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Long Vee Beam question
I'd like to put up a 40 to 60 wavelength (per leg) vee beam for 144
Mhz. It would be 12 feet off the ground and unterminated. My hope is that the angle between the wires can be smaller than normal becasue the legs are relatively long. How much gain (dbd) should I expect and about what take off angle will I have? It will be for transmitting only. With the transmitter located at the feedpoint, is there any need for exotic feed methods? What type of matching do I need to feed it with a 50 ohm output solid state power amp? Thanks, Art KY1K at pivot dot net -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Wed, 07 Jul 2004 17:44:58 -0400, Albert wrote:
I'd like to put up a 40 to 60 wavelength (per leg) vee beam for 144 Mhz. It would be 12 feet off the ground and unterminated. .... What type of matching do I need to feed it with a 50 ohm output solid state power amp? Hi Art, Too many details lacking. 40 to 60 covers too much turf (beyond the pun), you neglect how wide it will be at the ends too. I ran a couple of tests to see the gain from 17 to 22dBi and the Z anywhere from 200 Ohms to 2000 Ohms. One thing to consider is the "law of diminishing returns." The gain for a much smaller V is not much different from the giant one (think 3dB for each doubling - as a shorthand, I am sure this fails long before the 20, much less 50th, wavelength is reached). 73's Richard Clark, KB7QHC |
OK, thanks Richard.
How about this. 30 wavelengths per side, average conducting ground, 12 feet above ground. What angle do I need for best forward gain, what will the takeoff angle be at that angle? If I can get the entire antenna up to 20 feet above ground, how much better is the performance? Is there any way for me to do modelling myself without burdening you or someone with a modeling program? I am not aware of free software that will model rhombics or vee's. Thanks, Art ky1k at pivot dot net PS: Some additional info. This is for EME, which does not require much movement. Since it is so long, it won't be variable in the elevation angle anyway. But, I hope to move one wire a bit so I can steer the beam to the left or to the right a little (changing the angle of the vee at the same time). Hi Art, Too many details lacking. 40 to 60 covers too much turf (beyond the pun), you neglect how wide it will be at the ends too. I ran a couple of tests to see the gain from 17 to 22dBi and the Z anywhere from 200 Ohms to 2000 Ohms. One thing to consider is the "law of diminishing returns." The gain for a much smaller V is not much different from the giant one (think 3dB for each doubling - as a shorthand, I am sure this fails long before the 20, much less 50th, wavelength is reached). 73's Richard Clark, KB7QHC -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote:
OK, thanks Richard. How about this. 30 wavelengths per side, average conducting ground, 12 feet above ground. You still don't say how far apart the tips are. What angle do I need for best forward gain, what will the takeoff angle be at that angle? The angles were pretty consistant at 5 degrees at 12 feet up. Gain did not vary much either (5dB is a lot perhaps, but then there are so many variables to consider). If I can get the entire antenna up to 20 feet above ground, how much better is the performance? Height change up doesn't seem to be signficant. Is there any way for me to do modelling myself without burdening you or someone with a modeling program? I am not aware of free software that will model rhombics or vee's. Thanks, Art Hi Art, With monster size antennas and a desire for accuracy, this drives the model towards a lot of segments ($$$). You could use the free version of EZNEC but it will blow up. EME hmmm? Major lobe is 5 degrees off the horizon and 5 degrees wide. Isn't there a problem with an antenna looking at the moon so close to the horizon (ground temperature)? Also, the moon will only fill that box for, what, 15 minutes? You might find it simpler to build an array of dipoles such that they were all looking up at the same box. Combining them may be a bitch however, so I can see your desire for simplicity here. 73's Richard Clark, KB7QHC |
Art
Which brand of EME is this for? If for one of the new digital modes, you don't need that much gain, as you may already know. If it's for CW, having a ton of gain in one direction only gives you a very seldom available and very short window. You would be better off sticking up about 15dBd of rotatable gain and making a horizon sched with W5UN. The 2m EME net is on 14.345 at 11AM central saturdays and sundays. Net control is VE7BQH. Dave, W5UN, is almost always on. Dave also does the digi EME modes, and as a guess could probably work you on something like 10dBd with 100W on one of those. I know Ian is an EME're, so pipe in, Ian! tom K0TAR Albert wrote: OK, thanks Richard. How about this. 30 wavelengths per side, average conducting ground, 12 feet above ground. What angle do I need for best forward gain, what will the takeoff angle be at that angle? If I can get the entire antenna up to 20 feet above ground, how much better is the performance? Is there any way for me to do modelling myself without burdening you or someone with a modeling program? I am not aware of free software that will model rhombics or vee's. Thanks, Art ky1k at pivot dot net PS: Some additional info. This is for EME, which does not require much movement. Since it is so long, it won't be variable in the elevation angle anyway. But, I hope to move one wire a bit so I can steer the beam to the left or to the right a little (changing the angle of the vee at the same time). |
You still don't say how far apart the tips are. What angle do I need for best forward gain, what will the takeoff angle be at that angle? Hi Richard, When I asked 'what angle do I need for best forward gain', I was hoping you could give me an idea what the optimum distance between the tips was. My hope was that longer legs might allow me to use smaller angles although I have a big field to play in. Thanks, A -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Wed, 07 Jul 2004 19:14:03 -0500, Tom Ring
wrote: Art Which brand of EME is this for? If for one of the new digital modes, you don't need that much gain, as you may already know. If it's for CW, having a ton of gain in one direction only gives you a very seldom available and very short window. Thanks for the eme advice. But, I was asking about the gain of a vee beam because they are cheap to build and easy to put back up when the weather takes them out. We have major ice storms often here. Which "Brand of eme' is irreleveant, gain is gain regardless of which mode is used. Excess gain is never wasted, it either makes the QSO faster or allows one to work smaller stations or allows SSB instead of CW/digital modes. If the angle of the V makes little difference to the actaul gain, it can be steered by moving either leg, which is also easy to do. Although I can only work on my moon rise or moon set, more gain is always better. Since I can't elevate my yagi, a non elevatable vee beam for transmit only might give higher gain. The yagi would be for receive only. Having different receive and transmit antennas has benefits as well, such as no switching loss and no need to protect my gaasfet during transmit. Can you offer any suggestions regarding the original question? Thanks A -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Tom Ring wrote:
Art Which brand of EME is this for? If for one of the new digital modes, you don't need that much gain, as you may already know. If it's for CW, having a ton of gain in one direction only gives you a very seldom available and very short window. You would be better off sticking up about 15dBd of rotatable gain and making a horizon sched with W5UN. The 2m EME net is on 14.345 at 11AM central saturdays and sundays. Net control is VE7BQH. Dave, W5UN, is almost always on. Dave also does the digi EME modes, and as a guess could probably work you on something like 10dBd with 100W on one of those. I know Ian is an EME're, so pipe in, Ian! (Still sleepy and jet-lagged...) By coincidence, I was involved in just that kind of thing in 1979(?) when we made the very first 2m EME QSOs from G-land using amateur antennas at both ends of the path. One of our group was a farmer's son, so we were able to string a 600ft rhombic over a large field of pigs. The quick answer about feeding a very long V-beam on 2m is to use a "universal stub" - a half-wave open-wire stub with a shorting bar, and a 4:1 coax balun. Adjust the tapping points for the shorting bar and balun to get a good impedance match, and away you go. The universal stub is almost a lost art, but any *old* VHF handbook will show you how to make one. It's obviously much more convenient if you come down to ground level in high-grade open-wire feeder - not the store-bought stuff, but home-made, with close-spaced wires under tension and a minimum of insulators. You can then do the matching at ground level. 30 wavelengths per leg should be long enough to eliminate any termination requirements at the far end. Radiation "loss" from the forward-travelling wave will automatically ensure that the rear lobe is reduced. If you wish, you can terminate the far end of each leg with a 300R low-inductive resistor and two quarter-wave "radials" in a T configuration... but you'll probably not notice the difference. However, it's true that: having a ton of gain in one direction only gives you a very seldom available and very short window. This is a major inconvenience - you get maybe 20 minutes total operating time per day, on maybe 3-4 days per month maximum. And that's only if the direction of the beam is perfectly optimized. You need to lay out the antenna with an accuracy of about 1 degree maximum, so you'll need to borrow some serious surveying equipment. Guess-and-compass methods will not work, because even small azimuth errors could mean that you're operating on completely the wrong DAY! (As the one who did the calculations, I can still remember the feeling of relief on verifying that the moon really did set in front of the rhombic, and on the right day too.) Also, these moonrise or moonset windows will occur at arbitrary times of day or night. With absolutely no time to waste, you will be limited to making skeds... and not many sked partners may want to share that inconvenience with you. Overall, I agree with Tom - a large fixed antenna was the right thing to do 20+ years ago, but 2m EME is now in a very different place. A smaller steerable beam will trade raw gain for a huge increase in EME operating *time*, and with modern operating techniques, time is what you need the most. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hi Ian, Thanks for your comments. My hope was to use a v beam instead of a rhombic. The V is relatively easy to steer, especially if the performance doesn't change much if either leg is moved. Note that I hope to leave one leg of the beam fixed and steer the az by varying the position (and the V angle) of the other leg. Not sure how much changing the angle of the V impacts the performance, which is why I asked for someone with modeling software. My hope was to exploit the high gain, simple construction (but poor receive) of the V for transmit only and to use the modest sized FO yagi for receive only. If I can get 19 or 20 db from a big V beam, I can probably tollerate the limited operating time as well, Q's with big guns should take only minutes and medium sized stations should be workable in a 20 minute window. If the computer model predicts a usable gain, I'd like to try putting one up. Regards, Art -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote:
|OK, thanks Richard. | |How about this. | |30 wavelengths per side, average conducting ground, 12 feet above |ground. | |What angle do I need for best forward gain, what will the takeoff |angle be at that angle? Included angle from 12 to 16 degrees will give ~20 dBi at 6 deg elevation. | |If I can get the entire antenna up to 20 feet above ground, how much |better is the performance? ~ 2 dB more gain at 4 deg el. | |Is there any way for me to do modelling myself without burdening you |or someone with a modeling program? I am not aware of free software |that will model rhombics or vee's. 4nec2 is free, do a Google search Multinec is cheap and very flexible. | |Thanks, | |Art | |ky1k at pivot dot net | |PS: Some additional info. This is for EME, which does not require much |movement. Since it is so long, it won't be variable in the elevation |angle anyway. But, I hope to move one wire a bit so I can steer the |beam to the left or to the right a little (changing the angle of the |vee at the same time). Since I worked VK5MC to complete my 2-meter WAC and he used a slightly steerable rhombic, I'm not going to say that this won't work, but I must question why. If you're going to limit yourself to a few minutes of moon time a month, why not just put up a long fixed Yagi and use it for both transmit and receive. A forty-foot long Yagi will give the same gain as the vee, with a *huge* improvement in the pattern and it will have a much more tractable feedpoint impedance. Wes |
On Thu, 8 Jul 2004 09:33:45 -0400, "Tam/WB2TT"
wrote: Hi Richard, How about shooting for 112 Ohms, so he can match it with a 1/4 wave section of RG11. Tam/WB2TT Hi Tam, That would be an opening between 30° and 40° (106 to 122 Ohms) with a gain around 12dBi (11.8 @10° to 11.9 @20°). Impedance appears to change smoothly through out the full range; gain and angle less so (but no more than this spread of 10°). 73's Richard Clark, KB7QHC |
Albert I'm a little experienced with failure in trying EME on 2 meters. I'm not a scientist, but have some information that might help you decide which antenna is best for your situation. I'm not able to calculate the amount of steering thats available with a long V antenna. I couldn't even predict the antenna's radiation pattern shape. If the antenna pattern has about equal beamwidth in both elevation and azimuth, a 20 DB gain antenna will have about a 20 degree beamwidth (as I remember). You probably know all this, but-- The moon rises and sets at a slightly different azimuth each succeeding day. The azimuth direction to the moon on the horizon might vary by about 50 degrees per year. I consider the moon to be about 1/2 a degree wide. It will rise at "its diameter" each 2 minutes. It looks to me like you'd have a few minutes of 'max gain' each year, and theres no way for you to predict _*when*_ because the "takeoff angle" at 2 lambda is unpredictable. If my thinking about *where the moon is* is about right, I predict that you wont be satisfied with an antenna that isnt more stearable than the V for moon bounce. I've read about a guy who made a long yagi antenna by suspending it on ropes rather than 'booms'. I wonder if you might be more satisfied with a yagi that you could "roll up" when icy weather approaches. Jerry "Albert" wrote in message ... Hi Ian, Thanks for your comments. My hope was to use a v beam instead of a rhombic. The V is relatively easy to steer, especially if the performance doesn't change much if either leg is moved. Note that I hope to leave one leg of the beam fixed and steer the az by varying the position (and the V angle) of the other leg. Not sure how much changing the angle of the V impacts the performance, which is why I asked for someone with modeling software. My hope was to exploit the high gain, simple construction (but poor receive) of the V for transmit only and to use the modest sized FO yagi for receive only. If I can get 19 or 20 db from a big V beam, I can probably tollerate the limited operating time as well, Q's with big guns should take only minutes and medium sized stations should be workable in a 20 minute window. If the computer model predicts a usable gain, I'd like to try putting one up. Regards, Art -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Thu, 08 Jul 2004 09:10:41 -0700, Wes wrote:
On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote: |OK, thanks Richard. | |How about this. | |30 wavelengths per side, average conducting ground, 12 feet above |ground. | |What angle do I need for best forward gain, what will the takeoff |angle be at that angle? Included angle from 12 to 16 degrees will give ~20 dBi at 6 deg elevation. | |If I can get the entire antenna up to 20 feet above ground, how much |better is the performance? ~ 2 dB more gain at 4 deg el. | |Is there any way for me to do modelling myself without burdening you |or someone with a modeling program? I am not aware of free software |that will model rhombics or vee's. 4nec2 is free, do a Google search Multinec is cheap and very flexible. | |Thanks, | |Art | |ky1k at pivot dot net | |PS: Some additional info. This is for EME, which does not require much |movement. Since it is so long, it won't be variable in the elevation |angle anyway. But, I hope to move one wire a bit so I can steer the |beam to the left or to the right a little (changing the angle of the |vee at the same time). Since I worked VK5MC to complete my 2-meter WAC and he used a slightly steerable rhombic, I'm not going to say that this won't work, but I must question why. If you're going to limit yourself to a few minutes of moon time a month, why not just put up a long fixed Yagi and use it for both transmit and receive. A forty-foot long Yagi will give the same gain as the vee, with a *huge* improvement in the pattern and it will have a much more tractable feedpoint impedance. Wes Hi Wes, Thanks for the information, it's just what I needed. I am not sure how dbd converts to dbi, but a 100 foot 43 element rope yagi gives 19 dbd. My motivation for starting this was that I had the room for a large V beam and that it might be practical if I could get the same gain as the big rope yagi. The V beam is way easier to put up and much cheaper and simpler to construct. From looking at data from smaller rhombics, it appears they are poor for receiving as they have some major lobes that point towards the high noise temperature Earth, which is why I wanted the V for transmit only. I don't think the radiation off the back of the antenna will do any harm in transmit only, so my hope was that it does not need to be terminated. Your gain figures indicate this bears further investigation, even though I would be dependent on moon set or moon rise to make my Q's. But, people with yagis have had that limitation and live with it just fine. I'll look up 4nec2 and try to play with it myself. Hopefully, it will handle large V's. Regards, A -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Albert
I can't help on the V any more than others, such as Richard, are, but I can give you another cheap alternative that is as easy to repoint as your V, maybe easier. VE7BQH published a rope beam for EME a while ago. It is as cheap as rope and the elements plus TyWraps cost. The only downside I can see is making the match, but that's not really too tough, and you have to match to the V anyway. It will also have more gain than the V would for the same real estate. http://www.bigskyspaces.com/w7gj/longyagi.htm tom K0TAR Albert wrote: On Wed, 07 Jul 2004 19:14:03 -0500, Tom Ring wrote: Art Which brand of EME is this for? If for one of the new digital modes, you don't need that much gain, as you may already know. If it's for CW, having a ton of gain in one direction only gives you a very seldom available and very short window. Thanks for the eme advice. But, I was asking about the gain of a vee beam because they are cheap to build and easy to put back up when the weather takes them out. We have major ice storms often here. Which "Brand of eme' is irreleveant, gain is gain regardless of which mode is used. Excess gain is never wasted, it either makes the QSO faster or allows one to work smaller stations or allows SSB instead of CW/digital modes. If the angle of the V makes little difference to the actaul gain, it can be steered by moving either leg, which is also easy to do. Although I can only work on my moon rise or moon set, more gain is always better. Since I can't elevate my yagi, a non elevatable vee beam for transmit only might give higher gain. The yagi would be for receive only. Having different receive and transmit antennas has benefits as well, such as no switching loss and no need to protect my gaasfet during transmit. Can you offer any suggestions regarding the original question? Thanks A -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Wed, 07 Jul 2004 17:44:58 -0400, Albert wrote:
How much gain (dbd) should I expect and about what take off angle will I have? Hi Al, Contrary to Wes' results, I do not find much more than 13dBi, and certainly not from your proposed huge implementation running out towards 50 wavelengths. In fact, I find antennas that are a tenth of that (5M) have about as much gain as will be found. My matrix of testing shows that doubling to 10 wavelengths and doubling again to 20 wavelengths brings no further gain (except for some opportunistic outliers). As a variation upon a theme, I decided to play with uptilt on the 5 wavelength models, lifting the far ends by roughly 25 degrees. The feed point is at 3M, and the far ends are lifted 5M. With this configuration, there is some loss in gain, but the lobe looking at the horizon is easily four times broader. That is, there is an even gain of roughly 11dBi from 14 degrees above the horizon to 34 degrees above the horizon. This occurs for the tips being separated by 30 to 90 degrees (the gain falls to 9dBi with wider separation). When you separate further, out at 150 degrees between the tips, the broad characteristic collapse, but recaptures gain, and puts it out lower. At such a configuration you might observe 11dBi @ 4 degrees. That lobe is only 3 or 4 degrees tall however. Considering that common implementations of rhombics rarely go beyond a couple of wavelengths to several, it seems that 20 or 40 or 50 has no future. The law of diminish returns must occur somewhere as you are constantly losing power as it trucks down the length. At that far end, nothing added to little before it hardly piles up gain. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
On Wed, 07 Jul 2004 17:44:58 -0400, Albert wrote: How much gain (dbd) should I expect and about what take off angle will I have? Contrary to Wes' results, I do not find much more than 13dBi, and certainly not from your proposed huge implementation running out towards 50 wavelengths. In fact, I find antennas that are a tenth of that (5M) have about as much gain as will be found. My matrix of testing shows that doubling to 10 wavelengths and doubling again to 20 wavelengths brings no further gain (except for some opportunistic outliers). [...] Considering that common implementations of rhombics rarely go beyond a couple of wavelengths to several, it seems that 20 or 40 or 50 has no future. Those findings of "no further gain" and "no future" are very strange. In the real world, extreme-length rhombics on VHF do have high gain - and also a very distinguished past. The law of diminish returns must occur somewhere as you are constantly losing power as it trucks down the length. All types of antenna suffer from diminishing returns, in terms of gain versus absolute size; and it is conceded that extreme-length V-beams and rhombics take up a huge amount of real estate for the gain they generate. But what you seem to be finding is a "gain saturation" effect that is more severe than the normal diminishing returns. This is a puzzle: would you care to share some antenna files? -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Sun, 11 Jul 2004 10:49:30 +0100, "Ian White, G3SEK"
wrote: This is a puzzle: would you care to share some antenna files? Hi Ian, That currently runs to 62 files, some EZNEC+4 - give me something you are familiar with. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
On Sun, 11 Jul 2004 10:49:30 +0100, "Ian White, G3SEK" wrote: This is a puzzle: would you care to share some antenna files? Hi Ian, That currently runs to 62 files, some EZNEC+4 - give me something you are familiar with. I have EZNEC+ v4 too, but that is the only format I can presently read. Please can you zip and send a selection of files that seem to illustrate the point? -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Sun, 11 Jul 2004 07:53:54 GMT, Richard Clark
wrote: On Wed, 07 Jul 2004 17:44:58 -0400, Albert wrote: How much gain (dbd) should I expect and about what take off angle will I have? Hi Al, Contrary to Wes' results, I do not find much more than 13dBi, and certainly not from your proposed huge implementation running out towards 50 wavelengths. In fact, I find antennas that are a tenth of that (5M) have about as much gain as will be found. My matrix of testing shows that doubling to 10 wavelengths and doubling again to 20 wavelengths brings no further gain (except for some opportunistic outliers). As a variation upon a theme, I decided to play with uptilt on the 5 wavelength models, lifting the far ends by roughly 25 degrees. The feed point is at 3M, and the far ends are lifted 5M. With this configuration, there is some loss in gain, but the lobe looking at the horizon is easily four times broader. That is, there is an even gain of roughly 11dBi from 14 degrees above the horizon to 34 degrees above the horizon. This occurs for the tips being separated by 30 to 90 degrees (the gain falls to 9dBi with wider separation). When you separate further, out at 150 degrees between the tips, the broad characteristic collapse, but recaptures gain, and puts it out lower. At such a configuration you might observe 11dBi @ 4 degrees. That lobe is only 3 or 4 degrees tall however. Considering that common implementations of rhombics rarely go beyond a couple of wavelengths to several, it seems that 20 or 40 or 50 has no future. The law of diminish returns must occur somewhere as you are constantly losing power as it trucks down the length. At that far end, nothing added to little before it hardly piles up gain. 73's Richard Clark, KB7QHC Hi All, Following up with a series of 2 wavelength measurements, it is interesting to note that of the series of 19 tests, fully 13 of them evidenced HIGHER gain than those from the 20 wavelength series of measurements. The step from 2 wavelength to 5 wavelength showed gains consistent with doubling the length of the antenna size for many separations (e.g. 3dB gain, or thereabout). However, it appears that beyond 5 wavelengths (considering my next cardinal point was a doubling to 10 wavelengths) no further gain was observed as a general characteristic. If I were to judge this at the 180 degree spread and compare against ALL other designs; then the absolute greatest gain for a V design was observed to be slightly less than 4dB. In fact, the 2, 5, 10, and 20 wavelength designs configured as simple dipoles barely differed one from the other (1dB at most, and typically 9.9dBi). 73's Richard Clark, KB7QHC |
Richard Clark wrote:
The step from 2 wavelength to 5 wavelength showed gains consistent with doubling the length of the antenna size for many separations (e.g. 3dB gain, or thereabout). However, it appears that beyond 5 wavelengths (considering my next cardinal point was a doubling to 10 wavelengths) no further gain was observed as a general characteristic. Was this for a copper wire antenna? What happens when you choose wire loss = zero? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Mon, 12 Jul 2004 07:31:01 GMT, Richard Clark
wrote: [Earlier stuff snipped] | |Following up with a series of 2 wavelength measurements, it is |interesting to note that of the series of 19 tests, fully 13 of them |evidenced HIGHER gain than those from the 20 wavelength series of |measurements. | |The step from 2 wavelength to 5 wavelength showed gains consistent |with doubling the length of the antenna size for many separations |(e.g. 3dB gain, or thereabout). However, it appears that beyond 5 |wavelengths (considering my next cardinal point was a doubling to 10 |wavelengths) no further gain was observed as a general characteristic. | |If I were to judge this at the 180 degree spread and compare against |ALL other designs; then the absolute greatest gain for a V design was |observed to be slightly less than 4dB. In fact, the 2, 5, 10, and 20 |wavelength designs configured as simple dipoles barely differed one |from the other (1dB at most, and typically 9.9dBi). I must confess that I've tried to follow your path but clearly I'm lost. May I suggest that analysis at "180 degree spread", which I take to mean a dipole, has no relationship to a vee configuration, other than the wire length. If you will take each leg length and vary the apex angle to the optimum, you will (should) find that the gain *does* continue to increase with increased leg length, albeit at a sub-proportional rate. For each leg length there is an optimal apex angle. Leaving the angle fixed and varying the length is *not* a fair test of gain vs. length. Here are the results I obtained from a quick MultiNEC (NEC-2) analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4 deg. Source on the middle of a short (3 segment) wire. Len. (WL) Ang. (deg) Gain (dBi) 10.200 30 19.84 20.193 20 21.97 30.188 16 23.06 40.185 14 23.70 50.182 12 24.10 These data seem resonable and consistant with my expectations. Wes |
On Mon, 12 Jul 2004 10:24:01 -0700, Wes wrote:
Here are the results I obtained from a quick MultiNEC (NEC-2) analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4 deg. Source on the middle of a short (3 segment) wire. Len. (WL) Ang. (deg) Gain (dBi) 20.193 20 21.97 Hi Wes, For this particular design (except mine is elevated 5 wavelengths), I pushed for 1 degree resolution, with a split source, with tapered segments (1023 all told) and I still fall short, but also well ahead of my earlier reports: 18.5dBi @ 3 degrees w/3 degree lobe width 73's Richard Clark, KB7QHC |
This analysis should be done in free space, not over ground. Propagation
to the moon should avoid reflection from the ground, and in any case EZNEC's flat, infinite-extent ground model isn't representative of what the signal would encounter in real life. One other comment. As a dipole gets longer, the lobes move closer and closer to the direction of the wire. Bending the dipole into a vee shape aligns pairs of the lobes so they point in the same direction, i.e., along the vee axis. That's why the optimum angle becomes less and less as the wires get longer, and why a dipole model isn't necessarily representative of what a long vee will do. Roy Lewallen, W7EL Richard Clark wrote: For this particular design (except mine is elevated 5 wavelengths), I pushed for 1 degree resolution, with a split source, with tapered segments (1023 all told) and I still fall short, but also well ahead of my earlier reports: 18.5dBi @ 3 degrees w/3 degree lobe width 73's Richard Clark, KB7QHC |
On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen
wrote: Hello Roy, There are any number of problems with your comments: This analysis should be done in free space, not over ground. The application demands ground as an inescapable reality of design. Designing in free space, unless you expand upon your commentary, is meaningless. In other words, the product of a free space analysis offers no more insight that this blighted version. Your comments that follow fairly shout this as a wholly undecipherable problem. Propagation to the moon should avoid reflection from the ground, This has already been offered as a cautionary. However, as a cautionary it says nothing about the impact of application aside from the introduction of noise (ground temperature) which lies outside of EZNEC's constraints. There would undoubtedly be issues of ducting, much less diffraction at the air/space boundary - and these too are within the domain of propagation modelers which is not what I perceive EZNEC to be. However, propagation modelers do work from antenna characteristics and it would seem this work is adequate to that (anticipated) task. The propagation modelers I am used to seem to expect ground considerations rolled into the antenna characteristics. and in any case EZNEC's flat, infinite-extent ground model isn't representative of what the signal would encounter in real life. No doubt, but this still says nothing on which to hang a hat. The valuations offered range from 10 to 22dBi. Are these values off by 3dB, 10dB, 100dB? Rather than having a good low angle response, the actual response is straight up? What is the context of your warning? If they are not representative do we have an unimaginable response? If the EZNEC is sufficient for Rhombics at HF, certainly at VHF the wavelength horizon is much further off and earth appears that much flatter. Earth curvature exists for all applications and your warnings would suggest no model is useful. 73's Richard Clark, KB7QHC |
Roy Lewallen wrote:
This analysis should be done in free space, not over ground. Propagation to the moon should avoid reflection from the ground, and in any case EZNEC's flat, infinite-extent ground model isn't representative of what the signal would encounter in real life. Very long rhombics and vees (for any frequency) cannot ignore the existence of ground. On the contrary, they rely on it, so a model including ground reflection is correct for this antenna. This includes the use of these antennas for EME in former days. Apologies to Richard for not responding to the antenna files that he kindly sent across, a few days ago (other deadlines). The discussion has moved on in the meantime, so it makes more sense for me to jump in again here. One other comment. As a dipole gets longer, the lobes move closer and closer to the direction of the wire. Bending the dipole into a vee shape aligns pairs of the lobes so they point in the same direction, i.e., along the vee axis. That's why the optimum angle becomes less and less as the wires get longer, That would have been my first point in response to Richard's findings. As a single end-fed wire gets longer, its radiation pattern becomes predominantly X-shaped, with the four main lobes moving closer and closer to the line of the wire itself. The optimum angle for a V-beam is chosen to make the main lobes of the two wires overlap exactly, so they reinforce as strongly as possible in the forward direction. If the V angle is kept constant and only the leg length is increased, the maximum obtainable forward gain will not be realized. I don't have any of the classic references for optimum design of V-beams or rhombics to hand; but unless Richard and I are both doing it totally wrong, the V-beam does seem to show some reluctance to increase in gain by a whole 3dB for each doubling of the leg length (and the rhombic would do the same). However, that doesn't detract from the reputation of the rhombic in particular as "the king of HF antennas" - if you have the real estate and can tolerate the fixed direction, the rhombic can give several dB more forward gain than almost any other practical antenna. All of this prompted me to try to model the 50-wavelength-per-side rhombic that we used for EME, way back when. My recollection is that the included angle was 12deg, but the original notes are long gone. EZNEC+ v4 predicts the spectacularly narrow main lobe that one would expect, and it also confirms the well-known finding that if you terminate the rhombic at the far end, the pattern changes from bi-directional to unidirectional but the forward gain also drops by about 3dB. However, 30-40 years ago it was believed that it is not important to terminate an extremely long rhombic "because most of the forward-traveling wave has been lost to radiation before it arrives at the far end." The model categorically negates that belief - even at 50wl/side, termination has much the same effect as for shorter rhombics. Unfortunately the segmentation density in my model (7.5 segs/wl) is too sparse to be confident about the absolute value of the gain. Normally one should both increase and decrease the segmentation density to confirm that the predictions remain stable; but this is not possible because this enormous antenna has already used 1499 out of the 1500 allowable segments. Since I can't be confident about the gain predictions, there is no point in quoting and discussing them here - we already chase enough wild geese in this ng. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Sorry, I don't have the time or inclination to get into another
protracted "last man standing is the winner" diatribe. I'll leave it to the readers to evaluate what I've written and decide whether or not it makes sense, or whether they'll choose instead to be persuaded by your objections. Either is fine with me. Roy Lewallen, W7EL Richard Clark wrote: On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen wrote: Hello Roy, There are any number of problems with your comments: This analysis should be done in free space, not over ground. The application demands ground as an inescapable reality of design. Designing in free space, unless you expand upon your commentary, is meaningless. In other words, the product of a free space analysis offers no more insight that this blighted version. Your comments that follow fairly shout this as a wholly undecipherable problem. Propagation to the moon should avoid reflection from the ground, This has already been offered as a cautionary. However, as a cautionary it says nothing about the impact of application aside from the introduction of noise (ground temperature) which lies outside of EZNEC's constraints. There would undoubtedly be issues of ducting, much less diffraction at the air/space boundary - and these too are within the domain of propagation modelers which is not what I perceive EZNEC to be. However, propagation modelers do work from antenna characteristics and it would seem this work is adequate to that (anticipated) task. The propagation modelers I am used to seem to expect ground considerations rolled into the antenna characteristics. and in any case EZNEC's flat, infinite-extent ground model isn't representative of what the signal would encounter in real life. No doubt, but this still says nothing on which to hang a hat. The valuations offered range from 10 to 22dBi. Are these values off by 3dB, 10dB, 100dB? Rather than having a good low angle response, the actual response is straight up? What is the context of your warning? If they are not representative do we have an unimaginable response? If the EZNEC is sufficient for Rhombics at HF, certainly at VHF the wavelength horizon is much further off and earth appears that much flatter. Earth curvature exists for all applications and your warnings would suggest no model is useful. 73's Richard Clark, KB7QHC |
On Tue, 13 Jul 2004 05:49:08 GMT, Richard Clark
wrote: |On Mon, 12 Jul 2004 10:24:01 -0700, Wes wrote: | |Here are the results I obtained from a quick MultiNEC (NEC-2) |analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with |average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4 |deg. Source on the middle of a short (3 segment) wire. | |Len. (WL) Ang. (deg) Gain (dBi) | |20.193 20 21.97 | |Hi Wes, | |For this particular design (except mine is elevated 5 wavelengths), I |pushed for 1 degree resolution, with a split source, with tapered |segments (1023 all told) and I still fall short, but also well ahead |of my earlier reports: | 18.5dBi @ 3 degrees w/3 degree lobe width Keep trying, you're getting closer [g]. |
On Tue, 13 Jul 2004 01:07:20 -0700, Roy Lewallen
wrote: Sorry, I don't have the time or inclination to get into another protracted "last man standing is the winner" diatribe. I'll leave it to the readers to evaluate what I've written and decide whether or not it makes sense, or whether they'll choose instead to be persuaded by your objections. Either is fine with me. Roy Lewallen, W7EL Hi Roy, Are you really that bored by your customers? You could have said 80% less to equal the substance above, but instead you had time enough to choose the editorial over the technical. 73's Richard Clark, KB7QHC |
On Tue, 13 Jul 2004 09:53:16 -0700, Wes wrote:
Keep trying, you're getting closer [g]. Your advice contradicts other advice we have been offered [g]. |
On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen
wrote: |This analysis should be done in free space, not over ground. Propagation |to the moon should avoid reflection from the ground, Not true at all. "Ground gain" is routinely relied upon in EME, allowing marginal antenna systems to succeed on a rising/setting moon scenario. http://www.bigskyspaces.com/w7gj/smallemestn.htm http://www.qsl.net/oz1rh/gndgain/gnd...m#_Toc10586457 |and in any case |EZNEC's flat, infinite-extent ground model isn't representative of what |the signal would encounter in real life. Not exactly, but often good enough, especially for comparative purposes. The reflector at the other end isn't a smooth ball of green cheese either but it still looks pretty smooth at rf. [g] N7WS |
On Tue, 13 Jul 2004 09:02:40 +0100, "Ian White, G3SEK"
wrote: Sorry if this is untimely. My ISP has been up and down, mostly down, for the last three days. [snip] | |I don't have any of the classic references for optimum design of V-beams |or rhombics to hand; but unless Richard and I are both doing it totally |wrong, the V-beam does seem to show some reluctance to increase in gain |by a whole 3dB for each doubling of the leg length (and the rhombic |would do the same). Why would this not be so? The remote parts of the antenna have less energy to radiate. As the antenna becomes longer, it becomes less a standing-wave antenna and more a traveling-wave antenna. Kraus discusses Vs, terminated in their characteristic impedance, that offer increased front-to-back ratios because the reflected wave is suppressed. It is easily observed that as an unterminated V becomes longer, there is a modest increase in the front-to-back ratio and the real part of the feedpoint Z becomes nearly constant. The loss in the wires, both resistive and radiated, suppresses the reflected wave, in effect, self-terminating the wires. I believe that if it was not for this loss, the V would be totally bi-directional and the gain would be proportional to length. Kraus offers another V that has conductors of "considerable thickness" that produce a similar front-to-back improvement. | |However, that doesn't detract from the reputation of the rhombic in |particular as "the king of HF antennas" - if you have the real estate |and can tolerate the fixed direction, the rhombic can give several dB |more forward gain than almost any other practical antenna. | | |All of this prompted me to try to model the 50-wavelength-per-side |rhombic that we used for EME, way back when. My recollection is that the |included angle was 12deg, but the original notes are long gone. | |EZNEC+ v4 predicts the spectacularly narrow main lobe that one would |expect, and it also confirms the well-known finding that if you |terminate the rhombic at the far end, the pattern changes from |bi-directional to unidirectional but the forward gain also drops by |about 3dB. However, 30-40 years ago it was believed that it is not |important to terminate an extremely long rhombic "because most of the |forward-traveling wave has been lost to radiation before it arrives at |the far end." The model categorically negates that belief - even at |50wl/side, termination has much the same effect as for shorter rhombics. I'm surprised that you're seeing this. My modeling of even relatively short rhombics shows that removing the termination does not make the pattern symmetrically bi-directional and likewise I don't see anything near a 3 dB change. Wes N7WS |
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