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#1
March 11th 05, 06:36 PM
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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. Is it possible you are pointing to F/B for minimum radiation where I am refering to zero front to rear: Regards Art "Richard Harrison" wrote in message ... Art Unwin wrote: "I have just come to realise that if one drew a polygon of element phases in an array and all elements were 180 degrees to its companion element and excluding the driven elemment, the max gain and max front to back will occur at the same frequency!" Would an antenna made entirely of pairs of identical out of phase elements be a good antenna? A "polygon of element phases" must refer to the resultant current in each element and their combined effect at a point (P) for example in the far field. Art must have resolved and composed vectors or phasors at some time. The resultant of any number of vectors can all add to zero or to some other number and direction. A zero sum often happens in physics when systems are in equiblirium. Newton said that any action results in an equal and opposite reaction. Application of a new force often causes no loss in equilibrium, just a corresponding added reaction. The reflected wave from an antenna may change in magnitude in proportion to an incidebt wave yet be nearly exactly equal in magnitude and 180-degrees out of phase with the incident wave, if the reflection is perfect. A polygon is a closed plane bounded by straight sides. It can represenht forces. Art asked if there were anything written about complete front to back cancellation in two radiators carrying oppositely directed signals if I understood the question. Indeed Kraus of W8JK fame has a lot to say about the possibility. Kraus writes about an "Array of Two Driven 1/2-wavelength El;ements. General Case with Equal Currents of Any Phase Relation." in his 1950 edition of "Antennas". It includes on page 294, field patterns for physical spacings and feed phasings. For example, at a spacing of 1/8-wavelength and a phasing of 135-degrees, there is complete cancellation in one direction while there is maximum radiation in the opposite direction. That`s the good news. Now the bad. On page 297 Kraus says: "However, in the flat-top (an advantage placing all elements at maximum height) antenna such losses may have considerable effect on the gain (as the feedpoint resistance is very low). Therefore, the question of losses and of radiating efficiency will be treated in this section in connection with a discussion of arrays of two closely spaced, out-of-phase elements. The term "closely-spaced" will be taken to mean that the elements are spaced 1/4 wavelength or less." Then, Kraus shows another fly in the ointment on page 300: "Hence the Q for 1/8 wavelength spacing is about four times the Q for 1/4 wavelength spacing. Very large Q indicates a large amount of stored energy near the antenna in proportion to the energy radiated per cycle. This also means that the antenna acts like a sharply tuned circuit." So much for bandwidth! Best regards, Richard Harrison, KB5WZI |
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#2
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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#3
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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#5
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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#6
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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#7
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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#8
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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#9
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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#10
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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