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#21
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On Sep 27, 2:27?pm, AF6AY wrote:
If you are satisfied with your particular method of getting RF out of the transmitter and into some antenna, fine. Satisfaction is all part of the game. Such satisfaction is not the example to set for all. It seems to me that every- one's location is different and each presents a unique problem to solve for the more-optimum EM wavefront launch direction in that location. Anyone who says that one kind of antenna is the "best" or one should "always" use a certain kind of balanced transmission line isn't looking at the whole picture. They are probably describing just the only (or a few) antenna installations they used. Yes, some antennas "work better" than others. In a particular location. All that is certainly true. But I don't see anyone saying that one kind of antenna is "best" for all locations, or that one should "always" use a certain kind of transmission line in all applications. It's also not the whole story, either, because what must also be considered is the sort of radio operation that is being considered. Does the ham want only DX, or regional/national QSOs? Several bands, or only one or two? Will operation be confined to one part of a band, or spread out over the entire band? What time of day will most operating take place? Will there be ragchewing, net operations, contesting? All that and more have an effect on what the 'best' antenna is for a given location. For someone just starting out, I would suggest just a vertical for HF. Depending on a whole bunch of factors, that could be good advice, or very bad advice. It is the least obtrusive to neighbors Not always. It depends on the location. A wire antenna can be much less noticeable than an HF vertical in many situations. (can be described as a "flagpole") Perhaps, but I don't think anyone who has seen a typical manufactured amateur HF trap vertical would consider "flagpole" an accurate description. and most will perform adequately (to launch an EM wavefront) with a few radials for the "ground." Maybe - and maybe not. The performance of an HF vertical is dependent on many factors, such as the ground system, objects in the near field, how much loading is used to obtain resonance, ground losses in the Fresnel zone, etc. No, it won't win awards or work DX "better" than Brand Y using Brand T transmission line, but it WILL radiate adquately...and that's the whole name of the game, ain't it? :-) It may not radiate adequately. For example, on the lower HF bands such as 80/75 and 40 meters, the dimensions of a full-size quarter-wave vertical and radials may become impractical (60+ feet on 80/75, 30+ feet on 40 meters). Most trap vertical designs use a considerable amount of inductive loading on those bands, reducing the efficiency and radiation resistance as well as the SWR bandwidth. The lack of high-angle radiation from such a vertical may make it almost useless for daytime and closer-than-DX-but-farther-than-local communication on those bands. An amateur located in a valley, such as the one who started this thread, might prefer useful radiation that leaves the antenna at angles that would leave the valley. At this point in the sunspot cycle, the amateur bands above 11 MHz are often useless for ionospheric propagation much of the time, particularly during darkness hours. Having an effective antenna for the lower HF bands can be the difference between making QSOs and not making them. There's also the cost factor. Yes, "everyone's location is different and each presents a unique problem to solve". Which means that recommending a vertical antenna to someone just starting out could be very bad advice unless a lot more information was gathered first. And if Brand Y using Brand T transmission line works better, why not use it? IMHO, the "whole name of the game" is useful radio communication. IOW, making QSOs. I have seen situations where it was good advice to tell a ham starting out on HF to put up a vertical. I have also seen situations where that would be very bad advice. Same for dipoles of various kinds, loops, random wires, etc. 73 de Jim, N2EY |
#22
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On Sep 28, 9:29?am, Michael Coslo wrote:
wrote: On Sep 27, 10:53?am, Michael Coslo wrote: Why not, [use a 1/4 wave dipole] other than the fact that such a short dipole will present a feedpoint impedance that has a low resistive part and a high reactive part? That's a pretty good reason, though. 1/4 wavelength dipoles - make sure to read that as total antenna length, not 1/4 wavelength per leg, are just a troublesome combination that a lot of tuners can't tune very well. I'm not convinced. Depending on the transmission line impedance and length, the shack-end Z could be within the matching range of the Transmatch. I don't consider the G5RV and OCF to be 'trick' antennas. They're simply intelligent combinations of dipole and feed systems that have been worked out to present reasonable impedances so that line losses and Transmatch requirements are reasonable. They are very clever. The OCF especially is a joyful playground to work on with antenna design programs. It is a great way to learn both the programs and antenna theory. Agreed. But so are other implementations such as trap dipoles and the classic dipole-with-ladder-line feed, where you try different dipole and transmission-line lengths. Often the idea of "low SWR" is put out as if it is the sole criteria. Too often. In defense of SWR lovers, modern Rigs really hate High SWR, especially reactance of the capacitive kind. Well, that depends on how you define 'modern'... But a 50 ohm resistor has 1.1:1 VSWR, and some manufacturers have taken advantage of that sort of thing in the past. Not just in the past. Google "Maxx-comm matcher" (try different spellings). Those folks are still in business. The problem as I see it is that most new folks these days start out with an "all band" radio, and are inclined to want a antenna that is likewise all band. Which may or may not be a good idea. If the only bands that are open when you have time to operate are ones where your antenna works poorly, the result is going to be frustration. A lot of Western and Eastern EU, Great Britain, Iceland, Norway, most of South/Central America, about half of Africa, Israel, Australia, Antarctica, and a couple others. On which bands? No JA or far eastern countries, but I haven't tried really hard, I just work 'em if I happen to hear them. Of course from EPA, VK-land is about the antipodes. I once accidentally worked a fair part of a contest once on 75 meters on probably around 3 watts, mostly into California - I had tuned the antenna, and forgot to turn the power back up. I had works around 25 QSO's before catching that one. Situational awareness, that's all. With my rig, the power level is pretty obvious. Of course, that isn't quantified data, it's just anecdotal. But running at QRP levels does make for a more stern test of an antenna's abilities, especially if there isn't obvious signs of it, such as not getting calls answered. Sold me on the thing. The Ultimate Test is "what have you worked on it?" Theory is great but the real proof is in the QSOs. The technical details are that it is a 96 foot total length dipole, up around 55 feet, the center support is a short length of pvc tubing. I suspect that the 96 foot length was decided because that's what would fit in the available space. The ladder line is soldered to the respective dipole wire. Ladder line makes an almost straight drop to the Shack window. Definitely not the best thing going, but not too bad. Actually, what you describe is pretty close to optimum for a simple multiband antenna system in limited space, which I suspect is the main issue. Are you using true ladder line, or "window line" (Twin Lead with holes punched in the insulation)? True ladder line (heavy wire, wide spacing, mostly air insulation) has lower loss and less weather effects. If the line is short there's not much difference, but as frequency and line length increaseit can be worth changing out. Depending on the shack-end impedances, different Transmatches can have more or less loss. The worst-case scenario is where a 4:1 balun is used with a shack-end impedance that has low resistive and high reactive values. The poor Transmatch has to try to deal with one quarter of the resistive part! 73 de Jim, N2EY |
#23
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#24
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#25
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"Michael Coslo" wrote
A few years back I did a modeling of an antenna that was 1/4 wavelength long at I think 40 meters. The SWR of the antenna was approaching infinite. If I get the chance, I'll model it again tonight - I'm doing the mass mailing for the PAQSO party tonight, and if all goes well, I should have a little time. I'd think a strict 1/4 wave, regardless how it's fed, would be pretty horrific on that specific band. (But I couldn't explain why...it's just from what I've read.) That's why I made mine (55 feet) so that it was under 1/4 wave for 40, and more than 1/4 for all the higher bands. As for window line being affected by water... yes, it is, but I never found it a big deal. As I recall, I just retuned some of the transmatch settings to accommodate. I used both 450 and 300 ohm window line... and even tv twinlead. What fun I had one night when I heard a cat playing on my flat roof, and I could tell he was playing with the transmission line that was suspended about a foot off the roof... I transmitted 100 watts and heard him take off like a shot! BTW, I wonder what happened to the OP? Did we drive him away? ;-) Howard N7SO |
#26
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On Oct 2, 3:20?pm, Michael Coslo wrote:
wrote: On Sep 28, 9:29?am, Michael Coslo wrote: Depending on the transmission line impedance and length, the shack-end Z could be within the matching range of the Transmatch. A few years back I did a modeling of an antenna that was 1/4 wavelength long at I think 40 meters. The SWR of the antenna was approaching infinite. ??? With reference to what sort of feedline? SWR only has meaning wrt a particular line impedance. If I get the chance, I'll model it again tonight - TNX In defense of SWR lovers, modern Rigs really hate High SWR, especially reactance of the capacitive kind. Well, that depends on how you define 'modern'... Non-tube? Don't get me wrong, I LOVE tube rigs. There have been non-tube ham rigs for a couple decades now.... In a way, we have come full circle. Look back 50-odd years, and most ham rigs could match an SWR of 3 or 4 to 1 without a tuner, because they had fairly wide-range pi-networks. That was sacrificed to the desire for miniaturization, but even as late as the early 1980s, there were rigs with tube finals that could match 2:1 SWR or so. Meanwhile the SS rigs wanted 50 + j0 loads for their no-tune finals, so Transmatches became very common - so common, that rigs began to offer them built-in, so the rig could match SWR of 3 or 4 to 1. Only difference was automation. The more things change.... Google "Maxx-comm matcher" (try different spellings). Those folks are still in business. That's the one I was referring to. Hard to imagine they are still doing business. The ARRL Product Review where they tested one, then X-rayed it and opened it up to show the resistors was well over 20 years ago. The fact of the matter is that they *do* work - just not very well. Mostly 20 and 40. Just a few of the near out of country neighbors on 80. WInter nights are coming. That's when 80 and 40 really come into their own. I suspect that the 96 foot length was decided because that's what would fit in the available space. Yup, I tried a few dipoles that were longer, and made a Z shape. They worked okay, but were a lot more maintenance. Lots of tree whipping in storms here, and I almost made plastic pully's into the end insulators to combat the extra movement. But I went back to the 96 foot length IMHO, amateur radio antenna design is no more than 10% electrical engineering and no less than 90% mechanical engineering. In many cases it's 5%/95%. Actually, what you describe is pretty close to optimum for a simple multiband antenna system in limited space, which I suspect is the main issue. I agree. What really surprised me was that the performance on 75 meters was pretty acceptable. Not really a surprise to me. While short, the 96 footer and lowloss feed system will put significant RF in usable directions. I mostly get down there during contests, and have had nice results. Worked enough people to make it worthwhile. CW SS is a month away.... Are you using true ladder line, or "window line" (Twin Lead with holes punched in the insulation)? True ladder line (heavy wire, wide spacing, mostly air insulation) has lower loss and less weather effects. If the line is short there's not much difference, but as frequency and line length increase it can be worth changing out. I use the window line. I have heard of the advantages of the true ladder line, and certainly the higher impedance is one of them. Higher impedance in and of itself doesn't make the difference. What matters is the lower loss due to more copper and less dielectric. The ocarc transmission line loss calculator does balanced lines as well as coax. I've heard of some of the drawbacks of window line, such as it's performance when wet. I did take issue with the test method cited by many, in which the window line was dunked in water that included a wetting agent. My contention is that the experiment showed the effects on window line with wetted line. My experience has been that window line does not wet in this manner. When the experimenter has to add a chemical to coat the line with water, it is altering the conditions and producing results germane to only those conditions. I agree. A spray with the garden hose will adequately simulate a rainy day, I think. I think your biggest possible improvement would be to see how lossy your tuner/transmission line combo really is, and improve it if possible. 73 de Jim, N2EY |
#27
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