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"Richard Harrison" wrote in message ... Art, KB9MZ wrote: "O.K. Reg, I`ll take you up on the 50 dB F/B." It requires good balance for such cancellation. Yes, it will not come by accident but Reg has said that 50 db F/B is "silly"! Why would he say that without back up evidence? He also did not offer a max F/B that was obtainable in theory. I have seen computor results that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot be trusted? Obviously 50 db is hard to get but is it beyond the realms of possibility? The F/B that you offered is a bit unfair but then it is an antenna and Reg did not qualify his statement. I personaly modelled a antenna that comprised of a driven element with the remaining elements acting as reflectors only. And we all know that extra reflectors are a waste of time for H.F.( grin ) but they do give very high F/B figures. I wonder what a corner reflector antenna would provide Regards Art KB9MZ......XG Kraus gives the gain for a 100-meter dish near Bonn, Germany on page 676 of the 3rd edition of "Antennas". Gain is a function of frequency and varies from about 48 dB at 300 MHz to about 98 dB at 150 GHz. I don`t know if shielding has been added to make this a "high-performance" dish but I would wager that this enormous radio ear and mouth has an excellent front-to-back ratio. Probably exceeds forward gain at some azimuths and elevation angles. Arnold B. Bailey has a lot to say anout a "connected" (driven) element and a parasitic reflector, starting about on 447 of "TV and Other Receiving Antennas", Bailey says: "The optimum spacing for highest gain of a parasitic reflector is S=0.15 wavelength plus or minus 0.025. Here the relleector is operated at Q=+1 (longer than resonant) and the gain in direction 1 is approximately 5.5 dB. Reasonable compromise for a less critical system is to use a spacing of 0.2 wavelength and a parasitic element longer than resonant (at Q=+1). This case gives a gain in direction 1 of 5 dB." On page 440 Bailey says: The front-to-back ratio ---is 17.5 dB in this case, and the Y/X ratio only about 9 dB, where +X represents the optimum direction.---" A picture is worth 1000 words. Best regards, Richard Harrison, KB5WZI |
Just for the sake of curriosity: what if a RASER "Radio (frequency)
Amplification by Stimulated Emission of Radiation" device existed? 50 db would become acceptable and common place overnight, huh? MASER's exist, why now long wavelength? Sure the antenna at HF would be the size of the pentagon, but Bill Gates wouldn't blink an eye! grin Regards -- 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!!! " wrote in message news:P2o4e.22497$NW5.1099@attbi_s02... "Richard Harrison" wrote in message ... Art, KB9MZ wrote: "O.K. Reg, I`ll take you up on the 50 dB F/B." It requires good balance for such cancellation. Yes, it will not come by accident but Reg has said that 50 db F/B is "silly"! Why would he say that without back up evidence? He also did not offer a max F/B that was obtainable in theory. I have seen computor results that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot be trusted? Obviously 50 db is hard to get but is it beyond the realms of possibility? The F/B that you offered is a bit unfair but then it is an antenna and Reg did not qualify his statement. I personaly modelled a antenna that comprised of a driven element with the remaining elements acting as reflectors only. And we all know that extra reflectors are a waste of time for H.F.( grin ) but they do give very high F/B figures. I wonder what a corner reflector antenna would provide Regards Art KB9MZ......XG Kraus gives the gain for a 100-meter dish near Bonn, Germany on page 676 of the 3rd edition of "Antennas". Gain is a function of frequency and varies from about 48 dB at 300 MHz to about 98 dB at 150 GHz. I don`t know if shielding has been added to make this a "high-performance" dish but I would wager that this enormous radio ear and mouth has an excellent front-to-back ratio. Probably exceeds forward gain at some azimuths and elevation angles. Arnold B. Bailey has a lot to say anout a "connected" (driven) element and a parasitic reflector, starting about on 447 of "TV and Other Receiving Antennas", Bailey says: "The optimum spacing for highest gain of a parasitic reflector is S=0.15 wavelength plus or minus 0.025. Here the relleector is operated at Q=+1 (longer than resonant) and the gain in direction 1 is approximately 5.5 dB. Reasonable compromise for a less critical system is to use a spacing of 0.2 wavelength and a parasitic element longer than resonant (at Q=+1). This case gives a gain in direction 1 of 5 dB." On page 440 Bailey says: The front-to-back ratio ---is 17.5 dB in this case, and the Y/X ratio only about 9 dB, where +X represents the optimum direction.---" A picture is worth 1000 words. Best regards, Richard Harrison, KB5WZI |
|
What are "front" and "back"? If the maximum forward lobe is +10 dBi at
an elevation angle of 23 degrees and the best null is -30 dBi at an azimuth angle 160 degrees from the peak forward lobe, and at an elevation angle of 47 degrees, are you saying that by your definition the front/back ratio is 40 dB? If so, I guess that's interesting but I can't imagine what it might be good for. Who's "we"? Roy Lewallen, W7EL Jerry Martes wrote: Roy When I was working with antennas, we considered the antenna's F/B ratio used the max of the front compared to the max of the back. But, I get the impression that the rules are different now. Jerry |
Roy
I make no claim to being qualified to discuss antennas with you when we are in disagreement. I worked as an antenna design engineer for 15 years till 1968 when I was layed off from TRW. I was never a high level theorist but managed to hold a decent position with designing hardware. I did work with some highly qualified engineers from whom I thought I obtained alot of knowledge about antennas. Thats why I was bold enough to say "we". I still maintain a casual relationship with George Oltman who you might know from his association with antenna groups with IEEE. As for the F/B, I considered that to identify Front to Back of the antenna's radiation pattern. I would consider it appropriate to identify the radiation toward the "Front" as the max radiation to the front. Then, it seems that the numerical level used for the "F/B ratio" should be max to the Back. I make no argument that this definition I use is *the* way F/B is. But, dont we define "side lobe level" as the ratio of the main beam Max to the side lobe Max? Be aware, I dont write to correct your thinking. I did consider the F/B to be flawed when the main beam Max is compared with a rear radiation Min. I'll consider myself corrected and stop posting. Jerry "Roy Lewallen" wrote in message ... What are "front" and "back"? If the maximum forward lobe is +10 dBi at an elevation angle of 23 degrees and the best null is -30 dBi at an azimuth angle 160 degrees from the peak forward lobe, and at an elevation angle of 47 degrees, are you saying that by your definition the front/back ratio is 40 dB? If so, I guess that's interesting but I can't imagine what it might be good for. Who's "we"? Roy Lewallen, W7EL Jerry Martes wrote: Roy When I was working with antennas, we considered the antenna's F/B ratio used the max of the front compared to the max of the back. But, I get the impression that the rules are different now. Jerry |
Don't quit posting Jerry, you are an asset to the group.
It is just that courtesy is not a requirement in this group so some posts tend to be a bit sharp and personal Look forward to hearing from you again Regards Art "Jerry Martes" wrote in message news:xNr4e.3912$%b1.1814@trnddc08... Roy I make no claim to being qualified to discuss antennas with you when we are in disagreement. I worked as an antenna design engineer for 15 years till 1968 when I was layed off from TRW. I was never a high level theorist but managed to hold a decent position with designing hardware. I did work with some highly qualified engineers from whom I thought I obtained alot of knowledge about antennas. Thats why I was bold enough to say "we". I still maintain a casual relationship with George Oltman who you might know from his association with antenna groups with IEEE. As for the F/B, I considered that to identify Front to Back of the antenna's radiation pattern. I would consider it appropriate to identify the radiation toward the "Front" as the max radiation to the front. Then, it seems that the numerical level used for the "F/B ratio" should be max to the Back. I make no argument that this definition I use is *the* way F/B is. But, dont we define "side lobe level" as the ratio of the main beam Max to the side lobe Max? Be aware, I dont write to correct your thinking. I did consider the F/B to be flawed when the main beam Max is compared with a rear radiation Min. I'll consider myself corrected and stop posting. Jerry "Roy Lewallen" wrote in message ... What are "front" and "back"? If the maximum forward lobe is +10 dBi at an elevation angle of 23 degrees and the best null is -30 dBi at an azimuth angle 160 degrees from the peak forward lobe, and at an elevation angle of 47 degrees, are you saying that by your definition the front/back ratio is 40 dB? If so, I guess that's interesting but I can't imagine what it might be good for. Who's "we"? Roy Lewallen, W7EL Jerry Martes wrote: Roy When I was working with antennas, we considered the antenna's F/B ratio used the max of the front compared to the max of the back. But, I get the impression that the rules are different now. Jerry |
Jerry Martes wrote:
"When I was working with antwennas, we considered the antenna`s F/B ratio used the max of the front to the max of the back. But I get the impression that the erules are different now." Front-to-back ratio is defined as the ratio of power gain between front and rear of a directional antenna. A.W.P. King writes on page 209 of "Transision Lines, Antennas, and Wave Guides"*: "In practice, it is usually important to maximize the ratio of forward to backward field or the ratio of backward to forward field. If the ratio of forward to backward field is maximized, the parasite is called a reflector; if the ratio of backward to forward field is maximized, the parasite is called a director." I don`t think the above has changed since 1945. Best regards, Rihard Harrison, KB5WZI |
Richard
That rule is not all encompasing tho it may well suffice for yagi's and the like Regards Art "Richard Harrison" wrote in message ... Jerry Martes wrote: "When I was working with antwennas, we considered the antenna`s F/B ratio used the max of the front to the max of the back. But I get the impression that the erules are different now." Front-to-back ratio is defined as the ratio of power gain between front and rear of a directional antenna. A.W.P. King writes on page 209 of "Transision Lines, Antennas, and Wave Guides"*: "In practice, it is usually important to maximize the ratio of forward to backward field or the ratio of backward to forward field. If the ratio of forward to backward field is maximized, the parasite is called a reflector; if the ratio of backward to forward field is maximized, the parasite is called a director." I don`t think the above has changed since 1945. Best regards, Rihard Harrison, KB5WZI |
Art Unwin wrote:
"The particular case I was referring to was where multiple reflectors were used to dampen radiation to a maximum." It works. Taken to an extreme, you have a grid-dish reflector. Additional reflectors in the plane of the other Yagi-Uda elements don`t make best use of materials aloft. Placed ahead of the driven element, additional elements are in a strong field where they may work as directors to sharpen the forward gain. Placed behind the reflector which has already reversed most of the energy in its direction, a parasitic element is less effective as rhere is less energy to work with. Best regards, Richard Harrison, KB5WZI |
wrote:
I certainly would not call it "silly" which is exactly how "old wives tales" start. At first, Einstein thought quantum physics was silly (spooky). Entangled particles violate the spirit of Einstein's relativity, if not the letter (at least not yet). :-) -- 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 =---- |
"Richard Harrison" wrote in message ... Art Unwin wrote: "The particular case I was referring to was where multiple reflectors were used to dampen radiation to a maximum." It works. Taken to an extreme, you have a grid-dish reflector. Additional reflectors in the plane of the other Yagi-Uda elements don`t make best use of materials aloft. Generaly I can agree with that but it is NOT TRUE in all cases. 1 It can NEVER reflect all radiation from the rear 2 Coupling is reduced as the directors move away from the driven element to the point where little current flows.! A yagi does have an advantage in simplicity together with the ability to "focus" the forward lobe but not at the expense of removing rearward radiation. This post is starting to be remoniscent of the old days where Guru's stated that more than one reflector was pointless and unproductive because they could only think Yagis to the ommission of all others . Need W8TI to complete the full circle (On a prior post I brought up the corner reflector antenna but I had temporarily forgotten that the reflectors carried little current and thus could not be a good contender for high F/B) Placed ahead of the driven element, additional elements are in a strong field where they may work as directors to sharpen the forward gain. Placed behind the reflector which has already reversed most of the energy in its direction, a parasitic element is less effective as rhere is less energy to work with. HUM.... isn't F/B values retarded when maximum gain occurs? This suggests that max gain occurs when there is MORE energy to work with at the rear does it not? Regards Art Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote in message ... Art Unwin wrote: "The particular case I was referring to was where multiple reflectors were used to dampen radiation to a maximum." It works. Taken to an extreme, you have a grid-dish reflector. No, No, No Richard! I do not A grid-dish reflector is where the reflector assembly has voids in the reflector panel that R.F. cannot penetrate. A similar instance is the perforations in a transmitter housing. Maybe it was the term "dampen" that led you to think I had a "grid" type reflector which is not the case. My antenna consists of 8 - 1/2 wave elements of which one is fed. All others are resonant reflectors set on a parabolic curve as is the driven element, thus it does not "focus".This can be fabricated using standard straight dipole elements . Advantages are to be had using elements that are "shaped" to minimise "reactance" coupling and also the physical turning radius. It is this variation that I am building now to see if it becomes quieter especially when static is around. It is times like this that light telescopic fishing poles can be used to advantage together with aluminum tubular mast. You may remember that I sent a 60 foot boom yagi picture to you years ago with 13 elements ( 26 fishing poles) which was also light weight and thus desirable when testing or experimenting to find out where the knoweledge envelope can be pushed. Regards Art Additional reflectors in the plane of the other Yagi-Uda elements don`t make best use of materials aloft. Placed ahead of the driven element, additional elements are in a strong field where they may work as directors to sharpen the forward gain. Placed behind the reflector which has already reversed most of the energy in its direction, a parasitic element is less effective as rhere is less energy to work with. Best regards, Richard Harrison, KB5WZI |
Please don't consider yourself unqualified. I don't think anyone posting
on this newsgroup should, and with your background you certainly shouldn't. My question about who "we" meant was to establish a context for the definition you used, which you've supplied -- thanks. The definition you use isn't a bad one, although it might not be the most useful, provided that you restrict the analysis to free space and are speaking only of a single plane of the 3D pattern. This is commonly done in discussing Yagi arrays, for example. Perhaps your experience was largely in Yagi, log periodic, or other planar arrays which lend themselves to this simplification. The meaning of "back" is open to some interpretation, though. Sometimes it means the precise direction that's exactly 180 degrees from the main forward lobe. Sometimes, though, it refers to a range of angles, even as great as the whole rear semicircle. Let me give an example. Suppose an antenna nominally has a deep null directly to the rear of the front lobe. But a slight asymmetry in the antenna moves the lobe a few degrees to the side. This could easily degrade a strictly defined ("rear" meaning exactly to the rear of the peak of the front lobe) front/back ratio by 10 or 20 dB. It's hard to conceive the application where it would really change the usefulness of the antenna. But a very slightly asymmetrical antenna would look much worse on paper. In your experience, would you consider this to be a poor f/b ratio, or would you give the definition some slack and allow "rear" to vary a few degrees? If you'd give it some slack, then the next question is how much -- could the null be skewed 5 degrees? 10? more? In my limited experience, when the second convention is used (allowing the whole rear semicircle to count as "rear"), the "rear" figure often comes from the largest lobe in the "rear" region. So the gain in the precise direction opposite the front lobe doesn't matter, if there are lobes in other directions in the rear semicircle. This definition would be useful for amateur beam applications, because it tells you the minimum amount of attenuation you'll get from signals coming from any direction within the rear 180 degrees of the pattern. Who cares that you have a 50 dB null in one very narrow direction, if a few degrees away the response is 40 or 50 dB greater. The definition of front/back ratio seems flexible, sometimes used to make the measure more meaningful or useful, but sometimes, I'm sure, to obscure the quality of the pattern. In the example I mentioned in my earlier posting, though, of the complex pattern of an antenna over ground, the definition can get muddy indeed. So it's often necessary to carefully define the term and state exactly what you mean if you really want to communicate meaningful information when you quote a "front/back" ratio. Roy Lewallen, W7EL Jerry Martes wrote: Roy I make no claim to being qualified to discuss antennas with you when we are in disagreement. I worked as an antenna design engineer for 15 years till 1968 when I was layed off from TRW. I was never a high level theorist but managed to hold a decent position with designing hardware. I did work with some highly qualified engineers from whom I thought I obtained alot of knowledge about antennas. Thats why I was bold enough to say "we". I still maintain a casual relationship with George Oltman who you might know from his association with antenna groups with IEEE. As for the F/B, I considered that to identify Front to Back of the antenna's radiation pattern. I would consider it appropriate to identify the radiation toward the "Front" as the max radiation to the front. Then, it seems that the numerical level used for the "F/B ratio" should be max to the Back. I make no argument that this definition I use is *the* way F/B is. But, dont we define "side lobe level" as the ratio of the main beam Max to the side lobe Max? Be aware, I dont write to correct your thinking. I did consider the F/B to be flawed when the main beam Max is compared with a rear radiation Min. I'll consider myself corrected and stop posting. Jerry |
Cecil Moore wrote:
John Smith wrote: I cannot find a configuration on the gamma which brings the swr under that figure... As Richard H. said, find the point where the feedpoint resistance is 50 + jXL ohms. That must occur somewhere. Then tune out the reactance with a series capacitor. I've used EZNEC to model a gamma-fed 33' 20m dipole at 40' made out of 0.5" aluminum tubing. With a gamma element three feet long 6" below the 33' element and a series Xc=122 ohms, it indicates a feedpoint impedance of 49+j0.3 ohms. The EZNEC file can be downloaded by clicking on the link below. -- 73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ ----== 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 =--- |
Cecil:
Your work with EZNEC is greatly appreciated here. Your configuration is for a horz ant and I am working with a vertical--conversion is trivial. "A picture is worth a thousand words..." has real significance here. I am sure an analysis of your file will provide me with insight into the workings of EZNEC and accelerate my learning curve! THANKS!, 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: John Smith wrote: I cannot find a configuration on the gamma which brings the swr under that figure... As Richard H. said, find the point where the feedpoint resistance is 50 + jXL ohms. That must occur somewhere. Then tune out the reactance with a series capacitor. I've used EZNEC to model a gamma-fed 33' 20m dipole at 40' made out of 0.5" aluminum tubing. With a gamma element three feet long 6" below the 33' element and a series Xc=122 ohms, it indicates a feedpoint impedance of 49+j0.3 ohms. The EZNEC file can be downloaded by clicking on the link below. -- 73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ ----== 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 =--- |
John Smith wrote:
Your work with EZNEC is greatly appreciated here. Your configuration is for a horz ant and I am working with a vertical--conversion is trivial. "A picture is worth a thousand words..." has real significance here. I am sure an analysis of your file will provide me with insight into the workings of EZNEC and accelerate my learning curve! I knew I was confused about what you were trying to do. If you just tilt the horizontal dipole to a vertical position, it will still be center-fed. Are you trying to gamma-match a 1/2WL vertical base-fed against a radial system? If so, try this EZNEC file: -- 73, Cecil http://www.qsl.net/w5dxp/GAM20VHW.EZ ----== 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 =---- |
Cecil Moore wrote:
I knew I was confused about what you were trying to do. John, I just realized what is wrong with gamma feeding a 1/2WL vertical at the base. The currents in each side of the gamma feed section are pretty much out-of-phase so they don't radiate (much). Conclusion: Gamma feeding a 1/2WL antenna at the base results in a field strength comparable to a 1/4WL vertical and the minuscule amount of gain is not worth the effort. Antenna gurus probably already knew that but it just dawned on my concrete brain. In much the same way a Zepp or a J-Pole is not a 3/4WL radiator even though they are 3/4WL long, a gamma match on a base-fed 1/2WL vertical doesn't yield a 1/2WL electrical radiator. For this reason, the length of a gamma match should be considerably less than a wavelength. A good rule of thumb is probably in the ballpark of less than ~6% of a wavelength. Your idea seems to require about 16% of a wavelength. Moral: Physical length and RF electrical length are not always the same. What you are trying to do apparently requires a J-Pole or Zepp approach, i.e. a physical 3/4WL antenna. -- 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 =---- |
Cecil:
One element I do not understand in EZNEC is: say I end up with 225 ohms inductive reactance in the gamma rod(just an example figure, use any you please), how do I inform EZNEC I am inserting a 225 ohm capactive reactance, at the feedpoint to offset it? Or, I am all wet in considering this? 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!!! "John Smith" wrote in message ... Cecil: Your work with EZNEC is greatly appreciated here. Your configuration is for a horz ant and I am working with a vertical--conversion is trivial. "A picture is worth a thousand words..." has real significance here. I am sure an analysis of your file will provide me with insight into the workings of EZNEC and accelerate my learning curve! THANKS!, 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: John Smith wrote: I cannot find a configuration on the gamma which brings the swr under that figure... As Richard H. said, find the point where the feedpoint resistance is 50 + jXL ohms. That must occur somewhere. Then tune out the reactance with a series capacitor. I've used EZNEC to model a gamma-fed 33' 20m dipole at 40' made out of 0.5" aluminum tubing. With a gamma element three feet long 6" below the 33' element and a series Xc=122 ohms, it indicates a feedpoint impedance of 49+j0.3 ohms. The EZNEC file can be downloaded by clicking on the link below. -- 73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ ----== 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 =--- |
Cecil:
Yes Cecil, there is wisdom in your words... However, as I pointed out, I am now focusing on "received signal", its' quality and strength. After having given it some thought, that is what is most important. In an grave emergency I have the ability to bring multiple kilowatts on line (yes, I have the generator to support it.) What I can't do is guarantee that for the fellow at the other end of the contact. So, I am focusing my attention towards his/her benefit--in the refinement of my ability to pull sub-nano-watts from the ether... EZNEC and all other calculations are great, I appreciate what can get me to ballpark figures with matching, lengths, spacings, predictions, etc. But, in the end, the humble s-meter on a known transceiver, compared against know signals and similar conditions will be my final guide and the determining factor. To all which point out I "may only be"/am duplicating past work, while making futile attempts, or expelling great amounts of energy in this pursuit, I say: "It keeps me out of the bars and away from the wild women!" (saves on my Jim Beam/Viagra bill too!) 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: I knew I was confused about what you were trying to do. John, I just realized what is wrong with gamma feeding a 1/2WL vertical at the base. The currents in each side of the gamma feed section are pretty much out-of-phase so they don't radiate (much). Conclusion: Gamma feeding a 1/2WL antenna at the base results in a field strength comparable to a 1/4WL vertical and the minuscule amount of gain is not worth the effort. Antenna gurus probably already knew that but it just dawned on my concrete brain. In much the same way a Zepp or a J-Pole is not a 3/4WL radiator even though they are 3/4WL long, a gamma match on a base-fed 1/2WL vertical doesn't yield a 1/2WL electrical radiator. For this reason, the length of a gamma match should be considerably less than a wavelength. A good rule of thumb is probably in the ballpark of less than ~6% of a wavelength. Your idea seems to require about 16% of a wavelength. Moral: Physical length and RF electrical length are not always the same. What you are trying to do apparently requires a J-Pole or Zepp approach, i.e. a physical 3/4WL antenna. -- 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 =---- |
John Smith wrote: Cecil: Yes Cecil, there is wisdom in your words... However, as I pointed out, I am now focusing on "received signal", its' quality and strength. After having given it some thought, that is what is most important. In an grave emergency I have the ability to bring multiple kilowatts on line (yes, I have the generator to support it.) What I can't do is guarantee that for the fellow at the other end of the contact. So, I am focusing my attention towards his/her benefit--in the refinement of my ability to pull sub-nano-watts from the ether... EZNEC and all other calculations are great, I appreciate what can get me to ballpark figures with matching, lengths, spacings, predictions, etc. But, in the end, the humble s-meter on a known transceiver, compared against know signals and similar conditions will be my final guide and the determining factor. I think you'll eventually end up finding out you just need a big tower and a big beam. ac6xg |
Roy, W7EL wrote:
"Who cares that you have a 50 dB null in one particular very narrow direction, if a few degrees away the response is 40 or 50 dB greater." This is an example of Roy`s earlier post which noted differences between amateur requirements and commercial requirements. Roy is right. There are real differences. Commercial licensees operate on assigned frequencies and enjoy some protection from interference on their assignments. A broadcaster may as a condition of his license be required to have a null in one or more azimuths in his pattern to protect another broadcaster who was there first. In this broadcaster`s case, he is interested in the narrow null and may very well expect and hope his signal a few dgrees away is 40 or 50 dB greater. Best regards, Richard Harrison, KB5WZI |
Hmmmm, your analysis is interesting. The gamma looks very much like an
"auto-transformer" to me--the gama-rod like a variable primary, the secondary being the complete driven element. I suspected the action would be identical to the auto-transformer--where ALL the turns in the secondary deliver power to the load.... 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: I knew I was confused about what you were trying to do. John, I just realized what is wrong with gamma feeding a 1/2WL vertical at the base. The currents in each side of the gamma feed section are pretty much out-of-phase so they don't radiate (much). Conclusion: Gamma feeding a 1/2WL antenna at the base results in a field strength comparable to a 1/4WL vertical and the minuscule amount of gain is not worth the effort. Antenna gurus probably already knew that but it just dawned on my concrete brain. In much the same way a Zepp or a J-Pole is not a 3/4WL radiator even though they are 3/4WL long, a gamma match on a base-fed 1/2WL vertical doesn't yield a 1/2WL electrical radiator. For this reason, the length of a gamma match should be considerably less than a wavelength. A good rule of thumb is probably in the ballpark of less than ~6% of a wavelength. Your idea seems to require about 16% of a wavelength. Moral: Physical length and RF electrical length are not always the same. What you are trying to do apparently requires a J-Pole or Zepp approach, i.e. a physical 3/4WL antenna. -- 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 =---- |
Cecil:
Maybe I sent you the wrong .EZ file. Anyway, I went back and pondered over your example, DUH!, it was obivious, I simply had to splilt the driven element into two sections where the gamma joined it! Oh well, I might have made myself look stupid today, but not to such a great extent that I won't be able to top that tomorrow!!! 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!!! "John Smith" wrote in message ... Cecil: Here is an EZNEC file I am having a problem with, it is complaining about wires 5 and 2 NOT meeting at an end or segment? What corrects this? Thanks in advance, 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: I knew I was confused about what you were trying to do. John, I just realized what is wrong with gamma feeding a 1/2WL vertical at the base. The currents in each side of the gamma feed section are pretty much out-of-phase so they don't radiate (much). Conclusion: Gamma feeding a 1/2WL antenna at the base results in a field strength comparable to a 1/4WL vertical and the minuscule amount of gain is not worth the effort. Antenna gurus probably already knew that but it just dawned on my concrete brain. In much the same way a Zepp or a J-Pole is not a 3/4WL radiator even though they are 3/4WL long, a gamma match on a base-fed 1/2WL vertical doesn't yield a 1/2WL electrical radiator. For this reason, the length of a gamma match should be considerably less than a wavelength. A good rule of thumb is probably in the ballpark of less than ~6% of a wavelength. Your idea seems to require about 16% of a wavelength. Moral: Physical length and RF electrical length are not always the same. What you are trying to do apparently requires a J-Pole or Zepp approach, i.e. a physical 3/4WL antenna. -- 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 =---- |
have you tried this???
http://www.google.com/search?hl=en&q...ch+yagi+design lots of sources for design info, some software, plenty of pointers. basically once you have a yagi and decide to feed it with a gamma match you have the following variables you can work with (assuming you aren't going to change the diameter of the driven element)... 1. rod diameter 2. rod to driven element spacing 3. location of shorting strap 4. value of capacitance 5. length of driven element(don't forget this, it is very important part of adjustment!) now basically you pick a rod diameter, usually 1/3 to 1/2 the diameter of the driven element, but basically whatever you happen to have handy that is a reasonable length. then make a few brackets to space it a couple inches from the driven element, from about 1" on 2m to maybe 4-5" on 20m. so that fixes 1 and 2.. then you have 3 other variables you can adjust to get the match you want. if you can't find a good match after systematically running through the other 3 then try changing the spacing a bit and start again... but i've always been able to get a decent match by just picking what was handy for tubing and spacers. and don't overlook a T match. if you use a 1/2 wave phasing line and the right diameters and spacing you can get away without a capacitor. this makes for a fully dc grounded plumbers delight construction that is very rugged and simple to adjust. this was how all the old telrex monobanders were fed. "Jim Kelley" wrote in message ... John Smith wrote: Cecil: Yes Cecil, there is wisdom in your words... However, as I pointed out, I am now focusing on "received signal", its' quality and strength. After having given it some thought, that is what is most important. In an grave emergency I have the ability to bring multiple kilowatts on line (yes, I have the generator to support it.) What I can't do is guarantee that for the fellow at the other end of the contact. So, I am focusing my attention towards his/her benefit--in the refinement of my ability to pull sub-nano-watts from the ether... EZNEC and all other calculations are great, I appreciate what can get me to ballpark figures with matching, lengths, spacings, predictions, etc. But, in the end, the humble s-meter on a known transceiver, compared against know signals and similar conditions will be my final guide and the determining factor. I think you'll eventually end up finding out you just need a big tower and a big beam. ac6xg |
Cecil:
Here is the crux of my fruitless attempts to use EZNEC... With EZNEC, when I plug in the value of my gamma rod and -jxr, along with all other elements, EZNEC spits out an error! (something about too short a loop seems to bother EZNEC, but this short loop IS working!) However, I am looking at a REAL and functional antenna which appears to be well matched and receiving well. So, I end up throwing up my hands and just using the antenna and going about with the "cut-and-try" method! (probably just my inability to use the app) 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!!! "John Smith" wrote in message ... Cecil: One element I do not understand in EZNEC is: say I end up with 225 ohms inductive reactance in the gamma rod(just an example figure, use any you please), how do I inform EZNEC I am inserting a 225 ohm capactive reactance, at the feedpoint to offset it? Or, I am all wet in considering this? 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!!! "John Smith" wrote in message ... Cecil: Your work with EZNEC is greatly appreciated here. Your configuration is for a horz ant and I am working with a vertical--conversion is trivial. "A picture is worth a thousand words..." has real significance here. I am sure an analysis of your file will provide me with insight into the workings of EZNEC and accelerate my learning curve! THANKS!, 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!!! "Cecil Moore" wrote in message ... Cecil Moore wrote: John Smith wrote: I cannot find a configuration on the gamma which brings the swr under that figure... As Richard H. said, find the point where the feedpoint resistance is 50 + jXL ohms. That must occur somewhere. Then tune out the reactance with a series capacitor. I've used EZNEC to model a gamma-fed 33' 20m dipole at 40' made out of 0.5" aluminum tubing. With a gamma element three feet long 6" below the 33' element and a series Xc=122 ohms, it indicates a feedpoint impedance of 49+j0.3 ohms. The EZNEC file can be downloaded by clicking on the link below. -- 73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ ----== 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 =--- |
John Smith wrote:
"However, I am looking at a REAL and functional antenna which appears to be well matched and receiving well." Congratulationns! In a previous posting, I quoted Dr. Arnold King who wrote prior to 1945: "The accurate calculation of the (antenna) input impedance at AB of the modified antenna as a function of the impedance (presented to the matching section) and the (wire) lengths AC and BD has not been accomplished." If it were easy to calculate, they would have done it. They had excellent marhematicians prior to 1945, too. The implication is that you find the match by trial. John proved he could do it. Best regards, Richard Harrison, KB5WZI |
John Smith wrote:
Cecil: One element I do not understand in EZNEC is: say I end up with 225 ohms inductive reactance in the gamma rod(just an example figure, use any you please), how do I inform EZNEC I am inserting a 225 ohm capactive reactance, at the feedpoint to offset it? Use the "Loads" feature to install a lumped point load of any Q. For instance, a coil might be 10 + j300 ohms or a cap might be 2 - j500 ohms. The position of a load is similar to positioning a source. -- 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 =---- |
John Smith wrote:
Cecil: Maybe I sent you the wrong .EZ file. Anyway, I went back and pondered over your example, DUH!, it was obivious, I simply had to splilt the driven element into two sections where the gamma joined it! Oh well, I might have made myself look stupid today, but not to such a great extent that I won't be able to top that tomorrow!!! I dare say all of us EZNEC users have committed that infraction, some more than once. :-) -- 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 =---- |
John Smith wrote:
Cecil: Here is the crux of my fruitless attempts to use EZNEC... With EZNEC, when I plug in the value of my gamma rod and -jxr, along with all other elements, EZNEC spits out an error! (something about too short a loop seems to bother EZNEC, but this short loop IS working!) However, I am looking at a REAL and functional antenna which appears to be well matched and receiving well. So, I end up throwing up my hands and just using the antenna and going about with the "cut-and-try" method! (probably just my inability to use the app) Or maybe not. EZNEC apparently won't properly model the Lattin antenna. www.g3ycc.karoo.net/lattin.htm Somewhere I have a .ez file that shows 20 dBi omni- directional gain from an inverted-L antenna. Now that's what you need. :-) And you taught me something today. I didn't know one can attach .ez files to a newsgroup posting. -- 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 =---- |
Cecil Moore, W5DXP wrote:
"For instance, a coil might be 10+j300 ohms or a cap might be 2-j500 ohms." A small loop antenna with a perimeter only a small fraction of a wavelength has almost uniform current throughout. Its impedance is an inductive reactance in series with a resistance. The U.S. Bureau of Standards published inductance formulas for single-turn and several-turn loops in Bulletin 74. Results can`t take into account loss from radiation so they are approximate. Arnold King gives an approximate formula for the radiation resistance of a small one-turn loop in "Transmission Lines, Antennas, and Wave Guides": Re = 20(Beta)to the 4th power times A squared Beta=2pi/lambda A=area enclosed by the loop. Best regards, Richard Harrison, KB5WZI |
Cecil Moore wrote:
Or maybe not. EZNEC apparently won't properly model the Lattin antenna. www.g3ycc.karoo.net/lattin.htm . . . I took a look at it, and sure enough, it can't. The antenna is constructed from tubular 300 ohm twinlead, and EZNEC has no way to account for the dielectric between conductors. In that antenna, it looks like the velocity factor of the differential mode field between conductors would be important to its operation, and without the ability to model the dielectric between conductors, EZNEC wouldn't get the velocity factor right. When I see a claim that EZNEC can't model a particular antenna, I often find that the reason for the claim is that the antenna's inventor or seller has dreamed up some magical property to explain the impossibly good performance he's claiming for the antenna. EZNEC models the antenna just fine, it just doesn't model the magical property and validate the claims -- that is, it shows how the antenna really works, not how the huckster claims it works. But there are, certainly, some kinds of antennas which it really can't model properly for one reason or another. The Lattin antenna is one of those. Roy Lewallen, W7EL |
On Thu, 07 Apr 2005 10:05:04 -0700, Roy Lewallen
wrote: But there are, certainly, some kinds of antennas which it really can't model properly for one reason or another. The Lattin antenna is one of those. Hi Roy, Having model dozens of variations of this antenna, as well as what was described in the patent - I cannot think of one reason why it should work except through haphazard luck. Certainly the offered "theories" are no more credible than those for other antennas that defy modeling such as the single or double bazooka, the eh, the cfa. This antenna has many reports of its confounding expectation and then in the same breath those who are confounded expressing their sincere belief it works. It's one of those situations where the builder can't get it to sing, but has a brother-in-law who knows this fellow who lives next door to one who can, but who took it down ten years ago because it was so hard to tune. Perhaps you could widen your customer base if you added a "belief scale" to the available control settings for EZNEC. It could range from "agnostic" to "I believe in miracles." 73's Richard Clark, KB7QHC |
John Smith wrote:
Cecil: Take a look at this folded wave monopole... I am playing with a reflector and director to sandwich this driven element between.... This is the design I have in mind, NO groundplane radials when used as a stand-alone vertical monopole/end-fed. There are warnings in EZNEC about segments not lining up between close spaced wires. My 29 dBi omnidirectional antenna is an example of what happens when one ignores the segment alignment between closely spaced segments. I have modified your segmentation to try to follow the EZNEC guidelines and that file is attached. There is quite a change in the results. The feedpoint impedance went from 2.787+j15.47 to 0.7587+j22.69, the gain went from 7.23 dBi to 3.16 dBi, and the TOA went from 6 deg to 9 deg. I standardized on one foot per segment. I don't really understand what you are trying to do and am just the messenger. -- 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 =---- |
Cecil:
Did you remember to attach the file? I don't see it, but would love to study your changes... At ~2.7 ohms I can use a 1:16 balun (reversed direction from conventional use) to mate the antenna with 50 ohms, and get an acceptable SWR... Cecil, try to hear this in a positive manner: If I must conform to EZNEC, then any creativity must be abandoned... If I conform to all the rules of the past and the "Absolute Truths" encompassed by any application--how could I ever hope for a new discovery of geometry or configuration, or the possible use of un-used manipulations of existing rules? If one travels the same road, takes the same plane, rides the same rail, boards the same boat, walks the same path--he sees only the same sights, that which is common place.... even futile paths invoke my curriosity--as just a change of scenery can be enjoyed many times. If that argument is carried out to its nth degree, only one question would remain, "Why should I bother with any of this?", it would be quite easy for a programmer to create a program which quizzes you on ant type (monopole, dipole, yagi, 1/2 wave, 1/4 wave, center loaded, bottom loaded, etc, etc), the freq etc... then constructs the antenna for you. Including suggesting height, mounting brackets and a host of other varibles and conditions that tasks the mind to consider... In other words, once I give control and authority to EZNEC, why should it not "do it all?" But then, even if it did, I would still dilly around with these copper, aluminum and stainless bits and pieces. grin You have talked me back into abandoning EZNEC.... 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!!! "Cecil Moore" wrote in message ... John Smith wrote: Cecil: Take a look at this folded wave monopole... I am playing with a reflector and director to sandwich this driven element between.... This is the design I have in mind, NO groundplane radials when used as a stand-alone vertical monopole/end-fed. There are warnings in EZNEC about segments not lining up between close spaced wires. My 29 dBi omnidirectional antenna is an example of what happens when one ignores the segment alignment between closely spaced segments. I have modified your segmentation to try to follow the EZNEC guidelines and that file is attached. There is quite a change in the results. The feedpoint impedance went from 2.787+j15.47 to 0.7587+j22.69, the gain went from 7.23 dBi to 3.16 dBi, and the TOA went from 6 deg to 9 deg. I standardized on one foot per segment. I don't really understand what you are trying to do and am just the messenger. -- 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 =---- |
Richard H:
Do you happen to have a URL handy where I could view "Bulletin 74?" Thanks in advance, 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 Harrison" wrote in message ... Cecil Moore, W5DXP wrote: "For instance, a coil might be 10+j300 ohms or a cap might be 2-j500 ohms." A small loop antenna with a perimeter only a small fraction of a wavelength has almost uniform current throughout. Its impedance is an inductive reactance in series with a resistance. The U.S. Bureau of Standards published inductance formulas for single-turn and several-turn loops in Bulletin 74. Results can`t take into account loss from radiation so they are approximate. Arnold King gives an approximate formula for the radiation resistance of a small one-turn loop in "Transmission Lines, Antennas, and Wave Guides": Re = 20(Beta)to the 4th power times A squared Beta=2pi/lambda A=area enclosed by the loop. Best regards, Richard Harrison, KB5WZI |
John Smith wrote:
"Do you happen to have a URL handy where I could view "Bulletin 74?" I searched the web unsuccessfully, including some NIST sites which dictated schedules for clearing their files at certain file ages. But, I did find a reference to the famous bulletin in Keith Henney`s 1950 edition of "Radio Engineering Handbook". On page 132, Keith writes: "It is possible to calculate the true (l-f) inductance of some types of air-cored coils with a fair degree of accuracy. National Bureau of Standards Circular 74 has long been the most authorative source of such information. Some of the most useful contained therein are reproduced here." Pages of information follow which include design charts which I can`t reproduce due to my limitations. Such information is not copyrighted as it is assumed the taxpayer has paid for collection of the information already, and should not have to pay for it a second time. That was pre Bush policy, of course. You can probably borrow a copy of Keith Henney`s book in a library near you and delight ijn designing your own antenna coil. The publisher was McGraw-Hill. Best regards, Richard Harrison, KB5WZI |
|
On Sat, 9 Apr 2005 23:39:15 -0500, (Richard
Harrison) wrote: I searched the web unsuccessfully http://www.hills2.u-net.com/private/...4/CONTENTS.HTM contains partial page transcriptions. Employing the search term "Radio Instruments and Measurements" at: http://www.kartoo.com/ you can make use of an unique, cluster based search engine that represents the next wave of sifting information on the Web. You will find references to this tome (hardly a circular or bulletin) of more than 600 pages (you will also find book venders ready to sell this work for $20-$30). One NIST site contains a quote from Edison: " . . . This is the greatest book on this subject that I have ever read, and I want to congratulate you and your Bureau on its production." There are multiple authors. I cited one, George Clark Southworth, another was J. Howard Dellinger. There are hard copies available through NIST's Historical Archives (try to get one though). 73's Richard Clark, KB7QHC |
Richard:
Could these be them; or, are these not the same? http://www.fcc.gov/mb/audio/decdoc/r...bulletins.html Regards -- 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 Sat, 9 Apr 2005 23:39:15 -0500, (Richard Harrison) wrote: I searched the web unsuccessfully http://www.hills2.u-net.com/private/...4/CONTENTS.HTM contains partial page transcriptions. Employing the search term "Radio Instruments and Measurements" at: http://www.kartoo.com/ you can make use of an unique, cluster based search engine that represents the next wave of sifting information on the Web. You will find references to this tome (hardly a circular or bulletin) of more than 600 pages (you will also find book venders ready to sell this work for $20-$30). One NIST site contains a quote from Edison: " . . . This is the greatest book on this subject that I have ever read, and I want to congratulate you and your Bureau on its production." There are multiple authors. I cited one, George Clark Southworth, another was J. Howard Dellinger. There are hard copies available through NIST's Historical Archives (try to get one though). 73's Richard Clark, KB7QHC |
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