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opinions on an antenna idea
Hi, I just read instructions on building a "helically wound" antenna
using a broom stick. http://www.hard-core-dx.com/nordicdx.../bromstik.html After reading this, I had an idea. I'm doing more google searches for this type of configuration, but wanted to ask here as well to get some opinions. What if I took two broom sticks, and wound each with about 68 feet of wire. Each stick making half of a dipole antenna. Then attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave when connected to a tuner? Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? Jim |
opinions on an antenna idea
On Fri, 30 Nov 2007 08:06:02 -0800 (PST), James barrett
wrote: Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? Hi Jim, At the right frequency, the right height, and at night it might act like an aeronautical beacon. Or given the season, it might provide sufficient light to be used as the star of Bethlehem decoration. But if you want even more outlandish claims for performance (I like the part of adding capacity hats to lower noise), add the google term "Gaussian array." Good luck with making any sense of what passes for construction hints (you will have to wade through a lot of window dressing to find them first). OK, now for some real technical content: The first order of business is in the description for a tight wound coil. The proximity of windings drives efficiency down. Open up the windings. The short physical length of a radiator drives the efficiency down. Extend the winding pitch further. The short physical length of a radiator demands closer attention to matching (lower efficiency again). Lengthen the radiator. Using the entire length of a radiator to support a coil drives down the radiation current (this is really a combination of the earlier problems, but it is way cool to say it sucks again). Lengthen the radiator. The advice is sufficient for a receiving antenna, hence the source of Radio Habana Cuba's recommendation. It is especially poor for transmitting, hence its absence in Radio Habana Cuba's transmitting antenna farm. Those two clues should be sufficient. 73's Richard Clark, KB7QHC |
opinions on an antenna idea
On Nov 30, 11:25 am, Richard Clark wrote:
The advice is sufficient for a receiving antenna, hence the source of Radio Habana Cuba's recommendation. It is especially poor for transmitting, hence its absence in Radio Habana Cuba's transmitting antenna farm. Those two clues should be sufficient. Good enough for me. Maybe I'll experiment with a 2 meter receiving antenna just to compare it to a ground plane. I'll just stick with my plans to put up a long dipole with no coils. Jim |
opinions on an antenna idea
James barrett wrote:
Hi, I just read instructions on building a "helically wound" antenna using a broom stick. http://www.hard-core-dx.com/nordicdx.../bromstik.html After reading this, I had an idea. I'm doing more google searches for this type of configuration, but wanted to ask here as well to get some opinions. What if I took two broom sticks, and wound each with about 68 feet of wire. Each stick making half of a dipole antenna. Then attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave when connected to a tuner? Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? If you use the same amount of wire for the helix as you would for the wire dipole, the resonant frequency will not be the same. Feedpoint impedance wise, you can get it to respond like a 1/2WL dipole probably with a lower feedpoint resistance. Radiation wise, it will respond like a shortened antenna. The currents on opposite sides of the coil are close to 180 degrees out of phase and, like a transmission line, tend to cancel the fields. In addition, a broomstick is not the best dielectric to use for your coil form. -- 73, Cecil http://www.w5dxp.com |
opinions on an antenna idea
"Cecil Moore" wrote in message t... James barrett wrote: Hi, I just read instructions on building a "helically wound" antenna using a broom stick. http://www.hard-core-dx.com/nordicdx.../bromstik.html After reading this, I had an idea. I'm doing more google searches for this type of configuration, but wanted to ask here as well to get some opinions. What if I took two broom sticks, and wound each with about 68 feet of wire. Each stick making half of a dipole antenna. Then attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave when connected to a tuner? Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? If you use the same amount of wire for the helix as you would for the wire dipole, the resonant frequency will not be the same. Feedpoint impedance wise, you can get it to respond like a 1/2WL dipole probably with a lower feedpoint resistance. Radiation wise, it will respond like a shortened antenna. The currents on opposite sides of the coil are close to 180 degrees out of phase and, like a transmission line, tend to cancel the fields. In addition, a broomstick is not the best dielectric to use for your coil form. -- 73, Cecil http://www.w5dxp.com Helically wound antennas used to be more common some years ago. An outfit in CT used to sell HF beams that had elements slightly more than 1/2 normal length. More common were CB mobile antennas, which are probably still made. I have a CB vertical bought for RC use that is about 18 inches tall. Supposedly a shortened helically wound antenna has a wider bandwidth than a lumped inductor antenna of the same length. Tam/WB2TT |
opinions on an antenna idea
"Tam/WB2TT" wrote in
: about 18 inches tall. Supposedly a shortened helically wound antenna has a wider bandwidth than a lumped inductor antenna of the same length. Probably mostly due to the higher loss of the loading system. Better bandwidth, poorer efficiency. Owen |
opinions on an antenna idea
"Owen Duffy" wrote in message ... "Tam/WB2TT" wrote in : about 18 inches tall. Supposedly a shortened helically wound antenna has a wider bandwidth than a lumped inductor antenna of the same length. Probably mostly due to the higher loss of the loading system. Better bandwidth, poorer efficiency. Owen I don't think so. Tam |
opinions on an antenna idea
"James barrett" wrote in message ... Hi, I just read instructions on building a "helically wound" antenna using a broom stick. http://www.hard-core-dx.com/nordicdx.../bromstik.html After reading this, I had an idea. I'm doing more google searches for this type of configuration, but wanted to ask here as well to get some opinions. What if I took two broom sticks, and wound each with about 68 feet of wire. Each stick making half of a dipole antenna. Then attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave when connected to a tuner? Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? Jim Sounds a lot like the Slinky antenna, From what I've heard, using 2 slinkies (one on each side) will make a 40-meter dipole, and 2 on each side makes a 75-meter dipole (or was that a 20 and 40)?? But they used bamboo fishing poles as support instead of broomsticks. |
opinions on an antenna idea
People have an extremely strong tendency to simplify the mass of
incoming data into simply digested and understood binary categories: Is it good, or is it evil? Does the antenna work, or doesn't it? And here the binary choice is between a 136 foot dipole and a shorter dipole. The answer here, as it is to so may binary questions, is that it behaves in some ways like one, some ways like the other, and some ways like neither. The helically wound antenna can be made resonant. A 136 foot dipole is resonant, but a shorter dipole isn't, unless loaded. It will be inefficient, which is also usually characteristic of a short dipole and not a 136 foot one. A short dipole with a properly designed matching network could be made to be more efficient than the helically wound antenna. The input resistance at resonance will be between that of a 136 foot dipole and a straight dipole the length of the helical antenna, unless the loss is exceptionally high. The bandwidth will be between that of a 136 foot dipole and one the length of the helical antenna, unless the loss is exceptionally high. The pattern will be more like that of a short dipole than that of a 136 foot dipole, although you wouldn't be able to tell the difference. It would even be hard to measure using professional equipment. Roy Lewallen, W7EL James barrett wrote: Hi, I just read instructions on building a "helically wound" antenna using a broom stick. http://www.hard-core-dx.com/nordicdx.../bromstik.html After reading this, I had an idea. I'm doing more google searches for this type of configuration, but wanted to ask here as well to get some opinions. What if I took two broom sticks, and wound each with about 68 feet of wire. Each stick making half of a dipole antenna. Then attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave when connected to a tuner? Would it behave like a 136 foot dipole, or would it behave like a shorter dipole? Jim |
opinions on an antenna idea
Roy Lewallen wrote:
People have an extremely strong tendency to simplify the mass of incoming data into simply digested and understood binary categories: Is it good, or is it evil? Does the antenna work, or doesn't it? ... Uhh, yeah, that sums me up pretty much. You think I should be ashamed? JS |
opinions on an antenna idea
Alan Peake wrote:
If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan If room temperature super-conductors were available, do you even realize the shape antennas would take? My gawd man, share some of that material here! The thought alone is inspiring! Regards, JS |
opinions on an antenna idea
Alan Peake wrote:
If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan No. The answer can be found in any antenna textbook, because the lossless short dipole is a very good platform to illustrate a number of principles without the confounding additional consequences of loss. Briefly, -- The pattern of an infinitesimally short dipole is very similar to that of a half wave dipole. The difference is due to the triangular current distribution of the short dipole as opposed to the sinusoidal current distribution of the half wave dipole. Because the patterns are very similar and both antennas radiate all the applied power, the gain of the two antennas is nearly the same. The short dipole's pattern is a little fatter so it has slightly -- about a half dB -- less gain. But the pattern of even an infinitesimally short dipole retains the basic two-lobed dipole shape with around 1.7 dB gain over isotropic in its favored directions. -- The input resistance of the very short lossless dipole is very low and the capacitive reactance very high. The resistance approaches zero and the reactance negative infinity as the length approaches zero. There's no comparison to an isotropic radiator, since the latter is a purely fictional source with no even theoretical physical realization and therefore no definable input characteristics. -- The Q of the short dipole is very high, so the reactance varies very rapidly with frequency. A matched short antenna would have an extremely narrow bandwidth. Most of these properties of the dipoles can easily be observed with the free EZNEC demo program from http://eznec.com, and much more information about the properties of the short lossless dipole can be found in any antenna text. Roy Lewallen, W7EL |
opinions on an antenna idea
Roy Lewallen wrote:
... -- The Q of the short dipole is very high, so the reactance varies very rapidly with frequency. A matched short antenna would have an extremely narrow bandwidth. ... Roy Lewallen, W7EL And, here is where a DLM antenna is nice, keep the coils of low Q and bandwidth is "surprisingly wide." Regards, JS |
opinions on an antenna idea
Alan Peake wrote:
Roy Lewallen wrote: Alan Peake wrote: If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan No. The answer can be found in any antenna textbook, ... etc. Roy Lewallen, W7EL Thanks Roy. Unfortunately, since I retired, I no longer have access to Jasik, Kraus etc. So, thanks for the answer. I should have realised that a dipole of any length is still a dipole and as such will not radiate off it's ends. Mind you, Eznec shows the average dipole, less than half-wave above ground, goes pretty close to an isotropic radiator for all practical purposes :) Alan --------------- You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. Ed, NM2K |
opinions on an antenna idea
Alan Peake wrote:
Roy Lewallen wrote: Alan Peake wrote: If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan No. The answer can be found in any antenna textbook, ... etc. Roy Lewallen, W7EL Thanks Roy. Unfortunately, since I retired, I no longer have access to Jasik, Kraus etc. So, thanks for the answer. I should have realised that a dipole of any length is still a dipole and as such will not radiate off it's ends. Mind you, Eznec shows the average dipole, less than half-wave above ground, goes pretty close to an isotropic radiator for all practical purposes :) Alan --------------- You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. Ed, NM2K |
opinions on an antenna idea
Ed Cregger wrote:
Alan Peake wrote: Roy Lewallen wrote: Alan Peake wrote: If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan No. The answer can be found in any antenna textbook, ... etc. Roy Lewallen, W7EL Thanks Roy. Unfortunately, since I retired, I no longer have access to Jasik, Kraus etc. So, thanks for the answer. I should have realised that a dipole of any length is still a dipole and as such will not radiate off it's ends. Mind you, Eznec shows the average dipole, less than half-wave above ground, goes pretty close to an isotropic radiator for all practical purposes :) Alan --------------- You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. Ed, NM2K ------------- Bellsouth, now AT&T, is back to double posting everything again. Was having problems with their DSL service all last night. Wish they would get their act together. Ed, NM2K |
opinions on an antenna idea
The answer here, as it is to so may binary questions, is that it behaves in some ways like one, some ways like the other, and some ways like neither. .. .. .. Roy Lewallen, W7EL If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan |
opinions on an antenna idea
Ed Cregger wrote:
You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? No. It's not true that all the radiation "comes from one end of the dipole". Extrapolation from that mistaken premise will lead to invalid conclusions. Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. And there's the first one. . . Roy Lewallen, W7EL |
opinions on an antenna idea
Roy Lewallen wrote:
Ed Cregger wrote: You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? No. It's not true that all the radiation "comes from one end of the dipole". Extrapolation from that mistaken premise will lead to invalid conclusions. Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. And there's the first one. . . Roy Lewallen, W7EL ------------- You are the acknowledged expert here (we're not worthy!!!). What is the flaw in the proposed thinking? You have to admit that lots of the commercial antenna companies and ham publications either do, or used to, emphasize the point that "most of the radiation of a 1/4 wave ground plane antenna (half of a half wave) occurs near the feed point". Instead of just saying, no, this thinking is incorrect, how about teaching your students (includes me) precisely what is wrong with this line of thinking. Not at the engineering level necessarily (oodles of formulas), but in the analog/real world level. Please? Be merciful, oh great one. I'm on enough prescription drugs to put half a football team to sleep, so, occasionally, I get quite tangential to the topic at hand. I hope this isn't one of those times. G Thank you, oh merciful one. Ed, NM2K |
opinions on an antenna idea
Roy Lewallen wrote:
Ed Cregger wrote: You appear to be extrapolating, if I see this correctly, that since all of the radiation is believed to come from one end of the dipole, then the rest of the antenna is merely acting as the necessary reactances and resistance needed to obtain the proper feedpoint impedance at a given frequency. True? No. It's not true that all the radiation "comes from one end of the dipole". Extrapolation from that mistaken premise will lead to invalid conclusions. Following that line of reasoning, if the need for the aggregate reactances/resistances can be eliminated via superconducting elements, one will have just a single point source of radiation. Or, what is commonly known as an isotropic radiator. I suspect that the plasma antenna fellows are contemplating this too. And there's the first one. . . Roy Lewallen, W7EL ------------- You are the acknowledged expert here (we're not worthy!!!). What is the flaw in the proposed thinking? You have to admit that lots of the commercial antenna companies and ham publications either do, or used to, emphasize the point that "most of the radiation of a 1/4 wave ground plane antenna (half of a half wave) occurs near the feed point". Instead of just saying, no, this thinking is incorrect, how about teaching your students (includes me) precisely what is wrong with this line of thinking. Not at the engineering level necessarily (oodles of formulas), but in the analog/real world level. Please? Be merciful, oh great one. I'm on enough prescription drugs to put half a football team to sleep, so, occasionally, I get quite tangential to the topic at hand. I hope this isn't one of those times. G Thank you, oh merciful one. Ed, NM2K |
opinions on an antenna idea
Ed Cregger wrote:
You are the acknowledged expert here (we're not worthy!!!). What is the flaw in the proposed thinking? You have to admit that lots of the commercial antenna companies and ham publications either do, or used to, emphasize the point that "most of the radiation of a 1/4 wave ground plane antenna (half of a half wave) occurs near the feed point". Instead of just saying, no, this thinking is incorrect, how about teaching your students (includes me) precisely what is wrong with this line of thinking. Not at the engineering level necessarily (oodles of formulas), but in the analog/real world level. Please? Be merciful, oh great one. I'm on enough prescription drugs to put half a football team to sleep, so, occasionally, I get quite tangential to the topic at hand. I hope this isn't one of those times. G Thank you, oh merciful one. C'mon, now, I'm not the Great Guru. I'm just somebody who's interested in antennas and has spent a lot of time thinking and learning about them. As I said when I was in the service (as an enlisted man), "Don't call me 'sir'! My parents were married." The question of where radiation "comes from" is really a complicated one. Not long ago I came across a recent paper in the IEEE Transactions on Antennas and Propagation which addresses the issue, and it's one of many. One of the conclusions of the paper is that it's really not possible to assign any part or parts of an antenna as being responsible for a particular share of the radiation. A lot of people confuse the field generated by a current-carrying conductor with far field radiation. It's very well known and established that a field is created which is proportional to the current flowing on a conductor -- antenna analysis programs use this principle to produce very accurate results. This is certainly the source of claims that the middle of a half wave dipole or the bottom of a quarter wave monopole does most of the "radiating", because those points are where the current is highest and therefore the field most intense. However, the fields all parts of the antenna add together to become the radiation which "escapes" beyond the region close to the antenna. You can, for example, have two different parts of an antenna which each produce intense fields, but out of phase in some directions so they cancel completely or partially out of phase in such a way that they nearly cancel in all directions. If you could somehow make the field from one of those parts disappear without affecting the other, the contribution to the overall radiation from the other would increase. (However, the law of conservation of energy requires that radiation from somewhere else would have to decrease to keep the total the same.) So the radiation is the result of contributions from all parts of the antenna, but in a way that's not easy to apportion to individual parts. In the example, the two parts of the antenna, in combination, contribute little to the radiated field. But each one, by itself, would contribute quite a bit if it weren't for the other. An antenna has an infinite number of radiating parts which all sum together to produce the radiated field, so you can hopefully see the problem here. That being said, some professional papers do establish some sort of criteria for apportioning it. In ones I've seen, the radiation from half a dipole as a function of position looks sort of tub-shaped, with considerable radiation arising from all parts of the antenna, but having a somewhat larger amount coming from the center and ends. As far as I can tell, though, this depends on exactly how you define in what way a particular part of the antenna is responsible for each fraction of the total radiated power. The bottom line is that any simplified assignment of radiation as coming from one part of the antenna or another is too much of a simplification and will lead to erroneous conclusions. All I can say about what antenna publications and commercial antenna manufacturers say is that a very large fraction of it is just plain wrong. Consequently, they're very poor sources of information. Good information can be found in textbooks and professional publications, and very few other places. One exception (that is, one good source not in these categories) is the _ARRL Antenna Book_, since when Jerry Hall overhauled it (15th Edition if I recall correctly). The current editor, Dean Straw, is knowledgeable about antennas and very conscientious about correcting errors and misinformation. So it's become the only reference I know of which is fundamentally accurate while keeping explanations at a level which is easily understood by non-professionals. Hope this helped. Roy Lewallen, W7EL |
opinions on an antenna idea
Roy Lewallen wrote: Alan Peake wrote: If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan No. The answer can be found in any antenna textbook, .... etc. Roy Lewallen, W7EL Thanks Roy. Unfortunately, since I retired, I no longer have access to Jasik, Kraus etc. So, thanks for the answer. I should have realised that a dipole of any length is still a dipole and as such will not radiate off it's ends. Mind you, Eznec shows the average dipole, less than half-wave above ground, goes pretty close to an isotropic radiator for all practical purposes :) Alan |
opinions on an antenna idea
John Smith wrote: Alan Peake wrote: If one were to find lossless material (superconductors?) for the short antenna and it's corresponding matching network, what would happen as the antenna became shorter and shorter compared with the half-wave dipole? Would it simply approach an isotropic radiator? Alan If room temperature super-conductors were available, do you even realize the shape antennas would take? My gawd man, share some of that material here! The thought alone is inspiring! Regards, JS Don't know what shape it would be but I'm sure I wouldn't recognize it! Alan |
opinions on an antenna idea
Roy Lewallen wrote:
Ed Cregger wrote: You are the acknowledged expert here (we're not worthy!!!). What is the flaw in the proposed thinking? You have to admit that lots of the commercial antenna companies and ham publications either do, or used to, emphasize the point that "most of the radiation of a 1/4 wave ground plane antenna (half of a half wave) occurs near the feed point". Instead of just saying, no, this thinking is incorrect, how about teaching your students (includes me) precisely what is wrong with this line of thinking. Not at the engineering level necessarily (oodles of formulas), but in the analog/real world level. Please? Be merciful, oh great one. I'm on enough prescription drugs to put half a football team to sleep, so, occasionally, I get quite tangential to the topic at hand. I hope this isn't one of those times. G Thank you, oh merciful one. C'mon, now, I'm not the Great Guru. I'm just somebody who's interested in antennas and has spent a lot of time thinking and learning about them. As I said when I was in the service (as an enlisted man), "Don't call me 'sir'! My parents were married." The question of where radiation "comes from" is really a complicated one. Not long ago I came across a recent paper in the IEEE Transactions on Antennas and Propagation which addresses the issue, and it's one of many. One of the conclusions of the paper is that it's really not possible to assign any part or parts of an antenna as being responsible for a particular share of the radiation. A lot of people confuse the field generated by a current-carrying conductor with far field radiation. It's very well known and established that a field is created which is proportional to the current flowing on a conductor -- antenna analysis programs use this principle to produce very accurate results. This is certainly the source of claims that the middle of a half wave dipole or the bottom of a quarter wave monopole does most of the "radiating", because those points are where the current is highest and therefore the field most intense. However, the fields all parts of the antenna add together to become the radiation which "escapes" beyond the region close to the antenna. You can, for example, have two different parts of an antenna which each produce intense fields, but out of phase in some directions so they cancel completely or partially out of phase in such a way that they nearly cancel in all directions. If you could somehow make the field from one of those parts disappear without affecting the other, the contribution to the overall radiation from the other would increase. (However, the law of conservation of energy requires that radiation from somewhere else would have to decrease to keep the total the same.) So the radiation is the result of contributions from all parts of the antenna, but in a way that's not easy to apportion to individual parts. In the example, the two parts of the antenna, in combination, contribute little to the radiated field. But each one, by itself, would contribute quite a bit if it weren't for the other. An antenna has an infinite number of radiating parts which all sum together to produce the radiated field, so you can hopefully see the problem here. That being said, some professional papers do establish some sort of criteria for apportioning it. In ones I've seen, the radiation from half a dipole as a function of position looks sort of tub-shaped, with considerable radiation arising from all parts of the antenna, but having a somewhat larger amount coming from the center and ends. As far as I can tell, though, this depends on exactly how you define in what way a particular part of the antenna is responsible for each fraction of the total radiated power. The bottom line is that any simplified assignment of radiation as coming from one part of the antenna or another is too much of a simplification and will lead to erroneous conclusions. All I can say about what antenna publications and commercial antenna manufacturers say is that a very large fraction of it is just plain wrong. Consequently, they're very poor sources of information. Good information can be found in textbooks and professional publications, and very few other places. One exception (that is, one good source not in these categories) is the _ARRL Antenna Book_, since when Jerry Hall overhauled it (15th Edition if I recall correctly). The current editor, Dean Straw, is knowledgeable about antennas and very conscientious about correcting errors and misinformation. So it's become the only reference I know of which is fundamentally accurate while keeping explanations at a level which is easily understood by non-professionals. Hope this helped. Roy Lewallen, W7EL ------------ Thanks, Roy. Much appreciated. Ed, NM2K |
opinions on an antenna idea
Alan Peake wrote:
"---what would happen as the antenna became shorter and shorter compared with the half-wave dipole?" Terman answers that question on page 871 of his 1955 opus: "The directive gain of the elementary doublet =1.5." For a resonant wire of 0.5 lambda, the gain is 1.64. There`s not much difference in directivity as the doublet shrinks to a vanishingly small size. The gains shown are power ratios, not dB`s. Comparison antenna is the isotropic of which Terman says: "Although an isotropic radiator of coherent waves does not exist because it cannot satisfy Maxwell`s equations, the properties of such an imaginary antenna are easily visualized, and the concept of an isotropic radiator is often found useful in the analysis of antenna systems." (Page 871 in the 1955 opus.) Best regards, Richard Harrison, KB5WZI |
opinions on an antenna idea
On 4 Dec, 01:28, John Smith wrote:
Roy Lewallen wrote: ... -- The Q of the short dipole is very high, so the reactance varies very rapidly with frequency. A matched short antenna would have an extremely narrow bandwidth. ... Roy Lewallen, W7EL And, here is where a DLM antenna is nice, keep the coils of low Q and bandwidth is "surprisingly wide." Regards, JS Yup, My 160M antenna came at at a resistive 200 ohm plus resonance and with a bit of fiddling I now connect the coax direct and cover the whole band. Not sure if I would have been better off with keeping the high resistive impedance and using a transformer but snow is on the way so beggars can't be choosers. Regards Art KB9MZ........XG (uk) |
opinions on an antenna idea
In article , Roy Lewallen
wrote: All I can say about what antenna publications and commercial antenna manufacturers say is that a very large fraction of it is just plain wrong. Consequently, they're very poor sources of information. Good information can be found in textbooks and professional publications, and very few other places. One exception (that is, one good source not in these categories) is the _ARRL Antenna Book_, since when Jerry Hall overhauled it (15th Edition if I recall correctly). The current editor, Dean Straw, is knowledgeable about antennas and very conscientious about correcting errors and misinformation. So it's become the only reference I know of which is fundamentally accurate while keeping explanations at a level which is easily understood by non-professionals. Hope this helped. Roy Lewallen, W7EL You got that right, Roy. Do marketing departments ever talk to the engineers? At least I haven't seen a dial 1-800 TV commercial such as "Call right now and we'll include the matching network and balun free of charge. But call right now and we'll also include a CFA free!" Adding to what you said above how about a little gray box that can save you up to 25% on your electric bill (you can Google this one). Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
opinions on an antenna idea
J. B. Wood wrote:
In article , Roy Lewallen wrote: All I can say about what antenna publications and commercial antenna manufacturers say is that a very large fraction of it is just plain wrong. Consequently, they're very poor sources of information. Good information can be found in textbooks and professional publications, and very few other places. One exception (that is, one good source not in these categories) is the _ARRL Antenna Book_, since when Jerry Hall overhauled it (15th Edition if I recall correctly). The current editor, Dean Straw, is knowledgeable about antennas and very conscientious about correcting errors and misinformation. So it's become the only reference I know of which is fundamentally accurate while keeping explanations at a level which is easily understood by non-professionals. Hope this helped. Roy Lewallen, W7EL You got that right, Roy. Do marketing departments ever talk to the engineers? At least I haven't seen a dial 1-800 TV commercial such as "Call right now and we'll include the matching network and balun free of charge. But call right now and we'll also include a CFA free!" Adding to what you said above how about a little gray box that can save you up to 25% on your electric bill (you can Google this one). Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 ------------ I have quite a few engineering books on antennas (that I use G), so I can appreciate the value of good, solid engineering text/sources. However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. After all, superconductive radiating elements did not exist before and the math has not been done. Perhaps, their inclusion, will demand something more than a simple extrapolation of existing antenna theory. I believe this to be the point of the OP. I added the other type of radiating element, plasma radiators, as a part of the same discussion with the same reasoning behind it. Can you imagine an antenna ray that only manifests itself physically when needed? Wow! Ed, NM2K |
opinions on an antenna idea
On Wed, 05 Dec 2007 10:36:59 -0500, Ed Cregger
wrote: However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. Hi Ed, This is uni-dimensional thinking. "A new breakfast cereal could establish the need for a serious rethinking of sewing machine theory." There are probably more things possible ("could establish") than time to consider them - and probably on file pending patent. In that sense, patent publishing could establish the need for a serious rethinking of replacing burning oil for heat. "Could establish" ...this could establish a new form of gaming entertainment in this group. [and conforms to the usage of self-referential claims] 73's Richard Clark, KB7QHC |
opinions on an antenna idea
Ed Cregger wrote:
I have quite a few engineering books on antennas (that I use G), so I can appreciate the value of good, solid engineering text/sources. However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. After all, superconductive radiating elements did not exist before and the math has not been done. Perhaps, their inclusion, will demand something more than a simple extrapolation of existing antenna theory. I believe this to be the point of the OP. . . . And I disagree. The assumption of zero loss is implicit or explicit in nearly all the analyses in your antenna texts and mine. So no new math or "rethinking of antenna theory" is required to deal with lossless conductors. It is, in fact, the simplest case and so underlies virtually all the current theory. What it would do is cause a change in tradeoffs which would be made by engineers in the design of real antennas. However, superconductors (at least all known conventional and high-temperature superconductors) are lossless only at DC. Superconductor loss increases with frequency and, except at DC, with temperature. The resistivity of copper decreases quite dramatically with temperature, so it's not uncommon to find situations at very high frequencies and very cold temperatures where copper does better than a superconductor. Even high temperature superconductors have to be cooled to cryogenic temperatures to do reasonably well at very high frequencies. But again no new math or "rethinking of antenna theory" is necessary to deal with them -- the same electromagnetic principles apply and they can be treated like any other conductors with finite resistivity. Roy Lewallen, W7EL |
opinions on an antenna idea
Roy Lewallen wrote:
Ed Cregger wrote: I have quite a few engineering books on antennas (that I use G), so I can appreciate the value of good, solid engineering text/sources. However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. After all, superconductive radiating elements did not exist before and the math has not been done. Perhaps, their inclusion, will demand something more than a simple extrapolation of existing antenna theory. I believe this to be the point of the OP. . . . And I disagree. The assumption of zero loss is implicit or explicit in nearly all the analyses in your antenna texts and mine. So no new math or "rethinking of antenna theory" is required to deal with lossless conductors. It is, in fact, the simplest case and so underlies virtually all the current theory. What it would do is cause a change in tradeoffs which would be made by engineers in the design of real antennas. However, superconductors (at least all known conventional and high-temperature superconductors) are lossless only at DC. Superconductor loss increases with frequency and, except at DC, with temperature. The resistivity of copper decreases quite dramatically with temperature, so it's not uncommon to find situations at very high frequencies and very cold temperatures where copper does better than a superconductor. Even high temperature superconductors have to be cooled to cryogenic temperatures to do reasonably well at very high frequencies. But again no new math or "rethinking of antenna theory" is necessary to deal with them -- the same electromagnetic principles apply and they can be treated like any other conductors with finite resistivity. Roy Lewallen, W7EL ------------- All excellent points. I'm thinking - I'm thinking...G Ed Cregger |
opinions on an antenna idea
Roy Lewallen wrote:
Ed Cregger wrote: I have quite a few engineering books on antennas (that I use G), so I can appreciate the value of good, solid engineering text/sources. However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. After all, superconductive radiating elements did not exist before and the math has not been done. Perhaps, their inclusion, will demand something more than a simple extrapolation of existing antenna theory. I believe this to be the point of the OP. . . . And I disagree. The assumption of zero loss is implicit or explicit in nearly all the analyses in your antenna texts and mine. So no new math or "rethinking of antenna theory" is required to deal with lossless conductors. It is, in fact, the simplest case and so underlies virtually all the current theory. What it would do is cause a change in tradeoffs which would be made by engineers in the design of real antennas. However, superconductors (at least all known conventional and high-temperature superconductors) are lossless only at DC. Superconductor loss increases with frequency and, except at DC, with temperature. The resistivity of copper decreases quite dramatically with temperature, so it's not uncommon to find situations at very high frequencies and very cold temperatures where copper does better than a superconductor. Even high temperature superconductors have to be cooled to cryogenic temperatures to do reasonably well at very high frequencies. But again no new math or "rethinking of antenna theory" is necessary to deal with them -- the same electromagnetic principles apply and they can be treated like any other conductors with finite resistivity. Roy Lewallen, W7EL ------------- All excellent points. I'm thinking - I'm thinking...G Ed Cregger |
opinions on an antenna idea
Richard Clark wrote:
On Wed, 05 Dec 2007 10:36:59 -0500, Ed Cregger wrote: However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. Hi Ed, This is uni-dimensional thinking. "A new breakfast cereal could establish the need for a serious rethinking of sewing machine theory." There are probably more things possible ("could establish") than time to consider them - and probably on file pending patent. In that sense, patent publishing could establish the need for a serious rethinking of replacing burning oil for heat. "Could establish" ...this could establish a new form of gaming entertainment in this group. [and conforms to the usage of self-referential claims] 73's Richard Clark, KB7QHC -------------- So, rather than talk about the subject at hand, you would rather argue about the technically poor writing style I employed. No thanks. G Ed, NM2K |
opinions on an antenna idea
On Wed, 05 Dec 2007 16:34:50 -0500, Ed Cregger
wrote: So, rather than talk about the subject at hand, you would rather argue about the technically poor writing style I employed. No thanks. G Hi Ed, Talking already sputtered to the usual banal offerings so common with the glazed-eye "what if we could only reach that golden city on the hill," when I turned to commenting on the only thing left: the quality of entertainment. And going further with plasma antennas indeed! I remember plasma speakers. We've had reports of burning water that would rescue us from our dependence on Oil, -sigh- if only it didn't take more power lighting up a bottle of Evian than you got out of it. But even struggling through this doomed topic finds the cliff crumbling from beneath its heels and its only hope is that the inventors are making a living as scabs writing for daytime TV. 73's Richard Clark, KB7QHC |
opinions on an antenna idea
Ed Cregger wrote: However, the point that the OP was trying to make was that it is likely that superconductive radiating elements could establish the need for a serious rethinking of antenna theory. Actually, the only reason I mentioned superconductors (and thanks to Roy for putting me straight on that point!) was to examine a vanishingly short dipole without worrying about losses. I was not trying to alter existing antenna theory. Alan |
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