Re: Vertical antennas
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> 6. Don't expect very good performance on 80M. The antenna is short
> and you are unlikely to be able to put in a good enough ground system > to make it very efficient. Still, it is better than no antenna. Then again, you might have better performance than you expect. I don't have room on my roof for 80m radials, so I don't have any on my 6BTV. I put it up when I only had a KX1, so I didn't care. I got my K2 on the air shortly after the antenna, and started finding that 80m was sometimes useful when no other bands were. Performance isn't *great* by any means, but I worked VK9 with 5W last month! The Hustler antennas are compromise antennas. But they are better than no antenna ... a *lot* better. Very good value for money. 73 de chris K6DBG _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Fellow Elecrafters:
The discussion of verticals has inspired me to do a bit of EZNEC modeling. I plotted the azimuthal pattern at an 8 degree takeoff angle for several different 80 meter configurations. In all 3 cases, I've assumed average ground. The first case is the classical full size vertical, with a quarter wave monopole element and 128 quarter wave radials. I've assumed aluminum conductors on the theory that if I were really going to lay nearly 2 miles of wire on the ground, I'd use aluminum electric fence wire and not copper. Also, the monopole element would almost certainly be made from aluminum tubing; my EZNEC program does not support mixed conductor types. Hardly anyone would actually build such a costly configuration, but the performance does give a standard for comparison. Anyway, the pattern is an omni pattern with a signal strength at 8 degrees takeoff angle of -2.72 dBi. The second case is the Force 12 vertical dipole, with no radials. (I do not have the actual engineering data for the Force 12, but it is easy to approximate from the promotional materials.The Force 12 people do not recommend using radials, and for good reason. Cebik did a study that showed that radials under a vertical dipole do virtually no good whatsoever. The ground losses that affect its performance are hundreds if not thousands of feet from the antenna. That is why the spectacular results reported in the Force 12 promotional material are from operations right on the seashore.) I've assumed aluminum conductors. There is a very small note in the very fine print of the Force 12 promotional material that their patterns were run with the bottom of the antenna elevated 28 feet above the ground. I used that assumption in my simulation. (The trick with vertical dipoles is getting the current loop as high as possible above ground.) The signal at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. In other words, the Force 12 with its low end 28 feet above ground is an undetectable quarter dB worse than the ideal full size quarter wave configuration. The Force 12 appears to be just as good as the promotional material claims. How important is the mounting height? It matters. For the same configuration except with the bottom 1 foot above the ground, the signal strength at 8 degrees takeoff angle is -5.8 dBi. This is a quite noticeable >3db degradation from the full featured quarter wave configuration. The other configuration is an inverted L. This is a bit of a clunky design, but it is feasible tom build on my lot. It is a W3DZZ dipole, with one element vertical and one horizontal, and the feedline coming off normal to the plane of the L. The height of the feedpoint is 50 feet. The elements of a W3DZZ are longer than 50 feet, thus I've kinked out the part of the bottom element at a 45 degree angle (in the plane perpendicular to the horizontal element) so that the end barely clears the ground. (Yes, I know, if you have kids or dogs, put a fence around it.) I assume copper conductors, average ground, and take trap losses into account. Anyway, on 80 m at 8 degrees you get a near omni pattern that is -0.85 dBi in the strongest direction and -1.79 dBi in the weakest direction. Anyway, this is a cheap antenna (provided you happen to have 50 foot high trees at just the right spots) that outperforms both the full size vertical and the Force 12. Into the bargain, you get a near omni pattern on 40 meters that at 8 degrees takeoff angle is -1.2 dBi at its strongest direction and -3.15 dBi at its weakest direction. But wait, there's more; you get low SWR at both 80 and 40 with no need for a sophisticated matching scheme. The trick as always is that what really matters is getting the current loop as high above ground as possible, and configuring the elements such that the currents in them do not cancel each other out. As for slightly elevated ground planes with resonant radials, they work surprisingly well, but not as well as the three configurations above. However, that is another story for another day. 73, Steve Kercel AA4AK _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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I've not tried modeling 128 radials, or at that low of an elevation angle
but your results sound good, Stephen. Remember there are two sources of ground losses in verticals, near field and far field losses. Near field currents are those produced at the base of the antenna. Even fence wire is a vastly superior conductor to the wettest soil. The more radials dividing up that current, the less is left to "warm the earthworms". That results in more antenna current and more radiation. Most of us Hams focus on minimizing these losses because we can't do anything about far-field losses, but they are very significant. The other way we reduce lossy ground currents is to elevate the antenna and radials. The ground currents are induced currents, so doubling the distance between the radials and the earth reduces the induced currents by 75% assuming the same number of radials. Far field losses occur out a distance of wavelengths from a vertical antenna where currents induced in the lossy earth by the electromagnetic wave decreases the signal at low elevations. That's why a vertical shows sharply reduced levels below about 15 degrees above the horizon, under the best of conditions. A four radial configuration shows the major lobe at about 20 degrees above the horizon. That's a limitation we all have to live with. What varies most as the height is changed with a four-radial configuration is the overall gain as the induced grounds currents and losses decrease with height. With the more common 4 radial configuration, a near-the-ground ground plane antenna with the radials 10 feet up (to clear heads walking under them) will show just about 0 dBi at a 22 degree elevation above the horizon. That's why a horizontal dipole is usually preferred to a vertical if there's sufficient space to erect it. The change in orientation turns the ground into a reflector rather than absorbing so much RF current. It's a lossy reflector to be sure, but it's still effective. Even at a modest 30 foot height, a 40 meter dipole will show a gain of about 1.3 dBi at a 20 degree angle above the horizon, roughly the same as the vertical, with the bonus of a huge high-angle lobe produced by the ground reflection that the vertical lacks, giving superior short-skip performance. And, of course, those lucky Hams who can put their horizontal dipole up about 1/2 wavelength where it works best get a huge advantage. At 20 degrees it shows nearly 6 dB gain: equivalent to multiplying the transmitter power by four times! But most of us live with Marconi's problem, especially on the lower bands. Even if Marconi had understood the Hertz (dipole) antenna, for his transmitters operating near 100 kHz he'd have needed to string up 4,680 feet of horizontal wire at a height of over 4,900 feet to achieve optimum results. So he stayed with his tiny (in terms of wavelengths) top-loaded verticals with the best ground system he could devise and still got out well enough to prove that "wireless" worked and worked quite well. In the same way, those Hams who live without space for a decent horizontal radiator use verticals, some of them quite small, and continue to prove that we can still get out and work the world when conditions are right. By the way, I really admire Force 12's various comments about verticals. The readily agree they are a compromise between size and performance, and they note that their spectacular DX performance has nearly always been achieved on a beach at some rare DX site. Being on the edge of salt water reduces the far-field losses a great deal, and the signal the antenna is radiating is a rare DX call that attracts anyone who can hear it! It's no wonder that shipboard systems using the old 600 meter (about 400 - 500 kHz) marine band often logged large distances in spite of their tiny antennas. A shipboard antenna might be 200 feet long, but at 450 kHz that's hardly bigger than a mobile whip on 40 meters! The advantage they had was the world's best ground system for both near and far fields surrounding the ship in the middle of a salt water ocean. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Stephen W. Kercel Sent: Saturday, March 03, 2007 11:28 AM To: [hidden email] Subject: Re: [Elecraft] Vertical antennas Fellow Elecrafters: The discussion of verticals has inspired me to do a bit of EZNEC modeling. I plotted the azimuthal pattern at an 8 degree takeoff angle for several different 80 meter configurations. In all 3 cases, I've assumed average ground. The first case is the classical full size vertical, with a quarter wave monopole element and 128 quarter wave radials. I've assumed aluminum conductors on the theory that if I were really going to lay nearly 2 miles of wire on the ground, I'd use aluminum electric fence wire and not copper. Also, the monopole element would almost certainly be made from aluminum tubing; my EZNEC program does not support mixed conductor types. Hardly anyone would actually build such a costly configuration, but the performance does give a standard for comparison. Anyway, the pattern is an omni pattern with a signal strength at 8 degrees takeoff angle of -2.72 dBi. The second case is the Force 12 vertical dipole, with no radials. (I do not have the actual engineering data for the Force 12, but it is easy to approximate from the promotional materials.The Force 12 people do not recommend using radials, and for good reason. Cebik did a study that showed that radials under a vertical dipole do virtually no good whatsoever. The ground losses that affect its performance are hundreds if not thousands of feet from the antenna. That is why the spectacular results reported in the Force 12 promotional material are from operations right on the seashore.) I've assumed aluminum conductors. There is a very small note in the very fine print of the Force 12 promotional material that their patterns were run with the bottom of the antenna elevated 28 feet above the ground. I used that assumption in my simulation. (The trick with vertical dipoles is getting the current loop as high as possible above ground.) The signal at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. In other words, the Force 12 with its low end 28 feet above ground is an undetectable quarter dB worse than the ideal full size quarter wave configuration. The Force 12 appears to be just as good as the promotional material claims. How important is the mounting height? It matters. For the same configuration except with the bottom 1 foot above the ground, the signal strength at 8 degrees takeoff angle is -5.8 dBi. This is a quite noticeable >3db degradation from the full featured quarter wave configuration. The other configuration is an inverted L. This is a bit of a clunky design, but it is feasible tom build on my lot. It is a W3DZZ dipole, with one element vertical and one horizontal, and the feedline coming off normal to the plane of the L. The height of the feedpoint is 50 feet. The elements of a W3DZZ are longer than 50 feet, thus I've kinked out the part of the bottom element at a 45 degree angle (in the plane perpendicular to the horizontal element) so that the end barely clears the ground. (Yes, I know, if you have kids or dogs, put a fence around it.) I assume copper conductors, average ground, and take trap losses into account. Anyway, on 80 m at 8 degrees you get a near omni pattern that is -0.85 dBi in the strongest direction and -1.79 dBi in the weakest direction. Anyway, this is a cheap antenna (provided you happen to have 50 foot high trees at just the right spots) that outperforms both the full size vertical and the Force 12. Into the bargain, you get a near omni pattern on 40 meters that at 8 degrees takeoff angle is -1.2 dBi at its strongest direction and -3.15 dBi at its weakest direction. But wait, there's more; you get low SWR at both 80 and 40 with no need for a sophisticated matching scheme. The trick as always is that what really matters is getting the current loop as high above ground as possible, and configuring the elements such that the currents in them do not cancel each other out. As for slightly elevated ground planes with resonant radials, they work surprisingly well, but not as well as the three configurations above. However, that is another story for another day. 73, Steve Kercel AA4AK _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Ron:
See some interposed comments. 73, Steve, AA4AK At 04:05 PM 3/3/2007, Ron D'Eau Claire wrote: >I've not tried modeling 128 radials, or at that low of an elevation angle >but your results sound good, Stephen. > >Remember there are two sources of ground losses in verticals, near field and >far field losses. ************** NEC takes both into account. In fact, NEC lets you observe both the near field and far field radiation separately. This turns out to by highly useful if you're trying to troubleshoot your neighbor's TVI problems. *************** >Near field currents are those produced at the base of the antenna. Even >fence wire is a vastly superior conductor to the wettest soil. The more >radials dividing up that current, the less is left to "warm the earthworms". >That results in more antenna current and more radiation. Most of us Hams >focus on minimizing these losses because we can't do anything about >far-field losses, but they are very significant. The other way we reduce >lossy ground currents is to elevate the antenna and radials. ************ It is important to appreciate that these are two fundamentally different phenomena. With a radial system on the ground, you're trying to use the induced ground image as the second half of the antenna. Resonance of the radials is not critical. The principle is essentially that the more wire you have on the ground, the lower the effective ground losses. In the case of an elevated ground plane, the resonant radials serve to isolate you from the ground and its losses. The missing half of the vertical element is the effect of the resonant radials rather than the lossy ground. To get effective isolation from the ground, you need either higher elevation or more radials. That is why roof mounted CB antennas with four resonant radials are so effective; as a fraction of wavelength, they are high off the ground. On 80 meters, putting the ground plane at 15 feet elevation and using as few as 6 resonant radials yields surprisingly good results. As I'm sure you know, you can tell when the vertical antenna is performing better; the SWR goes up. A lossy vertical will have a low SWR because the high ground losses are in series with the radiation resistance and the sum comes out perversely close to 50 Ohms. A low loss vertical is around 30 Ohms. ************ >The ground >currents are induced currents, so doubling the distance between the radials >and the earth reduces the induced currents by 75% assuming the same number >of radials. > >Far field losses occur out a distance of wavelengths from a vertical antenna >where currents induced in the lossy earth by the electromagnetic wave >decreases the signal at low elevations. That's why a vertical shows sharply >reduced levels below about 15 degrees above the horizon, under the best of >conditions. A four radial configuration shows the major lobe at about 20 >degrees above the horizon. That's a limitation we all have to live with. *********** Although the major lobe peaks out at 20-25 degrees, there is still finite energy radiated at 6-10 degrees. On really long haul communications, it is that weak but finite low angle energy that propagates long distances. The higher energy starts out stronger, but makes more hops over a long path, and each ground reflection, especially on dry land, is extremely lossy. The lossy ground bounces matter. From Maine, I find that Hawaii on QRP (half the path is land and half is water) is a chip shot. Alaska, which is a thousand miles closer but entirely over land is virtually (but nor completely) impossible to work on QRP. *********** >What varies most as the height is changed with a four-radial configuration >is the overall gain as the induced grounds currents and losses decrease with >height. With the more common 4 radial configuration, a near-the-ground >ground plane antenna with the radials 10 feet up (to clear heads walking >under them) will show just about 0 dBi at a 22 degree elevation above the >horizon. > >That's why a horizontal dipole is usually preferred to a vertical if there's >sufficient space to erect it. *************** That's the gotcha. If you have a horizontal dipole at the same height as the top of a vertical dipole, in the broadside direction the horizontal wins hands down, provided you have two supports high enough to support the dipole. The current loop of the horizontal is twice as high as the current loop of the vertical. Of course, the vertical dipole has its current loop much higher than a ground mounted vertical or the typical elevated ground plane, and so will be the better performer. Of course, if you're going to implement a full size vertical dipole at frequencies below 14 MHz you need a really tall tree. *************** >The change in orientation turns the ground >into a reflector rather than absorbing so much RF current. It's a lossy >reflector to be sure, but it's still effective. Even at a modest 30 foot >height, a 40 meter dipole will show a gain of about 1.3 dBi at a 20 degree >angle above the horizon, roughly the same as the vertical, with the bonus of >a huge high-angle lobe produced by the ground reflection that the vertical >lacks, giving superior short-skip performance. > >And, of course, those lucky Hams who can put their horizontal dipole up >about 1/2 wavelength where it works best get a huge advantage. At 20 degrees >it shows nearly 6 dB gain: equivalent to multiplying the transmitter power >by four times! ********** All true. ********** >But most of us live with Marconi's problem, especially on the lower bands. >Even if Marconi had understood the Hertz (dipole) antenna, for his >transmitters operating near 100 kHz he'd have needed to string up 4,680 feet >of horizontal wire at a height of over 4,900 feet to achieve optimum >results. So he stayed with his tiny (in terms of wavelengths) top-loaded >verticals with the best ground system he could devise and still got out well >enough to prove that "wireless" worked and worked quite well. In the same >way, those Hams who live without space for a decent horizontal radiator use >verticals, some of them quite small, and continue to prove that we can still >get out and work the world when conditions are right. > >By the way, I really admire Force 12's various comments about verticals. The >readily agree they are a compromise between size and performance, and they >note that their spectacular DX performance has nearly always been achieved >on a beach at some rare DX site. Being on the edge of salt water reduces the >far-field losses a great deal, and the signal the antenna is radiating is a >rare DX call that attracts anyone who can hear it! *************** What I especially admire about Force 12 is that they've engineered their antennas to give real-world results that are very close to the idealized results predicted by theory. The enhancements of a seaside location and the "DX effect" are frosting on the cake. If they really want record breaking contest results, they should use a rare DX call, a seaside location and female operators on SSB. The female voice seems to give you the same effect as another 6 dB in transmitting power. We have a "go for blood" contesting group here in Maine that deliberately schedules as many female operators as possible on Field Day to run up their score; it really works. Of course this requires YLs who do not mind insects and outdoor plumbing, and those are few and far between. *************** >It's no wonder that shipboard systems using the old 600 meter (about 400 - >500 kHz) marine band often logged large distances in spite of their tiny >antennas. A shipboard antenna might be 200 feet long, but at 450 kHz that's >hardly bigger than a mobile whip on 40 meters! The advantage they had was >the world's best ground system for both near and far fields surrounding the >ship in the middle of a salt water ocean. > >Ron AC7AC > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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O.T. meaning "Off Topic" or "Old Time"!
This so "Off Topic" I have to wonder if Wayne or Eric were born yet! It's "Old Time": Ham radio in the 1950's/60's (or thereabouts). This is an excellent video for those with an interest in what Ham radio was like back then for homebrewing and operating with special emphasis on public service. You'll want a broadband connection: it runs over 20 minutes. After a rather sloppy interview at the opening there is a beautifully-produced public relations piece on Amateur Radio in the Philadelphia, Pennsylvania area of the USA. http://video.google.com/videoplay?docid=2943570522939177086&hl=en I had one of the mobiles in the photos, except mine was mounted in my Studebaker Champion and I was in southern California, chatting with buddies on 10 meter A.M. as I drove to and from classes at San Bernardino Valley College and work at Lockheed Aircraft. Most Hams I knew were involved in RACES (Radio Amateur Civil Emergency Service) that's so well demonstrated in the film. The technology has changed. We don't need to fill up the car with gear to run mobile and today's Hamshack can actually fit on a small desk. The people have changed. Hams today are no longer using an arcane technology completely beyond the imagination of our neighbors. A "wireless" anything was magic stuff of science fiction stories back then, not something everyone carries around in their pocket today. What I hope never changes is the attitude of the Amateur community. Toward that end, I see a lot of that old attitude right here on the Elecraft reflector and among the Elecraft owners. Let the good times roll on... Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Chris Kantarjiev K6DBG
> As I'm sure you know, you can tell when the vertical antenna is
> performing better; the SWR goes up. A lossy vertical will have a low > SWR because the high ground losses are in series with the radiation > resistance and the sum comes out perversely close to 50 Ohms. A low > loss vertical is around 30 Ohms. Not exactly. It more be more accurate to say that "you can tell when a short vertical (or otherwise loaded) antenna is performing better because the low-SWR bandwidth (e.g., the range over which the SWR is less than 2:1) decreases." Yes, I'm nit-picking here, but I think it's important to be clear on this. With proper loading and matching, pretty much any usable antenna can have 1:1 SWR on one specific frequency. But short antennas have narrow usable bandwidth unless they are very inefficient. >> That's why a horizontal dipole is usually preferred to a vertical if there's sufficient space >> to erect it. > That's the gotcha. If you have a horizontal dipole at the same height > as the top of a vertical dipole, in the broadside direction the > horizontal wins hands down, provided you have two supports high > enough to support the dipole. The current loop of the horizontal is > twice as high as the current loop of the vertical. >> And, of course, those lucky Hams who can put their horizontal dipole up >> about 1/2 wavelength where it works best get a huge advantage. At 20 degrees >> it shows nearly 6 dB gain: equivalent to multiplying the transmitter power >> by four times! I work mostly "local" North American stations on 20 and below, but once the 'spots come back I'll see what DX I can do on 10. 10 is about the only band where my yard can realistically handle the otherwise-mythical "half-wave dipole half a wavelength high". My G5RV and 20 meter Moxon are both up about 15 feet. It's difficult to give meaningful figure for G5RV height, partly because it slopes down somewhat like an inverted-vee at both ends, but mostly because the yard itself is not level and there is a sharp discontinuity more or less under the feedpoint. I think that there hasn't been enough discussion (in general, not specifically on the Elecraft list) on the benefits of low beams over low simple antennas. Too many articles on beams start out with the assumption that you can put them up a 1/2 wavelength high. Given the increasing urbanization of America and the trend toward smaller yards, fewer and few hams are going to be able to do this in the future. > If they really want record breaking contest results, they should > use a rare DX call, a seaside location and female operators on > SSB. The female voice seems to give you the same effect as another > 6 dB in transmitting power. We have a "go for blood" contesting > group here in Maine that deliberately schedules as many female > operators as possible on Field Day to run up their score; it > really works. Of course this requires YLs who do not mind insects > and outdoor plumbing, and those are few and far between. <smile> Perhaps I should try that some time. Since I work almost all digital modes and CW rather than SSB, I don't get any advantage. :-) Cathy N5WVR _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Ron D'Eau Claire-2
Every time I try this I get an error message. Any thoughts? Chris G3SJJ
> http://video.google.com/videoplay?docid=2943570522939177086&hl=en > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Stephen W. Kercel
Good info Steve,
I wonder a couple of things though. First, why 8 degrees for the takeoff angle? That puts it low enough that you're into the ground effect suck-out zone fairly heavily. I'd have though something like 25 degrees would be a more representative angle for typical communications use. Also, what is the effect on the Force 12 antenna if it is between 1 foot and 28 feet off the ground? What if it is 15 feet off the ground (i.e. on my garage roof)? - Keith N1AS - - K2 5411.ssb.100 - -----Original Message----- From: Stephen W. Kercel The discussion of verticals has inspired me to do a bit of EZNEC modeling. I plotted the azimuthal pattern at an 8 degree takeoff angle for several different 80 meter configurations. In all 3 cases, I've assumed average ground. The first case is the classical full size vertical, with a quarter wave monopole element and 128 quarter wave radials. *snip* the pattern is an omni pattern with a signal strength at 8 degrees takeoff angle of -2.72 dBi. The second case is the Force 12 vertical dipole, with no radials. ... bottom of the antenna elevated 28 feet above the ground. ... The signal at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. ... an undetectable quarter dB worse than the ideal full size quarter wave configuration. How important is the mounting height? It matters. For the same configuration except with the bottom 1 foot above the ground, the signal strength at 8 degrees takeoff angle is -5.8 dBi. This is a quite noticeable >3db degradation from the full featured quarter wave configuration. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Keith:
I agree that typical DX propagation more commonly occurs at 25 degrees than 8. However, my situation is untypical. I'm trying to get single band 80M WAZ. The zones that I am missing, 23, 24, 26, 27, 28, and 30 have one thing in common. They are all very far from Maine. My VOACAP simulations of 80 meter openings to these zones indicates that propagation occurs in the range 6-10 degrees. Thus an antenna that gives superior performance at 8 degrees is the more promising for enabling contacts with rare zones. Some comparative results at 8 degrees on 80m are as follows: My current setup: -6.3 dBi (Straight dipole 46 feet up running east-west, with one end bent down to fit on my lot) Force 12 at 1 foot above ground: -5.8 dBi (A $600+ antenna mounted low is not noticeably better than what I already have.) Force 12 at 15 feet above ground: -3.72 dBi (Almost 3 dB better than I have now. Worth $600? Maybe) Force 12 at 28 feet above ground: -3.09 dBi (The structure needed to get that extra 0.6 db might be pretty pricey) QW Vertical with 128 radials: -2.72 dBi (Gold standard: Hideously expensive and only marginally better than the high mounted Force 12) Inverted L kinked to fit on my lot: -1.79 dBi ( Cheap, but beats the the gold standard by a dB) Two inverted Ls fed out of phase, configured to fit on my lot: -0.15 dBi I can build two inverted L dipoles for a lot less than the $600+ that a Force 12 costs and get 6dB better (towards the Far East) than what I have now. Providentially, I have tall trees in just the right places. Admittedly, my solution would not work for everybody. 73, Steve AA4AK At 02:06 PM 3/5/2007, Darwin, Keith wrote: >Good info Steve, > >I wonder a couple of things though. First, why 8 degrees for the >takeoff angle? That puts it low enough that you're into the ground >effect suck-out zone fairly heavily. I'd have though something like 25 >degrees would be a more representative angle for typical communications >use. > >Also, what is the effect on the Force 12 antenna if it is between 1 foot >and 28 feet off the ground? What if it is 15 feet off the ground (i.e. >on my garage roof)? > >- Keith N1AS - >- K2 5411.ssb.100 - > >-----Original Message----- >From: Stephen W. Kercel > >The discussion of verticals has inspired me to do a bit of EZNEC >modeling. I plotted the azimuthal pattern at an 8 degree takeoff angle >for several different 80 meter configurations. In all 3 cases, I've >assumed average ground. > >The first case is the classical full size vertical, with a quarter wave >monopole element and 128 quarter wave radials. *snip* the pattern is an >omni pattern with a signal strength at 8 degrees takeoff angle of -2.72 >dBi. > >The second case is the Force 12 vertical dipole, with no radials. ... >bottom of the antenna elevated 28 feet above the ground. ... The signal >at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. ... an >undetectable quarter dB worse than the ideal full size quarter wave >configuration. > >How important is the mounting height? It matters. For the same >configuration except with the bottom 1 foot above the ground, the signal >strength at 8 degrees takeoff angle is -5.8 dBi. This is a quite >noticeable >3db degradation from the full featured quarter wave >configuration. > >_______________________________________________ >Elecraft mailing list >Post to: [hidden email] >You must be a subscriber to post to the list. >Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > >Help: http://mailman.qth.net/subscribers.htm >Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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