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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Stephen W. Kercel Sent: Saturday, March 03, 2007 11:28 AM To: elecraft@mailman.qth.net 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: Elecraft@mailman.qth.net 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: Elecraft@mailman.qth.net 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
