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OK, let's discuss dipoles vs length
On 10/12/2014 11:18 AM, Lostgallifreyan wrote:
John S wrote in : Something else might be interesting; include the effects of sag (centenary) in a wire antenna. Agreed. I was thinking about that possibility last night. Meaning 'catenary', perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so modelling it would be useful. This is an interesting modeling situation. After you get acquainted with your modeling software of choice, let's work on it to see what differences there are. We can compare notes, if you like. Sound like fun? If so, let's start another thread, yes? |
OK, let's discuss dipoles vs length
John S wrote in :
On 10/12/2014 11:19 AM, Lostgallifreyan wrote: John S wrote in : Catenary instead of what I posted. Yes. I should have read your second post before my first reply.. No problem. Your reply was completely appropriate. We might be just a bit out of sink (he, he) sync. Let's not get too serious. I try not to. It's one reason I had a bit of off-topic fun with Douglas Adams last night when the opportunity arose. :) |
OK, let's discuss dipoles vs length
John S wrote in :
On 10/12/2014 11:18 AM, Lostgallifreyan wrote: John S wrote in : Something else might be interesting; include the effects of sag (centenary) in a wire antenna. Agreed. I was thinking about that possibility last night. Meaning 'catenary', perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so modelling it would be useful. This is an interesting modeling situation. After you get acquainted with your modeling software of choice, let's work on it to see what differences there are. We can compare notes, if you like. Sound like fun? If so, let's start another thread, yes? I won't be up to speed that fast, but once I have something that doesn't look like it will waste people's time I'll have a go. I did look up catenary curves some time back for some forgotten purpose (actually, I think is was to do with loads on lengthy beams in a PV installation) so I have some idea where to start looking, maybe. I'm assuming that the pysical properties would relate to the electrical ones in some way, but it's not something I've thought through. |
OK, let's discuss dipoles vs length
On 10/12/2014 12:09 PM, Lostgallifreyan wrote:
John S wrote in : Excellent! I like to explore and I encourage everyone to do so whether it be with math tools or getting your hands dirty. Keep it up. Indeed. :) That synth I mentioned, it took me a good chunk of lifetime waiting for someone to make so I could pay for it. I gave up waiting. :) Jeff just posted about the perils of antenna towers and hillsides vs NEC2's flat earth, and many earlier things (liek modellign with Sketchup prior to engineering in metals and plastics, have taught me caution. measure thrice, cut once. THis is how and why I want to use NEC, to save me from falling into expensive and time consuming traps, which I think it will do very well. Jeff seems to be a very experience man with RF. I don't doubt his comments about what is possible with a practical situation. I, however, am playing with some fundamentals (free space, antenna characteristics, etc) so that I understand how to catch up to the gurus here. My toy is the modeling program. I do like to compare theory to practical situations, however. |
OK, let's discuss dipoles vs length
On 10/12/2014 12:13 PM, Lostgallifreyan wrote:
John S wrote in : On 10/12/2014 11:19 AM, Lostgallifreyan wrote: John S wrote in : Catenary instead of what I posted. Yes. I should have read your second post before my first reply.. No problem. Your reply was completely appropriate. We might be just a bit out of sink (he, he) sync. Let's not get too serious. I try not to. It's one reason I had a bit of off-topic fun with Douglas Adams last night when the opportunity arose. :) I'm sorry, I don't know that person. Should I? |
OK, let's discuss dipoles vs length
John S wrote in :
Jeff seems to be a very experience man with RF. I don't doubt his comments about what is possible with a practical situation. I, however, am playing with some fundamentals (free space, antenna characteristics, etc) so that I understand how to catch up to the gurus here. My toy is the modeling program. I do like to compare theory to practical situations, however. Yep, while nowt is perfect, there is always worth in something as a guide. I got spoiled by op-amps, it's a different thing imagine stuff with actual transistors. Just one example of how tough it can be without the guides. |
OK, let's discuss dipoles vs length
John S wrote in :
I try not to. It's one reason I had a bit of off-topic fun with Douglas Adams last night when the opportunity arose. :) I'm sorry, I don't know that person. Should I? Writer of The Hitch-Hiker's Guide To The Galaxy, and others. He's fairly popular in technical and scientific circles so he gets mentioned at surprising and sometimes very funny moments. Aquired taste actually, but I was lucky enough to aquire it early. |
OK, let's discuss dipoles vs length
On 10/12/2014 12:26 PM, Lostgallifreyan wrote:
John S wrote in : I try not to. It's one reason I had a bit of off-topic fun with Douglas Adams last night when the opportunity arose. :) I'm sorry, I don't know that person. Should I? Writer of The Hitch-Hiker's Guide To The Galaxy, and others. He's fairly popular in technical and scientific circles so he gets mentioned at surprising and sometimes very funny moments. Aquired taste actually, but I was lucky enough to aquire it early. I know of Hitch-Hiker's Guide To The Galaxy, but I have never watched. Probably my loss. In any case, let's do the antenna thing. Yes? |
OK, let's discuss dipoles vs length
On 10/12/2014 12:17 PM, Lostgallifreyan wrote:
John S wrote in : On 10/12/2014 11:18 AM, Lostgallifreyan wrote: John S wrote in : Something else might be interesting; include the effects of sag (centenary) in a wire antenna. Agreed. I was thinking about that possibility last night. Meaning 'catenary', perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so modelling it would be useful. This is an interesting modeling situation. After you get acquainted with your modeling software of choice, let's work on it to see what differences there are. We can compare notes, if you like. Sound like fun? If so, let's start another thread, yes? I won't be up to speed that fast, but once I have something that doesn't look like it will waste people's time I'll have a go. I did look up catenary curves some time back for some forgotten purpose (actually, I think is was to do with loads on lengthy beams in a PV installation) so I have some idea where to start looking, maybe. I'm assuming that the pysical properties would relate to the electrical ones in some way, but it's not something I've thought through. Ok. Putting the catenary into the simulator will take some thought from me as well. I'll do my best, but don't wait for me. Pursue it as you wish. (Or anybody else reading the thread) Can somebody start another thread if you are interested? |
OK, let's discuss dipoles vs length
Lostgallifreyan wrote:
John S wrote in : Something else might be interesting; include the effects of sag (centenary) in a wire antenna. Agreed. I was thinking about that possibility last night. Meaning 'catenary', perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so modelling it would be useful. I doubt you will see any significant difference. I've done a lot of modeling with V's and inverted V's. Except for a slight diffenence due to ground effects at low heights, there is no difference between them. As the angle goes from 180 degrees, i.e. a dipole, the impedance goes down and and the pattern spreads out. As the angle gets smaller, the gain goes down, the pattern becomes almost circular like a vertical, and the antenna starts looking like a transmission line, which it becomes when the angle gets to 0, with some spacing between the wires of course. And like an ordinary dipole, height over ground has a major effect on the pattern. Example: At 108 degrees, the impedance is about 65 Ohms and the broadside null of the dipole is now only about 7 dB down from the main lobe. But as the inverted V is a popular antenna, the pattern with common leg angles would be instrutive. -- Jim Pennino |
OK, let's discuss dipoles vs length
Jeff Liebermann wrote:
snip Somewhere on my computah is a series of models that I built for a simple 20 meter dipole, that started with an idealized free space model, and progressed towards a real installation which by coincidence resembles my house. I threw in everything that might have an effect on the pattern to see what might happen. I got stuck at including the sloping hillside because NEC2 seems to only include a flat earth. I've also done similar studies for commercial antennas mounted on very real and quite cluttered towers. Doing these incrementally is an excellent introduction into the difference between ideal antenna patterns, as found in the literature, and the nightmarish reality of real antenna installations. Yes, this is another thing typically ignored by amateurs. There are lots of plans out there for J-poles made of pipe where one of the legs is extended at the bottom to provide the mounting, typically by bolting it to a mast. The assumption is there is no current in that bottom leg. If you model that you find that there really is current in that bottom leg and it can REALLY screw up the antenna characteristics. -- Jim Pennino |
OK, let's discuss dipoles vs length
wrote in message ... I've done a lot of modeling with V's and inverted V's. Except for a slight diffenence due to ground effects at low heights, there is no difference between them. As the angle goes from 180 degrees, i.e. a dipole, the impedance goes down and and the pattern spreads out. I have not played with the programs , but often wondered what the effect is on antennas that are suported on the ends and the middle is dropped in a U or V shape and not the inverted V shape. Many antenna books and articals seem to show the flat top and inverted V paterns, but I have not seen any with the actual V type. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
On 10/12/2014 1:21 PM, wrote:
John S wrote: On 10/11/2014 12:51 PM, wrote: John S wrote: snip OK, so lets analyze my results: Conditions are free space, wire is #14 gauge but may have zero ohms where noted. The antenna is a dipole with the source connected at the center, F=7MHz. I'm using EZNEC with a source of 1 watt. Antenna resonance plays no part in this. # segments = 99 unless otherwise noted. Lambda Wire Rin Gavg(dbi) Gmax(dbi) Efficiency 0.5 zero 80 0 2.16 100% 0.5 #14 73.6 -.09 2.08 98% 0.25 zero 13.2 0 1.85 100% 0.25 #14 13.7 -.17 1.69 96% 0.125 zero 3 0 1.78 100% 0.125 #14 3.25 -.33 1.45 93% 0.05 zero .464 0 1.76 100% 0.05 #14 .556 -.78 0.98 83% Rin is the terminal resistance only. Gave is the average gain integrated over the pattern, Gmax is the highest gain detected. Unless I have done something wrong, I see that a dipole that is .05 wavelengths long is within 20% of being as efficient as a half-wave dipole. Even including wire resistance. I invite discussion in any case. The diameter of #14 solid wire is 0.0641"; how about a line for #8, which is 0.1285"? 0.05 #8 0.515 -.41 1.36 91.1% Free space, no ground loss. So it seems that a .05 lamda dipole is only about 7% less efficient than a full size dipole wit suitable wire... So much for "short antennas are not efficient". (snip extraneous input) Yes, Jim, that is so. In fact, that was the hidden reason for the exercise. I was hoping this would provide an example to let others know that it is not the antenna length that is the problem as Gareth proposed. I was hoping that others would take the investigation into their own hands as a result. I noted that you tried to foul me up with the unreasonable wire size. EZNEC has a nice warning feature to take care of it. |
OK, let's discuss dipoles vs length
John S wrote in :
I know of Hitch-Hiker's Guide To The Galaxy, but I have never watched. Probably my loss. In any case, let's do the antenna thing. Yes? I think you may need someone better at it than I am right now. :) I've just been looking at pjotos of a vertical whip to consider making the junction of fences it's mounted over into somethign better approximating thre radials of goodish length, and it's very crude work, very basic practical salvage of an awkward location. Apart from an FM dipole for listening, and a portanle longwire to experiment with a Beverage antenna on open land I have access to. This one is the likely sagging antenna, hence my interest in catenary effects, but I'm not well placed to try modelling anything yet. I'll want to finish the thing and get it out there just to try listening to stuff a while first. |
OK, let's discuss dipoles vs length
John S wrote in :
I know of Hitch-Hiker's Guide To The Galaxy, but I have never watched. Probably my loss. Not really. :) It's much better read, or maybe best of all, heard in the original programs that went out on Radio 4 in the early evenings in the late 70's. Those are famous, likely easily had in various ways. I've seen the movie, but it doesn't work for me so well. Some of it is great, but they totally failed to get Marvin right, and that means they lost a lot of the depth of it. I liked their Vogons though. |
OK, let's discuss dipoles vs length
John S wrote in :
Ok. Putting the catenary into the simulator will take some thought from me as well. I'll do my best, but don't wait for me. Pursue it as you wish. (Or anybody else reading the thread) Can somebody start another thread if you are interested? Sweconded. I'll admit at this point welcoming an easy start, and a prepared example of this would be a hell of an incentive for me to get into exploring NEC too. |
OK, let's discuss dipoles vs length
John S wrote:
On 10/12/2014 1:06 PM, wrote: Lostgallifreyan wrote: John S wrote in : Something else might be interesting; include the effects of sag (centenary) in a wire antenna. Agreed. I was thinking about that possibility last night. Meaning 'catenary', perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so modelling it would be useful. I doubt you will see any significant difference. Do you have any data or a simulation which supports your position? (snip irrelevant portion) Reread the portion you snipped as irrelevant. -- Jim Pennino |
OK, let's discuss dipoles vs length
Lostgallifreyan wrote in
: Seconded. I'll admit at this point welcoming an easy start, and a prepared example of this would be a hell of an incentive for me to get into exploring NEC too. Actually to be fair, with ,y longwire tests I'll likely be using very thin strong stainless wire and pulling it tight enough to reduce my need to worry about it much. STill interesting though, I've seen heavy-looking HF antennas strung over the apex of a roof in a valley near here, and that had a pronounced sag that may or may not have been bothersome to whoever owned it. |
OK, let's discuss dipoles vs length
Ralph Mowery wrote:
wrote in message ... I've done a lot of modeling with V's and inverted V's. Except for a slight diffenence due to ground effects at low heights, there is no difference between them. As the angle goes from 180 degrees, i.e. a dipole, the impedance goes down and and the pattern spreads out. I have not played with the programs , but often wondered what the effect is on antennas that are suported on the ends and the middle is dropped in a U or V shape and not the inverted V shape. Many antenna books and articals seem to show the flat top and inverted V paterns, but I have not seen any with the actual V type. Like I said, there is no difference in free space between a V and an inverted V. There are slight differences when close to ground. -- Jim Pennino |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
In message ,
writes Look at this for an overview of NEC. http://en.wikipedia.org/wiki/Numeric...magnetics_Code At the bottom under External links you will find both free and commercial implementations. EZNEC by W7EL is popular among hams and has a free demo version that is fully functional but limited in how complex a model you can generate. NEC itself just crunches and produces numbers, but there are several versions, including EZNEC, which have graphical interfaces to make it easier to build the model and view the results. I've used EZNEC+ for years. I use 4NEC2 - it's free ! Arie Voors used to post here, but I've not seen anything from him for some time. MMANA is good too, but there's a lot it can't do. 73 Brian GM4DIJ -- Brian Howie --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
John S wrote:
On 10/12/2014 1:21 PM, wrote: John S wrote: On 10/11/2014 12:51 PM, wrote: John S wrote: snip OK, so lets analyze my results: Conditions are free space, wire is #14 gauge but may have zero ohms where noted. The antenna is a dipole with the source connected at the center, F=7MHz. I'm using EZNEC with a source of 1 watt. Antenna resonance plays no part in this. # segments = 99 unless otherwise noted. Lambda Wire Rin Gavg(dbi) Gmax(dbi) Efficiency 0.5 zero 80 0 2.16 100% 0.5 #14 73.6 -.09 2.08 98% 0.25 zero 13.2 0 1.85 100% 0.25 #14 13.7 -.17 1.69 96% 0.125 zero 3 0 1.78 100% 0.125 #14 3.25 -.33 1.45 93% 0.05 zero .464 0 1.76 100% 0.05 #14 .556 -.78 0.98 83% Rin is the terminal resistance only. Gave is the average gain integrated over the pattern, Gmax is the highest gain detected. Unless I have done something wrong, I see that a dipole that is .05 wavelengths long is within 20% of being as efficient as a half-wave dipole. Even including wire resistance. I invite discussion in any case. The diameter of #14 solid wire is 0.0641"; how about a line for #8, which is 0.1285"? 0.05 #8 0.515 -.41 1.36 91.1% Free space, no ground loss. So it seems that a .05 lamda dipole is only about 7% less efficient than a full size dipole wit suitable wire... So much for "short antennas are not efficient". (snip extraneous input) Yes, Jim, that is so. In fact, that was the hidden reason for the exercise. I was hoping this would provide an example to let others know that it is not the antenna length that is the problem as Gareth proposed. I was hoping that others would take the investigation into their own hands as a result. I noted that you tried to foul me up with the unreasonable wire size. EZNEC has a nice warning feature to take care of it. What "unreasonable wire size"? #8 wire is readily available and often used to make antenna elements, as is 1/8 th aluminum, which is only a few thousands of an inch different. Or are you refering to issues with segmentation and fat, short wires which I thought I had warned you about? -- Jim Pennino |
OK, let's discuss dipoles vs length
Lostgallifreyan wrote:
Lostgallifreyan wrote in : Seconded. I'll admit at this point welcoming an easy start, and a prepared example of this would be a hell of an incentive for me to get into exploring NEC too. Actually to be fair, with ,y longwire tests I'll likely be using very thin strong stainless wire and pulling it tight enough to reduce my need to worry about it much. STill interesting though, I've seen heavy-looking HF antennas strung over the apex of a roof in a valley near here, and that had a pronounced sag that may or may not have been bothersome to whoever owned it. You might want to read this before you put up stainless steel wi http://www.mwrs.org.au/2011/06/23/an...eel-vs-copper/ There is a link to a report in there. -- Jim Pennino |
OK, let's discuss dipoles vs length
Lostgallifreyan wrote:
wrote in : I doubt you will see any significant difference. I've done a lot of modeling with V's and inverted V's. Except for a slight diffenence due to ground effects at low heights, there is no difference between them. Fair enough. I've seen antennas loaded down with feedline but given that V forms are used well, the slight V in those cases, let alone some sag, might not be a concern for me. My main interest came out of my wish to try a befavare antenna I can install almost as fast as I can run the distance. That one would have no support over at least a half mile, and I don't yet know whether it will be mechanically sound till I finish designing and making it. A half mile is a long distance. I hope you have the equations for the sag and tension on the wire at hand and run through them first. As I recall, both charts and calculators can be found on the 'net. -- Jim Pennino |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
wrote in :
You might want to read this before you put up stainless steel wi http://www.mwrs.org.au/2011/06/23/an...eel-vs-copper/ There is a link to a report in there. I will, but I have also seen several claims that at least for listenign purposes, stainless isn't so bad, and that except when driving an antenna with power, it may not matter what metal is used. No worries though, it's cheap, I've got it, and I'll try it. :) If it fails, I'll get some hard-drawn copper. |
OK, let's discuss dipoles vs length
Lostgallifreyan wrote:
wrote in : You might want to read this before you put up stainless steel wi http://www.mwrs.org.au/2011/06/23/an...eel-vs-copper/ There is a link to a report in there. I will, but I have also seen several claims that at least for listenign purposes, stainless isn't so bad, and that except when driving an antenna with power, it may not matter what metal is used. No worries though, it's cheap, I've got it, and I'll try it. :) If it fails, I'll get some hard-drawn copper. Claims are like belly buttons; everyone has one. These guys did an actual study. If nothing else, the stainless could be used as a support for the copper. -- Jim Pennino |
OK, let's discuss dipoles vs length
Ian Jackson wrote:
In message , writes Ralph Mowery wrote: wrote in message ... I've done a lot of modeling with V's and inverted V's. Except for a slight diffenence due to ground effects at low heights, there is no difference between them. As the angle goes from 180 degrees, i.e. a dipole, the impedance goes down and and the pattern spreads out. I have not played with the programs , but often wondered what the effect is on antennas that are suported on the ends and the middle is dropped in a U or V shape and not the inverted V shape. Many antenna books and articals seem to show the flat top and inverted V paterns, but I have not seen any with the actual V type. Like I said, there is no difference in free space between a V and an inverted V. There are slight differences when close to ground. Assuming it's a halfwave dipole, I would have thought it was better to concentrate on getting the centre (where the current is) as high as possible, rather than the ends. [Obviously this depends on whether you have a suitable sky-hook available for and at the centre.] However, I believe that there is at least one commercial loaded dipole (essentially for portable use, I think) that has a short mast, and the ends of the V are up in the air. Model it let me know what you find. Also, regarding the radiation pattern, as the ends are dropped, doesn't the signal that starts coming off the ends have an increasingly vertically-polarised component? As I recall, yes, the vertical component increases with decreasing angle. -- Jim Pennino |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
"Ian Jackson" wrote in message ... In message , writes Like I said, there is no difference in free space between a V and an inverted V. There are slight differences when close to ground. Assuming it's a halfwave dipole, I would have thought it was better to concentrate on getting the centre (where the current is) as high as possible, rather than the ends. [Obviously this depends on whether you have a suitable sky-hook available for and at the centre.] However, I believe that there is at least one commercial loaded dipole (essentially for portable use, I think) that has a short mast, and the ends of the V are up in the air. In free space I would not think there would be much of any differance. Near the ground where most of us are is what we have to live with. In my back yard I have enough room to string wire dipoles for 80 meters and shorter. I don't have a way to support the antennas in the middle. I can go up about 60 feet or so at the ends. One of my antennas is sort of flat as it is a windom and the long leg runs by my tower and was drooping to about 40 feet, so I put a piece of PVC pipe at the 50 foot level to help support it. The OCF antenna is suported so the ends are on some pulleys and a cinder block on the ground with a rope so that as trees swing the block will sometimes raise off the ground with high wind. I have put a spectrum analizer/tracking generator and return loss bridge on it a few times and watched the SWR/RL go up and down as the wind blew. Really interisting as the RL did not change much in amplitude, but the frequency shifted. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
Ralph Mowery wrote:
"Ian Jackson" wrote in message ... In message , writes Like I said, there is no difference in free space between a V and an inverted V. There are slight differences when close to ground. Assuming it's a halfwave dipole, I would have thought it was better to concentrate on getting the centre (where the current is) as high as possible, rather than the ends. [Obviously this depends on whether you have a suitable sky-hook available for and at the centre.] However, I believe that there is at least one commercial loaded dipole (essentially for portable use, I think) that has a short mast, and the ends of the V are up in the air. In free space I would not think there would be much of any differance. Near the ground where most of us are is what we have to live with. In my back yard I have enough room to string wire dipoles for 80 meters and shorter. I don't have a way to support the antennas in the middle. I can go up about 60 feet or so at the ends. 60 feet is good for 30M, marginal for 40M, and crap for 80M and below for a dipole. It is also too high for 15M and up. For best results, a diple should be at .5 labda. Then again, a marginal 40M dipole is better than no antenna at all. -- Jim Pennino |
OK, let's discuss dipoles vs length
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OK, let's discuss dipoles vs length
wrote in message ... the ground where most of us are is what we have to live with. In my back yard I have enough room to string wire dipoles for 80 meters and shorter. I don't have a way to support the antennas in the middle. I can go up about 60 feet or so at the ends. 60 feet is good for 30M, marginal for 40M, and crap for 80M and below for a dipole. It is also too high for 15M and up. For best results, a diple should be at .5 labda. Then again, a marginal 40M dipole is better than no antenna at all. I have been putting out a lot of furtlizer around the trees, but they do not seem to be growning much.. Most ideal antennas for most hams are just not practictal or cost too much. I will agree with what you are saying, especially about they should be up .5 wavelength if possiable. Just difficult to get an 80 meter dipole up 130 feet or so for most hams. The 50 to 60 feet of my 80 meter antenna seems to work well for what I do. Mostly talk to a couple of friends within 100 miles from my house. It should send most of the signal up at a high angle for local coverage. I stopped at 60 feet for my triband beam antenna as I did not want to climb any higher. Not afraid of heigths,but not physically able to do much more. Also the spacing of the guy wires worked out just right for that height. Like you say, an antenna at 10 or 20 feet is still beter than no antenna. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
OK, let's discuss dipoles vs length
"Lostgallifreyan" wrote in message . .. Lostgallifreyan wrote in : Actually to be fair, with ,y longwire tests I'll likely be using very thin strong stainless wire and pulling it tight enough to reduce my need to worry about it much. STill interesting though, I've seen heavy-looking HF antennas strung over the apex of a roof in a valley near here, and that had a pronounced sag that may or may not have been bothersome to whoever owned it. Looking at a chart in an old ARRL antenna handbook gives a rough estiment of a length of 500 feet and a tension of 400 pounds a wire of around 12 to 14 gauge will drop about 10 feet if Idid it right. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
OK, let's discuss dipoles vs length
On Sun, 12 Oct 2014 18:13:39 -0000, wrote:
There are lots of plans out there for J-poles made of pipe where one of the legs is extended at the bottom to provide the mounting, typically by bolting it to a mast. The assumption is there is no current in that bottom leg. If you model that you find that there really is current in that bottom leg and it can REALLY screw up the antenna characteristics. I don't like J-Poles. Various reasons, some of what are non-technical. I'll spare you the details. Our local radio club had a meeting where everyone was suppose to build a twinlead J-Pole or a Slim Jim and have it tested by the experts and their MFJ antenna analyzers. It wasn't too difficult for me to predict that things were not going to work according to the science fiction found in the collection of construction articles, so I elected to just attend and watch. I arrived late and found about 12 J-Poles in various stages of construction and testing in groups of 2 to 4 members. Many J-Poles were not even close to tuning onto the target frequencies. All of them changed tuning when the antenna was moved. Measurements were stable unless something moved (including people), where everything changed. Someone had the bright idea of hanging the antennas from the metal framework supporting the acoustic tile ceiling, thus adding the framework to the antenna. There were no baluns in sight, so when I recommended adding some additional coax cable just to see what happens, the tuning again changed radically. Running my hand up and down the coax cable did much the same thing, demonstrating that a balun was probably a good idea. Minimum VSWR improved when I convinced two builders to add series trimmer caps to tune out the inductance of the feed tap wire, but usable bandwidth decreased. In the past, someone would usually bring a copper pipe J-pole with the coax feed and ground reversed, and wonder why it wouldn't work. Not this time. I tried to drag out my laptop and show some interested builders an NEC2 model of their J-Pole, but couldn't figure out how to deal with the velocity factor of elements made from twinlead. The IS card in NEC4 handles it easily, but there's no equivalent in NEC2. Incidentally, I use an insulated PVC pipe for the "handle", large diameter elements, a balun, and a series tuning cap (Gamma match style), when building a J-Pole. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
OK, let's discuss dipoles vs length
Ralph Mowery wrote:
wrote in message ... the ground where most of us are is what we have to live with. In my back yard I have enough room to string wire dipoles for 80 meters and shorter. I don't have a way to support the antennas in the middle. I can go up about 60 feet or so at the ends. 60 feet is good for 30M, marginal for 40M, and crap for 80M and below for a dipole. It is also too high for 15M and up. For best results, a diple should be at .5 labda. Then again, a marginal 40M dipole is better than no antenna at all. I have been putting out a lot of furtlizer around the trees, but they do not seem to be growning much.. Most ideal antennas for most hams are just not practictal or cost too much. I will agree with what you are saying, especially about they should be up .5 wavelength if possiable. Just difficult to get an 80 meter dipole up 130 feet or so for most hams. The 50 to 60 feet of my 80 meter antenna seems to work well for what I do. Mostly talk to a couple of friends within 100 miles from my house. It should send most of the signal up at a high angle for local coverage. Yes, it does. There is a discussion here worth reading: http://en.wikipedia.org/wiki/Near_ve...idence_skywave I stopped at 60 feet for my triband beam antenna as I did not want to climb any higher. Not afraid of heigths,but not physically able to do much more. Also the spacing of the guy wires worked out just right for that height. Like you say, an antenna at 10 or 20 feet is still beter than no antenna. The highest I can get at my QTH is about 25 feet and for years I wondered why I could never work anyone other than locals with my dipoles on the lower bands. Then I did some study on the effects of ground and switched to verticals. That was when I made my very first contact on 160 after many years of being licenced. -- Jim Pennino |
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