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#1
March 11th 05, 07:03 PM
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wrote:
Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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#2
March 11th 05, 08:06 PM
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Yes Cecil, a cardioid pattern ,which cannot produce zero radiation at the
rear 180 degrees of the feed point. Remember we started of with a figure 8 or two balloon pattern so for total reversal of radiation the front lobe must finish up as a perfect circle . Said another way, the two ballons are merged thus making a larger single balloon Best Regards Art "Cecil Moore" wrote in message ... wrote: Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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#4
March 11th 05, 11:05 PM
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A cardioid pattern has radiation in the 180 degree portion behind the feed
point Regards Art "Gene Fuller" wrote in message ... Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. A detailed description of all of this is in Kraus' Antennas, 2nd Ed., in Chapter 11. He uses equations, and he does not mention coordination of balloon patterns, so perhaps you have another new invention. 73, Gene W4SZ wrote: Yes Cecil, a cardioid pattern ,which cannot produce zero radiation at the rear 180 degrees of the feed point. Remember we started of with a figure 8 or two balloon pattern so for total reversal of radiation the front lobe must finish up as a perfect circle . Said another way, the two ballons are merged thus making a larger single balloon Best Regards Art "Cecil Moore" wrote in message ... wrote: Richard I looked thru The ARRL antenna book and I cannot find any antenna let alone the two element phased array that showed ZERO radiation to the rear of the feed point. In my ARRL Antenna Book, 15th edition, page 8-6: Two phased verticals with 1/8WL spacing and phased at 135 degrees shows a perfect cardioid with zero radiation in a direction 180 degrees from the direction of maximum gain, i.e. directly to the rear. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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#5
March 12th 05, 02:14 AM
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wrote:
A cardioid pattern has radiation in the 180 degree portion behind the feed point You want zero radiation in an entire hemisphere? Arecibo probably meets that specification. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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#6
March 12th 05, 03:14 AM
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Yup.
They have now upgraded the mountain road so you don't have to take a spare rear axle with you now. Art "Cecil Moore" wrote in message ... wrote: A cardioid pattern has radiation in the 180 degree portion behind the feed point You want zero radiation in an entire hemisphere? Arecibo probably meets that specification. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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#7
March 14th 05, 05:40 AM
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Gene Fuller wrote:
Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
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#8
March 14th 05, 03:51 PM
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Hi Roy,
I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
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#9
March 14th 05, 04:11 PM
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Gene,
At a matter of interest during the 80s I tried to get to zero radiation at 180 degree point since Lawson stated it was possible. After covering the half acre under the long boom with a ground screen, in fraustration, I finally gave up with the pursuit. On one of my present models the rear radiation never exceed 40db for more than 180 degrees but as Roy pointed out earlier you still have to deal with the higher angles which was the case with my model in that when the angle reached 30 degrees elevation we were back to 20 db.. The center "plume" radiation seems difficult to eradicate. I think I will try your suggetion of radiators with radiators of 0.01 diameter to see what happens Regards Art "Gene Fuller" wrote in message ... Hi Roy, I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
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#10
March 17th 05, 04:09 AM
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Yes, as the wire diameter goes to zero, the current distribution
approaches the same on all elements. But in some cases (where the element height is in the vicinity of a half wavelength) the wires have to get impossibly thin to achieve good f/b with equal magnitude and correctly phased base currents. I guess you could categorize needing a finite diameter wire as a "real world effect" and a zero diameter wire as "theoretically perfect". As I mentioned, it's not hard to do well at a quarter wavelength height, but much harder at heights approaching a half wavelength. For example, I took the EZNEC Cardioid.ez example file and increased the element heights to 0.4 meter (0.4 wavelength) using 25 segments/element. With wire diameter of 10^-15 mm, the front/back ratio was still 32 dB. With the original wire diameter of about 0.24 mm, the front/back was less than 15 dB. And things get worse yet as the elements get closer to a half wavelength high. But in practice, even at a quarter wavelength height, people using phased towers might encounter an unexpectedly low f/b ratio. For anyone who's interested, I've posted the Technical Correspondence piece on my web site. You can get it at http://eznec.com/Amateur/Articles/Current_Dist.pdf. Roy Lewallen, W7EL Gene Fuller wrote: Hi Roy, I have read many of your articles, and I have no doubt you are correct. However, in the ideal case, specifically in the limit as the wire diameter goes to zero, the current perturbation from mutual inductance vanishes. (The mutual inductance does not vanish, only its impact on current distribution.) I just spent a few minutes playing around with EZNEC 3, and I was able to achieve a null of -52 dBi (-57 dBmax) for two half-wave elements, with nominal 90 degree spacing and 90 degree phasing. The wire size was as small as possible. This null was in the symmetry plane and directly in the anti-end-fire direction of course. I expect with more computational precision, and perhaps fine tuning frequencies and dimensions this null could be driven farther. The reported current imbalance was a maximum of 0.2%, mid-way between the center and the ends of the wires. The phase imbalance between the wires was a maximum of 0.2 degrees. I am not trying to say this is practical. I was just pointing out the Art's use of polygons and canceling phasors was not particularly unique. We have since learned that what Art is trying to accomplish is to eliminate all radiation in the back hemisphere. The cardioid example is obviously moot for his quest. 73, Gene W4SZ Roy Lewallen wrote: Gene Fuller wrote: Art, Why not? The cardioid pattern from a two-element array was reported back as least as far as 1937, by the famous George H. Brown. In the ideal case (free space, no losses, etc.) the radiation directly to the rear is precisely zero. If you add various real world effects then the back lobe is not precisely zero, and this is shown in the ARRL Antenna Book referenced by Cecil. . . . Actually, this isn't quite true. If you manage to get perfectly phased and equal magnitude currents in two identical elements where the phase angle equals 180 degrees minus the element spacing (such as the classic 90-degree fed, 90-degree spaced cardioid), you don't get an infinite front-back ratio. In the case of the cardioid with typical diameter quarter wavelength elements, you end up with around a 35 dB front/back ratio. With longer elements, close to a half wavelength, the front/back ratio can deteriorate to less than 10 dB when base currents are identical in magnitude and correctly phased. The reason is that the mutual coupling between elements alters the current distribution on the elements. The mutual coupling from element 1 to element 2 isn't the same as the coupling from element 2 to element 1 (the mutual Z is the same, but the coupled voltage and coupled impedance aren't). The net result is that the two elements have different current distributions, so despite having identical magnitude base currents the two elements don't generate equal magnitude fields. The overall fields from the two elements end up being imperfectly phased, also. This occurs for theoretically perfect and perfectly fed elements, and isn't due to "real world" effects. I published some comments about this effect in "Technical Correspondence" in July 1990 QST ("The Impact of Current Distribution on Array Patterns"). I'm certainly not the first to have observed it -- some papers published as early as the '40s are referenced in my article. But I had never seen its effect on front/back ratio of cardioids mentioned before. Modern versions of the ARRL Antenna Book clearly show the small reverse lobe of a typical antenna with quarter wavelength elements. I stumbled across it when doing some modeling with ELNEC, the predecessor of EZNEC, and originally thought it was an error in the program. You'll see it in a plot from the Cardioid.EZ EZNEC example file (which is also included with the demo program), and a brief explanation in the corresponding Antenna Notes file. A theoretically infinite front/back ratio can be achieved by modification of the base currents. The amount of modification required depends on the length and diameter of the elements. Only a small modification is needed if elements are a quarter wavelength high and small diameter, but in that case, real world effects will probably have at least as much and likely more of an effect on the front/back than the current distribution phenomenon. Rather drastic modification is required of the base currents of elements approaching a half wavelength high, however, as elaborated in the "Technical Correspondence" piece. Roy Lewallen, W7EL |
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