted that MARS and Amateur Radio are a "Service to the Nation..." MARS Chief Says Otherwise | Policy | |||
Rare Books on Electronics and Radio and Commmunications | Equipment | |||
Rare Books on Electronics and Radio and Commmunications | Equipment |