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On Thu, 31 Mar 2005 23:11:04 -0800, "John Smith"
wrote: My gawd! Is this the same Richard Clark which I just reported to mental health for a three day observation??? Hi Brett, Catch a particularly interesting movie? A good one for programmers from Argentina: "Una Sombra Ya Pronto Seras" comes to mind. However, in a diverse crowd at the end of the road and for those willing to truck 2 tons of nitro over the Andes: "Le Salaire De La Peur." For Art, all of this is like sitting through "The Saragossa Manuscript." ;-) 73's Richard Clark, KB7QHC |
Err, Richard, I like the other personality better, yanno, the technical one
which imitated sanity in a believable manner? Hey, just how many of those multiple personalities do you have? But, only one sane one? Regards -- Hay, if'n ya'll cun't konstructivly partecipete in this har disscusion, haw aboot speel-checkin it fer me? "Richard Clark" wrote in message ... On Thu, 31 Mar 2005 23:11:04 -0800, "John Smith" wrote: My gawd! Is this the same Richard Clark which I just reported to mental health for a three day observation??? Hi Brett, Catch a particularly interesting movie? A good one for programmers from Argentina: "Una Sombra Ya Pronto Seras" comes to mind. However, in a diverse crowd at the end of the road and for those willing to truck 2 tons of nitro over the Andes: "Le Salaire De La Peur." For Art, all of this is like sitting through "The Saragossa Manuscript." ;-) 73's Richard Clark, KB7QHC |
On Fri, 1 Apr 2005 06:33:17 -0800, "John Smith"
wrote: Hey, just how many of those multiple personalities do you have? But, only one sane one? Hi Brett, How many screen names do you have? Is your serial multiple personality somehow better than what you see as parallel multiple personality? What about false personality? What you perceive as a talent in yourself is projected to be the mark of a fool upon another. In this group, we observe the laws of what are called symmetry and reciprocity - two very strong, scientific principles. The responsibility of posting to a newsgroup is that only you choose what you think is important to discuss. Of your choices to go technical or metaphysical (or CB truckers), I have no control over. I have responded to your technical comments and your sidebars (as now) equally. Upon my analysis of the Top Hat on J-Pole you could have acknowledged the results, asked questions about particulars of implementation, or challenged the results to greater scrutiny - and simply ignored side topics. I observe you have not shown any interest in obtaining the model used. We share models as a commonplace expectation of peer review. Any of this would have constituted technical correspondence. Instead, the side-topics attracted your notice and you took that route instead. I welcome all correspondence, but I am not responsible for what you respond to when there are choices - especially when you can add topics as well (a privilege I exercise, you present above, and I do not object to in anyone). So, as I offered so long ago: 1. Adding a coil offers loss [demonstrated]; 2. Physical size in relation to wavelength dominates launch characteristics [demonstrated]; 3. It barely matters [less than 1dB]. The launch gain of a half wave radiator did not appear simply because a much shorter antenna is ginned up to be antiresonant [demonstrated]. This also returns me to an unanswered question (also technical): The rubber duckie antenna is certainly useful for some situations, however, it is better if it stays on the walkie-talkie and we use another design for portable or base use. Given this opprobrium, what makes its cousin for 20M more suitable? OR I am both content, and competent to discuss those movies that would have been more enjoyable than the alternative of sitting through the snooze of finding gain from a small antenna. OR I am content discussing how to obtain the most gain from a small antenna, or simply pointing you at 300 pages of results I've done years ago. 73's Richard Clark, KB7QHC |
You seem to have a problem with my name...
Would what I say have more importance to you if my name were "George Walker Bush?" How about "Princess Dianna?", "Lord Balder Wash?" How about an alcoholic PhD. down on his luck--Edward Noble, PhD., in the gutter? Is a rose not still a rose by any other name? Is logic not still the same logic no matter who speaks it? I have hoisted the pirate's flag in rebellion of fools who place importance on bloodline, circumstance of life or the self-importance of their being or--even for the sake of control alone--BUCKO! However, instead of a skull and bones, the symbol on my banner looks surprisingly like--JOHN SMITH!!! (who was that masked man anyway? (hey, that voice sounded like Tonto!)--well, maybe it looks even a bit like Alfred E. Newman! I'll tell ya what, pick a noble sounding name, the sound of which inspires shock and awe in those who fall victim to my speech--I'll change my gmail account for you....) You have already tempted me to replace my Russian made "California Kilowatt" (well, 5.5 KW--but it is hard to get true linear performance at reduced input power and drive grin) and "keenwood", with a 6X2 push pull in parallel, 1500w+ Chicken Band linear! And, that 225 amp 13.8 v power supply gets hot when I just think about a key down! (the sound of all those cooling fans are annoying, don't cha think?) Somewhere in the attic is an old Johnson 11 meter with increased output to drive the 1500!!! I can hear the "Anyone got a Radio Check", "Break channel 19", "10-4 GOOD BUDDY", "AUUDDDDIIIIIOOOOO!" and "What's up ahead southbound?" coming off the 11 meter band now!--it brings a tear to my eye I tell ya, the memories.... and the ham bashing sessions we used to engage in---felonious I tell ya! My only fear is the feds detecting me by the strange purplish glow emanating from my antenna, while I kick back and shoot skip--sipping a Corona... But then, I am NOT normal--I like to have my bit of fun...or, at least I did in the past... If your test of intellect is whether one agrees with you or not--or, whether someone plays your game of insane obfuscation--or whether one withholds his/her identity for protection from the insane (however, I did just indulge you a bit, didn't I?), or whether one "used to be"/"still is" a part-time "Chicken Bander"--I have failed, be prepared for the masses which will follow... Regards -- Hay, if'n ya'll cun't konstructivly partecipete in this har disscusion, haw aboot speel-checkin it fer me? "Richard Clark" wrote in message ... On Fri, 1 Apr 2005 06:33:17 -0800, "John Smith" wrote: Hey, just how many of those multiple personalities do you have? But, only one sane one? Hi Brett, How many screen names do you have? Is your serial multiple personality somehow better than what you see as parallel multiple personality? What about false personality? What you perceive as a talent in yourself is projected to be the mark of a fool upon another. In this group, we observe the laws of what are called symmetry and reciprocity - two very strong, scientific principles. The responsibility of posting to a newsgroup is that only you choose what you think is important to discuss. Of your choices to go technical or metaphysical (or CB truckers), I have no control over. I have responded to your technical comments and your sidebars (as now) equally. Upon my analysis of the Top Hat on J-Pole you could have acknowledged the results, asked questions about particulars of implementation, or challenged the results to greater scrutiny - and simply ignored side topics. I observe you have not shown any interest in obtaining the model used. We share models as a commonplace expectation of peer review. Any of this would have constituted technical correspondence. Instead, the side-topics attracted your notice and you took that route instead. I welcome all correspondence, but I am not responsible for what you respond to when there are choices - especially when you can add topics as well (a privilege I exercise, you present above, and I do not object to in anyone). So, as I offered so long ago: 1. Adding a coil offers loss [demonstrated]; 2. Physical size in relation to wavelength dominates launch characteristics [demonstrated]; 3. It barely matters [less than 1dB]. The launch gain of a half wave radiator did not appear simply because a much shorter antenna is ginned up to be antiresonant [demonstrated]. This also returns me to an unanswered question (also technical): The rubber duckie antenna is certainly useful for some situations, however, it is better if it stays on the walkie-talkie and we use another design for portable or base use. Given this opprobrium, what makes its cousin for 20M more suitable? OR I am both content, and competent to discuss those movies that would have been more enjoyable than the alternative of sitting through the snooze of finding gain from a small antenna. OR I am content discussing how to obtain the most gain from a small antenna, or simply pointing you at 300 pages of results I've done years ago. 73's Richard Clark, KB7QHC |
On Fri, 1 Apr 2005 14:47:31 -0800, "John Smith"
wrote: You seem to have a problem with my name... Hi Brett, one withholds his/her identity for protection You could change your name to Pollyanna with that kind of naivete. Perhaps this is your cue to another movie. "The Whole Nine Yards?" Cute, but the sequel sucked big time. be prepared for the masses which will follow... Say a few Hail Marys too. ;-) 73's Richard Clark, KB7QHC |
Richard:
(Psalms 9999:9999) You fill mine and others threads with your garbage... You are irksome like a small self-centered, temper throwing child... You name and call is on the verge of filling me with loathing... If you have any education, if your up-bringing occurred in the civilized world--it should be sufficient to just ask you to cease... But, you have convinced me that is out of the realm of possibility. However, if you won't quit for sheer decency... quit for your own benefit. As surely you must realize, the behavior you exhibit here is almost certainly NOT localized to here and others will KNOW this, do not be a fool for yourself. You give away the fact that in REAL life you have problems getting along with others; here is just a continuation of that. You lead one to believe that your neighbors hold you in contempt, that you purposely hinder and set to conflict all which you involve yourself in. I even would bet law enforcement agencies know you personally and dread your calls. You seem to take pride in appearing as a bully and expect others to enjoy your performance; only idiots and other bullies will enjoy you. You lead me to pity your fellow workers--especially if one happens to invoke your ire. I pray you have no wife... Halt your present course man! Human memory is fleeting, what shreds of dignity are left you--KEEP!!!! Regards -- Hay, if'n ya'll cun't konstructivly partecipete in this har disscusion, haw aboot speel-checkin it fer me? "Richard Clark" wrote in message ... On Fri, 1 Apr 2005 14:47:31 -0800, "John Smith" wrote: You seem to have a problem with my name... Hi Brett, one withholds his/her identity for protection You could change your name to Pollyanna with that kind of naivete. Perhaps this is your cue to another movie. "The Whole Nine Yards?" Cute, but the sequel sucked big time. be prepared for the masses which will follow... Say a few Hail Marys too. ;-) 73's Richard Clark, KB7QHC |
On Fri, 1 Apr 2005 16:22:38 -0800, "John Smith"
wrote: it should be sufficient to just ask you to cease... But, you have convinced me that is out of the realm of possibility. Hi Brett, You must've had some great difficulty with those kill file instructions. I'm sure if you posted them again, someone might help you. Perhaps if they were written in assembler. :-) 73's Richard Clark, KB7QHC |
Hmmmm. The top had does NOT seem to be such a good idea, at least not in
combination with the helical coil--with total element length as a helical and evenly spread out. If I just move a wrench in the yard around the antenna I get wide SWR swings! (OK, maybe that is exaggerating a bit--but you get my meaning.) The top hat seems to give the antenna an almost "magical ability" to couple into any metal object of sufficient size, cars are noteworthy examples!!! Regards, John -- I would like to point out, I do appreciate the "Been there--done that!" posts. Indeed, now your observations, comments and discourse should be filled with wisdom--I am listening!!! "Richard Clark" wrote in message ... On Fri, 1 Apr 2005 16:22:38 -0800, "John Smith" wrote: it should be sufficient to just ask you to cease... But, you have convinced me that is out of the realm of possibility. Hi Brett, You must've had some great difficulty with those kill file instructions. I'm sure if you posted them again, someone might help you. Perhaps if they were written in assembler. :-) 73's Richard Clark, KB7QHC |
On Sun, 10 Apr 2005 08:14:38 -0700, "John Smith"
wrote: Hmmmm. The top had does NOT seem to be such a good idea, at least not in combination with the helical coil--with total element length as a helical and evenly spread out. Hi Brett, If it's not a good idea, it's not a bad idea either. A top hat is simply a means to an end - or in terms of good and bad, it makes the best of a poor situation. If I just move a wrench in the yard around the antenna I get wide SWR swings! (OK, maybe that is exaggerating a bit--but you get my meaning.) The top hat seems to give the antenna an almost "magical ability" to couple into any metal object of sufficient size, cars are noteworthy examples!!! You may have a future in Home security and monitoring electronics. 73's Richard Clark, KB7QHC |
John Smith wrote:
If I just move a wrench in the yard around the antenna I get wide SWR swings! Use a different length wrench. :-) -- 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 =---- |
Use a different length wrench. :-)
================== Or use one made of well-varnished dry wood. |
Richard:
Your sense of humor is greatly enjoyed :) Warmest regards, John -- I would like to point out, I do appreciate the "Been there--done that!" posts. Indeed, now your observations, comments and discourse should be filled with wisdom--I am listening!!! "Richard Clark" wrote in message ... On Sun, 10 Apr 2005 08:14:38 -0700, "John Smith" wrote: Hmmmm. The top had does NOT seem to be such a good idea, at least not in combination with the helical coil--with total element length as a helical and evenly spread out. Hi Brett, If it's not a good idea, it's not a bad idea either. A top hat is simply a means to an end - or in terms of good and bad, it makes the best of a poor situation. If I just move a wrench in the yard around the antenna I get wide SWR swings! (OK, maybe that is exaggerating a bit--but you get my meaning.) The top hat seems to give the antenna an almost "magical ability" to couple into any metal object of sufficient size, cars are noteworthy examples!!! You may have a future in Home security and monitoring electronics. 73's Richard Clark, KB7QHC |
I am planing on side mounting a 2 meter J-Pole part way up on a tower.
How far from the tower should I mount the antenna? I do plan on making the final adjustment of the feed point with the antenna mounted up a few feet on the tower. The radiation pattern is not a big concern. In fact I would prefer that it favor one quadrant. Thanks for any advice. Ron - K0QVF http://www.southslope.net/~ronton/ |
Ron, K0QVF wrote:
"How far from the tower should I mount the antenna?" Supposing you want the least effect on the pattern and impedance, place the antenna as far as possible from the tower.. Satisfaction is obtained with much closer spacing. The tower is non-resonant. Its reactance will impede induced current from a radiator which is really close by. If there is little induced current there is little re-radiated energy to foul the pattern. The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in field intensity versus spacing from a flat reflector. At 0.5 wavelength the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone which has about 2.14 dB gain over an isotropic. The graph shows a gain of about 2.14 dBi with a spacing of only 0.1 wavelength spacing. So, anything greater than 0.1 wavelength from the tower should be fine. That`s 20 cm in the 2-meter band, or about 8 inches. More distance means less coupling and should be better. Best regards, Richard Harrison, KB5WZI |
The only way to get 0 dBi maximum gain from a lossless antenna is to
have a perfectly circular pattern in all azimuths and elevations. It's not possible to have a maximum gain (that is, gain in the best direction) less than 0 dBi unless loss is present. This is something everyone with a basic understanding of antennas should know. So it seemed to me very unlikely that a dipole spaced a half wavelength from a reflector would have a perfectly circular pattern and, therefore, it must have gain greater than 0 dBi in some direction. I don't have Kraus' third edition (yet), but there's a diagram on p. 546 of the second edition which I suspect is the same as the one Richard is referring to. The caption under the graph clearly says that the gain at 0.5 wavelength is 0 dB *relative to a half wave dipole in free space*, or about 2.15 dBi, not 0 dBi. If the third edition really says that the gain of a half wave antenna spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and should be brought to the editor's attention so it can be corrected. I also believe that while you might draw some possible parallels, you can't directly apply the characteristics of an antenna in proximity to an infinite reflecting plane (as in Kraus) to those of an antenna in proximity to a tower. Roy Lewallen, W7EL Richard Harrison wrote: . . . The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in field intensity versus spacing from a flat reflector. At 0.5 wavelength the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone which has about 2.14 dB gain over an isotropic. The graph shows a gain of about 2.14 dBi with a spacing of only 0.1 wavelength spacing. So, anything greater than 0.1 wavelength from the tower should be fine. That`s 20 cm in the 2-meter band, or about 8 inches. More distance means less coupling and should be better. Best regards, Richard Harrison, KB5WZI |
Roy Lewallen wrote:
I also believe that while you might draw some possible parallels, you can't directly apply the characteristics of an antenna in proximity to an infinite reflecting plane (as in Kraus) to those of an antenna in proximity to a tower. Roy is gallantly trying to be non-commercial about this, so someone else had better say it: Download the free demo version of any NEC-based antenna modeling program, and model the effects of the actual tower geometry. It's the only way to see what's really happening. You'll get a good first approximation by modeling the tower as its three very long vertical legs. Since you're only interested in the effects of the tower on the J-pole's omnidirectional pattern, you can use any kind of vertical omni to model the antenna itself - a center-fed dipole for simplicity. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
I agree, although I have joked with Roy, his EZNEC 4.0 is a serious and
"state of the art" application. Although my experience with applications of this nature is limited (I have not examined EVERY one) it is a well designed and thought out work. His efforts are notable... Regards, John "Roy Lewallen" wrote in message ... The only way to get 0 dBi maximum gain from a lossless antenna is to have a perfectly circular pattern in all azimuths and elevations. It's not possible to have a maximum gain (that is, gain in the best direction) less than 0 dBi unless loss is present. This is something everyone with a basic understanding of antennas should know. So it seemed to me very unlikely that a dipole spaced a half wavelength from a reflector would have a perfectly circular pattern and, therefore, it must have gain greater than 0 dBi in some direction. I don't have Kraus' third edition (yet), but there's a diagram on p. 546 of the second edition which I suspect is the same as the one Richard is referring to. The caption under the graph clearly says that the gain at 0.5 wavelength is 0 dB *relative to a half wave dipole in free space*, or about 2.15 dBi, not 0 dBi. If the third edition really says that the gain of a half wave antenna spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and should be brought to the editor's attention so it can be corrected. I also believe that while you might draw some possible parallels, you can't directly apply the characteristics of an antenna in proximity to an infinite reflecting plane (as in Kraus) to those of an antenna in proximity to a tower. Roy Lewallen, W7EL Richard Harrison wrote: . . . The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in field intensity versus spacing from a flat reflector. At 0.5 wavelength the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone which has about 2.14 dB gain over an isotropic. The graph shows a gain of about 2.14 dBi with a spacing of only 0.1 wavelength spacing. So, anything greater than 0.1 wavelength from the tower should be fine. That`s 20 cm in the 2-meter band, or about 8 inches. More distance means less coupling and should be better. Best regards, Richard Harrison, KB5WZI |
Roy Lewallen
, W7EL wrote: "I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of thye second edition which I suspect is the same as the one Richard is referring to." I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the identical groph is on page 327 in it. If you look at the patterns of a 1/2-wavelength antenna at spacings of 1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby, they are all nearly circular, indicating little distortion in their unblocked direction. Best regards, Richard Harrison, KB5WZI |
Roy, I looked at the graph and get a different interpretation. Every spacing
except 1/2 wave length spacing shows gain. That being the case the pattern must be distorted for all cases except .5 wavelength. I have Kraus 1950 edition "Richard Harrison" wrote in message ... Roy Lewallen , W7EL wrote: "I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of thye second edition which I suspect is the same as the one Richard is referring to." I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the identical groph is on page 327 in it. If you look at the patterns of a 1/2-wavelength antenna at spacings of 1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby, they are all nearly circular, indicating little distortion in their unblocked direction. Best regards, Richard Harrison, KB5WZI |
*Chuckle* Yes, they're nearly circular for close spacings (1/8 and 1/16
wavelength), but those aren't "circular patterns". The antenna isn't at the center of the circle -- it's on the circumference(*). I hope you're not seriously presenting this as evidence that the gain can be zero dBi. The patterns and gains are, or should be, identical to those of half of a W8JK array with twice the spacing. For example, the pattern and gain of a dipole 1/8 wavelength above a perfect reflector is exactly the same as half the pattern of a W8JK array made of two of those dipoles spaced 1/4 wavelength. And those are far from an isotropic pattern. (*) The patterns in the book also are circular because of the scale factor which was chosen. If some other scale factor were chosen, they wouldn't be circular. A truly circular pattern (one with the antenna at the center) is circular regardless of the scale. You can illustrate this with the EZNEC demo or standard program. Open the dipole1.EZ example file and click FF Plot to generate a 2D pattern. Notice that the two lobes are roughly circular in shape (with the antenna at the circumference, as in Kraus' diagrams). Leaving the 2D plot on the screen, in the main window Options menu, select 2D Plot Scale and choose Linear dB. Note how the shape of the two lobes changes. You can get a wide variety of shapes by changing the scale -- this technique is very useful to antenna manufacturers to make their beam lobes look extra narrow. In the main window, change the Plot Type to Elevation. Due to the orientation of the antenna, you'll now get a plot of the pattern looking end-on to the dipole. This will be a truly circular pattern. Click FF Plot to generate the pattern. Change the 2D Plot Scale back to ARRL Type and note that it remains circular. An isotropic antenna has a circular pattern like this regardless of the orientation; the 3D plot is a sphere. And that's the only antenna which can have a free space gain as low as 0 dBi when there's no loss. Roy Lewallen, W7EL Richard Harrison wrote: Roy Lewallen , W7EL wrote: "I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of thye second edition which I suspect is the same as the one Richard is referring to." I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the identical groph is on page 327 in it. If you look at the patterns of a 1/2-wavelength antenna at spacings of 1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby, they are all nearly circular, indicating little distortion in their unblocked direction. Best regards, Richard Harrison, KB5WZI |
I believe that all three people (Richard, Roy, and Fred) who have
commented on this topic have misread the figure in Kraus. The gain at one-half wave spacing from the reflector is *zero*, not zero dB (relative to a dipole), nor zero dBi. As expected from the simple mirror image model there is complete far field cancellation in the phi = 0, theta = 0 direction. That is the only direction referenced in the figure. 73, Gene W4SZ Roy Lewallen wrote: The only way to get 0 dBi maximum gain from a lossless antenna is to have a perfectly circular pattern in all azimuths and elevations. It's not possible to have a maximum gain (that is, gain in the best direction) less than 0 dBi unless loss is present. This is something everyone with a basic understanding of antennas should know. So it seemed to me very unlikely that a dipole spaced a half wavelength from a reflector would have a perfectly circular pattern and, therefore, it must have gain greater than 0 dBi in some direction. I don't have Kraus' third edition (yet), but there's a diagram on p. 546 of the second edition which I suspect is the same as the one Richard is referring to. The caption under the graph clearly says that the gain at 0.5 wavelength is 0 dB *relative to a half wave dipole in free space*, or about 2.15 dBi, not 0 dBi. If the third edition really says that the gain of a half wave antenna spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and should be brought to the editor's attention so it can be corrected. I also believe that while you might draw some possible parallels, you can't directly apply the characteristics of an antenna in proximity to an infinite reflecting plane (as in Kraus) to those of an antenna in proximity to a tower. Roy Lewallen, W7EL Richard Harrison wrote: . . . The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in field intensity versus spacing from a flat reflector. At 0.5 wavelength the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone which has about 2.14 dB gain over an isotropic. The graph shows a gain of about 2.14 dBi with a spacing of only 0.1 wavelength spacing. So, anything greater than 0.1 wavelength from the tower should be fine. That`s 20 cm in the 2-meter band, or about 8 inches. More distance means less coupling and should be better. Best regards, Richard Harrison, KB5WZI |
I have to apologize. I also misinterpreted the graph. The confusing
graph is, in the Second Edition, Fig. 12-4 on p. 546. I've uploaded it temporarily to http://eznec.com/misc/Kraus2_Fig_12-4.JPG. The caption says that the gain is relative to a half wave dipole in free space with the same power input. The numbers on the left side Y axis are the numerical gain, 0 to 3, relative to a dipole. 0 represents a numerical gain mulitplier of zero, or no field intensity at all. 1 is the gain of a half wave dipole in free space (about 2.15 dBi). A value of 2 represents a gain in field intensity by a factor of 2, or 6 dB relative to a dipole. The right hand side Y axis labels are the gain in dBi. Note that 2.1 dBi corresponds approximately with the value of 1.0 on the left side. The bottom horizontal line corresponds to zero field strength -- a gain of minus infinity dBi -- *not* zero dBi as Richard said, or zero dB relative to a dipole, which I initially assumed. What I missed was that the gain is "in direction [phi] = 0", quoting from the caption. So this isn't a graph of the maximum gain, but the *gain in one specific direction* -- normal to the reflecting plane. At 0.5 wavelength spacing, the "gain in field intensity" (left set of Y axis labels) is a *factor* of zero, meaning that the field strength is zero, or minus infinity dBi. Sure enough, if you model the antenna, or two elements spaced one wavelength, you find that the pattern has a null directly broadside to the antenna ([phi] = 0). It has gain in other directions, but that's not what the graph is showing. Of course, any lossless antenna has a gain of 0 dBi in some directions. In the case of the element and reflecting plane, the gain directly broadside to the antenna has a gain of 0 dBi at spacings of roughly 0.425 and 0.575 wavelengths. There's no particular significance to this -- the maximum gain is greater in other directions. These gains and patterns can easily be seen with any modeling program, including the EZNEC demo, by modeling a dipole over perfect ground. You can also model two elements fed 180 degrees out of phase at twice the spacing and no ground and see that the pattern is identical except for being bidirectional. Roy Lewallen, W7EL Fred W4JLE wrote: Roy, I looked at the graph and get a different interpretation. Every spacing except 1/2 wave length spacing shows gain. That being the case the pattern must be distorted for all cases except .5 wavelength. I have Kraus 1950 edition "Richard Harrison" wrote in message ... Roy Lewallen , W7EL wrote: "I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of thye second edition which I suspect is the same as the one Richard is referring to." I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the identical groph is on page 327 in it. If you look at the patterns of a 1/2-wavelength antenna at spacings of 1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby, they are all nearly circular, indicating little distortion in their unblocked direction. Best regards, Richard Harrison, KB5WZI |
You're absolutely correct.
Roy Lewallen, W7EL Gene Fuller wrote: I believe that all three people (Richard, Roy, and Fred) who have commented on this topic have misread the figure in Kraus. The gain at one-half wave spacing from the reflector is *zero*, not zero dB (relative to a dipole), nor zero dBi. As expected from the simple mirror image model there is complete far field cancellation in the phi = 0, theta = 0 direction. That is the only direction referenced in the figure. 73, Gene W4SZ |
Gene Fuller, W4SZ wrote:
"As expected from the simple mirror image model there is complete far field cancellation in the phi = 0. theta = 0 direction." Right. Real and virtual imaqges are equidistant and 180-degrees out of phase perpendicular to the reflector. The more distant the radiator from the reflector, the smaller its image appears. The phase difference between the radiator and its image varies with the distance between them. At 1/2-wavelength from the reflector, a round-trip takes 360-degrees and leaves the phase unchanged. However, the reflection inverts the phase. Incident and reflected waves cancel perpendicular to the reflector. Two antenna elements, or one with its image in a reflectoe, will be directional. An unobtainable isotropic is the reference, so te antenna has gain. There is no way to mount a J-pole next to a tower and preserve its omni characteristics. The question is which directions would you favor and which would you diminish? You know the tower may be impenetrable, and the vector sum of incident and reflected signals directly opposite the tower depends on the antenna to tower spacing. An antenna modeling program should be able io show you your options. You know thet 1/2-wave spcing cancels in the diretion of the antenna mounted on the tower face. Add or subtract a 1/4-wave to the spacing, and the incident and reflected waves reinforce instead of cancel. As I think Roy said, you pay your money and take your choice. Best regards, Richard Harrison, KB5WZI |
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