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But this leads me
back to my previous post, which no one has commented on yet. To me, the upper meter is actually measuring a point above the coil. But it hard to tell from the pix exactly where the coupler is mounted. The main question I would like answered is, would the presence of the capacitive whip above the coil effect the current reading you got, being you seem to be measuring slightly above the coil? To me, it seems it would being you are not measuring inside the coil windings themselves, but slightly above the coil. If this capacitance is the cause of the decreased reading, then yes, it would be totally normal to see the same results if you flipped the coil and meters. MK There is no coupler involved. Meter is inserted between the end of the coil and remaining mast or whip. Thermocouple meters have negligible insertion effect, they act as perhaps an inch of wire inserted in the circuit, which is easily compensated for by retuning either the antenna or moving the frequency. Their meter mechanism is virtually immune to any RF field distortion. They are specially designed to measure the RF current with minimum impact on the measured circuit and to be interfered with. If you can grab one at the flea market get it! You could measure current on each turn if you managed to cut it and insert the ammeter. It would show cosine curve decrease across the coil. Measuring it at the first turn, end of the coil or inch or two above or below the coil produces virtually the same results, difference in the current there is really minuscule. Another close way of measuring the current is to fashion the current probe/coupler made of (split) ferrite ring, have few turns of pickup wire, rectifier and small meter. (There is a description on one of G something web pages.) You could slide this contraption up and down the radiator and measure the current. Of course you have to back off and not to touch anything in vicinity, otherwise you will detune the antenna setup and get erroneous results. The most accurate and practical way is the way W9UCW did it, he read the meters with binoculars from the distance. Yet another way is to use thermal effect, use thermal strips, paste it along the coil, put some power to it and watch the colors change. Not terribly accurate, but proof that meters do not disturb the circuit or distort the measurements. You can't use probes, scopes or anything with wires attached to it, it detunes the antenna and gives useless results Yuri, K3BU.us |
Mark Keith wrote:
How much though? What would be an average ratio difference you would be likely to see on a 8 ft center loaded whip? A lot on 75m. Not much on 12m. Or lets go one better...What would be a likely "worse case" scenario? The worse case I can think of is a short center-loaded whip on 160m. :-) The coil is almost all of the necessary 1/4WL. Will this vary from antenna to antenna? I would think so. Of course. It is all capable of being calculated. Is it your opinion that the modeling we now see with these antennas and coils is quite flawed? The antenna current reported by EZNEC is inaccurate because of simplified assumptions. EZNEC assumes that the current doesn't change through the single point inductive load. Therefore, EZNEC cannot be used to prove that the current doesn't change. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Yuri Blanarovich wrote:
in other words, the highest current point on the structure is at the inductor. That's what W8JI calculated in EZnec, does it make sense? Like 2+2 is 4.5? Why would inductor "suck" the current up? We should then use "those" inductors to suck the current all the way to the top of the whip - perfect antenna? Cecil, can you 'splain that? Again, the current can either stay the same, increase, or decrease through an inductor depending upon where it is located. Has that statement sunk in on anyone? If you install a coil 1/8WL up on a 1/2WL vertical, the current through the coil will *INCREASE*. If you install it in the center, the current magnitude will be the same in and out of the coil and opposite in phase. If you install it 1/8WL from the top, the current will decrease through the coil like it does on a 1/4WL mobile antenna. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Again, the current can either stay the same, increase, or decrease through an inductor depending upon where it is located. Has that statement sunk in on anyone? Yes, to be more precise, we are actualy arguing about the case of resonant quarter wave vertical, as a typical mobile antenna. Other losses, such as ground conditions, poor contacts, color of eyes are not considered here. Yuri |
Of course it doesn't account for phase shifts of current, since there
aren't any. It does account for voltage phase shift. It uses the same equations I learned in freshman circuits class. Perhaps they taught those same equations in Texas, too, but I can't be sure. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: I haven't had the time to participate in this, but in a brief look, it looks pretty silly. Of course EZNEC shows no current difference across a load. The EZNEC model of a load has zero length, so the current at both terminals has to be the same. It appears that EZNEC also doesn't account for phase shifts across a zero length coil. |
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Roy Lewallen wrote:
Of course it doesn't account for phase shifts of current, since there aren't any. It does account for voltage phase shift. It uses the same equations I learned in freshman circuits class. Perhaps they taught those same equations in Texas, too, but I can't be sure. Roy, We are talking about distributed networks. Of course, there is a phase shift in the current as well as the voltage. You and W8JI seem to be using lumped circuit analysis when you should be using distributed network analysis. The center loading coil for a 75m mobile antenna is an appreciable percentage of an electrical wavelength so you cannot use your lumped circuit analysis without introducing errors. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Mark,
This term of reactance "sucking" or "drawing" current up the antenna, I have never heard of reactances described like this before. Has an article been written lately with these terms since I saw another poster use the same terminology? Regards Art (Mark Keith) wrote in message . com... oSaddam (Yuri Blanarovich) wrote in message ... NM5K: NO , I didn't see the pictures. Like I said, they didn't load on that site. All the pix load except his. Then you are missing a lot. I don't know what the problem with eHam.net site is, I uploaded all the pictures the same way, some showed, some not. This is why I posted link to my page http://www.k3bu.us/loadingcoils.htm which has the pictures, also RIGHT drawings from ON4UN book, latest modeling of W5DXP stub loaded G5RV and some selected comments explaining the phenomena. Check it out. I found them earlier... Again, put on the fricken Hustler 80m resonator, feed it 100W and feel it! No meters, no hokus pokus, just "naked" antenna. What would that mean? The hustler coil is known to be flawed, and makes a poor coil to be tested in this manner. The effects of it's metal end caps, and overall design problems are fairly well known. I refuse to use such a lousy coil. You can feed my homebrew coil 100w all day long, and you won't feel anything except the finger on the mike button hand start to go numb from holding the key down. Another "very well known fact" from W8JI's teachings? No, just personal experiences. But I never had a heat issue with a hustler. I never used one long enough to develop any heat...:/ Metal caps are at both ends, top would feel the same then, it doesn't. Hustler coil is "lousy" because uses aluminum wire to achieve "match" through some loss and that is additional loss. Well that also. But I think the windings are also overly tight. But from the point of view as inductor, it is uniformly wound solenoid, same wire and diameter, so according to I2R law, the heat developed in it is proportional to the square of current. If there is more heat at the bottom, then irrefutably there is more current flowing at the bottom. Well, not irrefutably. Obviously the other guy that tested had a case that was excess resistence at the base of the coil. I'm not saying this was a problem in your case, but he seemed to have no more heating issues after he fixed the connection. Here is the shocker: When W9UCW bunch was measuring various coils, they also compared that perfect coil as you and I have (heavy wire, proper form factor, good connections) to "lousy" Webster Banspanner sliding coil (aka cheap screwdriver) they found negligible difference in measured signal strength. They rechecked everything scratched their heads, but that was it. Actually, thats fairly old news. I had heard of that test months ago. Maybe from more than one person. I think others have found the same thing. And it doesn't surprise me one bit when mobiles are involved. Ground losses almost always overshadow coil losses on a mobile. So another myth about the quality of coil (resistance of course applies, but is minuscule) importance. Well, it's not totally a myth. Coil losses can eat your lunch if you want first rate performance. The hustler coils are a case in point. They are pathetic compared to my coils. The difference on the air is like night and day. But! If you have so much ground loss that it overshadows the coil loss, even in that case, a better coil won't help much. In other words, the better the ground system, the more any coil deficiencies will show up. Now when we look at it from the point of view of effect of the coil on the efficiency of antenna, it is explanatory. Coil replaces portion of radiator that is not there anymore, so the significance of its quality is not as important as the position of the coil on the radiator (area under current curve). Of course the ohmic losses are a factor, but that is minuscule (ohms or two) versus reduction in current flowing in remaining radiator. I like and have big fat coils, but looks like they can be optimized better, perhaps heavier wire in first few turns, slimmer construction, less wind load, but placed higher up on the mast. So there is another one you don't have to believe, I sure was surprised. Measure it!!! I have. I've been building different homemade mobile coils for about 13 years. I've come to the conclusion wire size is generally not very important at all, AS LONG as the proper pitch/winding ratio is used. Don't wind them too close together. My current coil uses thinner wire than my old one. I much prefer a lighter coil. But I'm fairly sure it actually works as well if not better than the heavier old one. It is wider than the old one. "3 inch vs 2 inch" I am not a salesman, I will not try to convince you of anything. I can elaborate and answer some questions and it is up to you to believe it or not. You can believe engineers with education, their experience and results, or you can believe some "technical impostor" as K7GCO phrased it. I'm not believing anyone. I'm going by my own experiences, and my own gut hunches. "BS filter" :) W8JI has very little to do with any of my ideas. His was just one viewpoint out of many. You could see almost constant current across the coil if the coil is at the base of quarter wave radiator, has heavy windings and is replacing relatively short electrical length of the radiator. Did he mention what coil, where was the coil placed? We have methods and pictures of W9UCW tests on various bands at different positions, we have yet to get objections or pointed out errors in his setup. I think a lot of that is only a few lurking on there actually know how, or have the equipment to make an accurate measurement. So most wouldn't know if the setup could cause problems or not. I don't say this lightly, as it appears to be quite a "hook" prone undertaking. I sure don't have the setup to do it, or have even tried something like that. I do know from messing around with fluorescent tubes and the mobile antennas, that the electric field around the coil seems to be very steady across, and abruptly decreases once the stinger begins. This while quite possibly an erroneous assumption, led me to believe the current across the coil is also fairly constant in direct relation to that. You believe what you want. As I mentioned we will write concise article on the subject and you can take it from there or stick with Rauchians. Why do you keep involving Tom in this? I have nothing to do with him. Frankly, I find comments like "Raunchians" and "Flat Earthers" etc, kind of tacky. So far I haven't accused you all of any "voodoo" antenna magic, like I do say the EH bunch. I just want to make sure all the bases are covered as far as the accuracy of the measurments, and also the exact locations of the couplers. I still question the accuracy of hooking the top coupler on the lower end of the stinger. No matter how close the coil is.... I sure enjoyed this exercise, learned from it a thing or two and I am looking forward to implement some of the stuff (measure it too) in the design of new loaded mobile and low band antennas. As they say on FreeRepublic.com, this is series and hugh :-) There is no doubt that elevating the coil on a low band antenna improves the current distribution. I've tested it many times, over and over again. BUT! This will not model worth a hoot if you look at gain numbers alone. Note Wes's recent post. If I remember right, most programs will leave the gain pretty much the same, as you vary coil height. They sure don't show the real world increases anyway...But you want to include top loading if you want the best of the best. Omit that, and it'll never happen. BTW, to my thinking, the capacitance is what draws the current through the coil. And I haven't tested it, but I bet the current taper above the coil is much more abrupt with a short stinger, than a longer one. There is no doubt a short stinger does a poor job of drawing current through the coil. Thats what kills the average ham sticks with the short stubby stingers. There is little capacitance to draw current through the coil. A large enough top hat will give you a fairly steady current distribution up the whip, regardless of the coil placement. Raising the coil in addition to the hat helps a bit more. But I think less so than with a non hatted antenna. Coil placement is critical if you run the low bands, and don't use a top hat. But thats nothing really new to people that fart with mobile antennas all the time. I think what you are doing is a good thing, and worthwhile. But I'd chill as far as Tom is concerned. You seem to be taking his criticisms a bit personal, and respond in kind. This all distracts greatly from the original point of the excercise. It's like the parasitic thing with the amps between him and Measures. Even after all the bickering, I'm still not totally sure what to believe as far as parasitic bangs in tube amps. The best answers seemed to get bogged down in the quagmire of constant bickering. :/ If I'm doing something I *think* is right, and someone disagrees, I'll assess it, and if I still think I'm right, I just ignore them. I think that would be your best course. It's his right to disagree with you. Nothing you can really do about it, and no point wasting energy, or stirring discontent over it. MK |
Roy Lewallen wrote:
Of course it doesn't account for phase shifts of current, since there aren't any. You seem to be disagreeing with John Devoldere's "Bible" - "ON4UN's Low Band DXing", 3rd Edition, on page 9-34 at: http://www.k3bu.us/loadingcoils.htm A little thought should prove there is a current phase shift (delay) through the coil. Let's look at an 8 foot long center-loaded mobile antenna for 75m. The 4 feet below the coil gives a phase shift of about 5 degrees. Assume zero phase shift through the coil. The 4 feet above the coil gives a phase shift of another 5 degrees for a total of 10 degrees at the end reflection point. It's an open circuit, so a 180 degree phase shift takes place. That puts the reflected current at 190 degrees. Add the 10 degrees coming back and we see the reflected current arrives mostly out of phase with the forward current at the feedpoint. Since the feedpoint impedance is known to be around 15 ohms, these superposed currents cannot possibly be out of phase and must necessarily be in phase. The phase shift (delay) of the current simply cannot be the same with and without the coil. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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