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#211
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Ian White, G3SEK wrote:
I don't know what the full detailed explanation is. But I do know that, in order to be correct for all possible cases, it *must* include the feature that as the physical dimensions of the coil tend towards zero, the difference in current between its two ends falls towards zero also. Any "explanation" that denies this fundamental physical fact is guar-an-teed to be wrong. So let's agree on that, and then we can move forward to find an explanation. OK, let's agree on that and take another look at the example I just presented on another thread involving half a dipole. wire coil wire ---------------////--------------- 22.5 deg ? deg 22.5 deg Assuming a lumped inductance, presumably the current into the coil will have the same phase as the current out of the coil. Zero phase shift through the coil means this antenna has the same current phasing as a 40m dipole used on 75m, i.e. the feedpoint current is at a current minimum point. We know that is NOT the case in reality. So what's the answer? -- 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! =----- |
#212
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G3SEK wrote:
But first we all need to agree that for an ideal lumped loading coil, the current at the top and bottom terminals will be the same. If the current is not the same at both ends, then the coil cannot be behaving as an ideal lumped inductor. We are back to this line of "reasoning": ......and if we rip all four legs off the frog and say "Frog jump", frog doesn't jump. The "conclusion" is - Froggie is deaf :-) Is it a big science secret that coils can cause delay, phase shift? Can anyone also measure the fricken thing and THEN argue???? We have the effect, W9UCW measured it, ON4UN and W5DXP provided explanations, what's missing? Being wrong, recognizing it and admitting? (Sorry) Just because software (for now?) cannot digest it, it can't be? This is becoming amusing to see how serious this misconception was out there and who is on the bandwagon. Let's see "better" explanation. The difference is THERE doesn't matter how anyone denies it!!! Knowing about it will help to design and optimize the crippled antennas, including fricken fracktals (with coils). I am gathering material for test setup allowing to measure RF current in every foot of the loaded antenna, I have two 8 amp meters, so I will have to use some power for full deflection, but it will be just another level of the same thing. You are all invited to witness! I will document it, take pictures and video. If there are no answers, pointing wrong to the points I have raised in my other posting, then I am done, can't do anything more, just will do the measurements and present the results. Yuri, K3BU.us |
#213
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Cecil Moore wrote:
OK, let's agree on that and take another look at the example I just presented on another thread involving half a dipole. wire coil wire ---------------////--------------- 22.5 deg ? deg 22.5 deg Assuming a lumped inductance, presumably the current into the coil will have the same phase as the current out of the coil. Zero phase shift through the coil means this antenna has the same current phasing as a 40m dipole used on 75m, i.e. the feedpoint current is at a current minimum point. Sorry, I don't understand that last statement... -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
#214
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Ian White, G3SEK wrote:
Cecil Moore wrote: OK, let's agree on that and take another look at the example I just presented on another thread involving half a dipole. wire coil wire ---------------////--------------- 22.5 deg ? deg 22.5 deg Assuming a lumped inductance, presumably the current into the coil will have the same phase as the current out of the coil. Zero phase shift through the coil means this antenna has the same current phasing as a 40m dipole used on 75m, i.e. the feedpoint current is at a current minimum point. Sorry, I don't understand that last statement... A 40m 1/4WL vertical used on 40m is 90 degrees long. A 40m 1/4WL vertical used on 80m is 45 degrees long and we know its characteristics. If the coil above has zero phase shift, the antenna above is also 45 degrees long and will exhibit the feedpoint characteristics of a 40m 1/4WL vertical used on 80m but we know it doesn't exhibit those characteristics. ERGO, the phase shift through the coil is not zero. -- 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! =----- |
#215
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"Ian White, G3SEK" wrote in message ...
Yuri wrote: I am just amazed that with all the "theoretical" arguing going on, why none of the "learned" experts measure, answer or explain the following REAL effects or show where I (we) are wrong: [...] Yuri is making some fair points - those practical observations do need to be explained. But first we all need to agree that for an ideal lumped loading coil, the current at the top and bottom terminals will be the same. If the current is not the same at both ends, then the coil cannot be behaving as an ideal lumped inductor. I believe that Tom took this stance early on when he explained variances of slight current change by virtue of the capacitive component of the real world inductor when wound on a scale that is used on a vertical radiator. I suspect that if he brought Q into the equation /discussion people would not of gotten off track so quickly in the first place! People have become so enamourd with modeling technics they are willing to let their guard down and accept what their results show and throw caution to the wind. I think I will follow Roy and get out of this one Art Now the vainess of man does not allow one to admit error, so the hole digging continues with faces pointed down despite pleas from the faces above Art That tells us that the explanation has to involve the non-zero physical dimensions of the coil. In other words, the coil is no longer just a pure inductor - it also has some antenna-like distributed properties, which do allow (and indeed require) a current variation along the length. I don't know what the full detailed explanation is. But I do know that, in order to be correct for all possible cases, it *must* include the feature that as the physical dimensions of the coil tend towards zero, the difference in current between its two ends falls towards zero also. Any "explanation" that denies this fundamental physical fact is guar-an-teed to be wrong. So let's agree on that, and then we can move forward to find an explanation. |
#216
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Cecil Moore wrote:
Ian White, G3SEK wrote: Cecil Moore wrote: OK, let's agree on that and take another look at the example I just presented on another thread involving half a dipole. wire coil wire ---------------////--------------- 22.5 deg ? deg 22.5 deg Assuming a lumped inductance, presumably the current into the coil will have the same phase as the current out of the coil. Zero phase shift through the coil means this antenna has the same current phasing as a 40m dipole used on 75m, i.e. the feedpoint current is at a current minimum point. Sorry, I don't understand that last statement... A 40m 1/4WL vertical used on 40m is 90 degrees long. A 40m 1/4WL vertical used on 80m is 45 degrees long and we know its characteristics. If the coil above has zero phase shift, the antenna above is also 45 degrees long and will exhibit the feedpoint characteristics of a 40m 1/4WL vertical used on 80m but we know it doesn't exhibit those characteristics. ERGO, the phase shift through the coil is not zero. OK, I see what you're getting at, but by changing the frequency you are losing sight of some important points. Let's stay on the *same* frequency, halve the physical height of the antenna and insert a lumped loading coil at the midpoint. Instead of being 90deg tall, our vertical monopole is now only 45deg tall (physically). Drawn on its side, and fed against ground at one end, it now looks like: wire coil wire ---------------////--------------- 22.5 deg ? deg 22.5 deg (just as you drew it) Where you're going astray (I suspect) is in believing that the coil literally "replaces" the 45deg of antenna that was lost when we halved the height. It doesn't - the loading coil drastically changes the current distribution. Fed with 1.0A at the base, the original quarter-wave has a roughly cosine-shaped current distribution, so the current at a point 22.5deg from the top is 1A * cos(90-22.5) = 0.38A. Now feed the loaded antenna with 1.0A at the base. The current distribution is now radically different: in the bottom 22.5deg of the antenna, the current hardly changes - let's say it's 0.9A at the bottom of the loading coil. Since we have assumed a lumped loading coil, the current at the top of the coil is the same 0.9A. But now the current distribution in the top 22.5deg is very different from the full-size case: it tapers much more sharply, from 0.9A down to zero at the very top. This is all standard stuff. My reason for walking through it is to emphasize that shortening the antenna and loading it changes many things about the current distribution, both above and below the loading coil. And here are two other important differences: the feed impedance of the loaded antenna is much lower than that of the full-size; and the much sharper reduction in current is associated with a much higher E-field over the same length of top section. To sum up, shortening and loading the antenna creates so many important differences that you're misleading *yourself* if you say that the loading coil simply "replaces" the missing length of antenna. Two footnotes: 1. The diagram for current distribution with center loading in 'Low Band DXing' is based on the same incorrect assumption that loading coil somehow fully "replaces" the missing 45deg of antenna. (Fortunately the method later in the same chapter for calculating the inductance of loading coils is still OK, because it doesn't depend on any assumptions about current distribution.) 2. I haven't thought about an answer to Cecil's problems of what happens to the "missing" 45deg, and what happens to the forward and reflected waves of voltage and current. Since it's Cecil who chooses to think about antennas in such ways, he'll have to solve his own problems! My only point is that a correct solution can *not* involve a difference in the currents at the two ends of an idealized lumped inductor. Such a difference simply cannot be... so the true solution will be that bit harder for Cecil to find. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
#217
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#218
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Cecil Moore wrote in message ...
Mark Keith wrote: This is the way I view it, and why I couldn't automatically endorse the fairly large difference they saw. Unless the coil is very long, say as in a helical whip, I see it acting pretty much as a lumped inductor. Sure, the current may vary some through the coil when it's a foot or so long bugcatcher coil, but I don't see it being a major issue. I wouldn't normally expect to see a sharp current taper across the coil. I see that type of coil acting much as a "one piece" lumped inductor, not as a many turned rf roller coaster ride. Not a perfect lumped inductor, but close enuff for average gov work. MK The round trip current phase shift in an electrical 1/4WL vertical must total 360 degrees. If the coil doesn't perform part of that phase shift, what does? What if it does though? If the coil is still a fairly small portion of the overall length, I don't see the change as severe. Heck, if current is supposed to be the same on each end of the coil, it should be the same going in both directions.. Seems to me it would pretty much equal out in the overall scheme of things. Even if one ends up with a bit more current, being the coil is not that large overall, the difference should not be drastic. Or to my thinking anyway..I think it's quite possible a coil mounted higher than 1/2 the total length shows a sharper cutoff of current at it's end. To me , it's because of the shorter overall length of the stinger above the coil. The current taper at the top of the coil and above is more abrupt than if the coil were center loaded. Or thats the way I see it. Whether it's right or wrong remains to be seen... MK |
#219
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However, we get into issues of your having done work at the 100mA levels and we thus turn to my earlier comments about accuracy. 100mA on an 8 Ampere full scale 3.5 inch meter is slightly more than 1% deflection (less than the width of the needle). Richard, you got the "wires crossed" - mixing two cases. W9UCW measured data I have been quoting in my article at K3BU.us, he SET the power for the bottom meter to read full scale on his 100 mA meter. Then he read the top meter which showed readings in the 40 to 60 % down. I have not done my measurements besides "hand test" and frying the Hustler coils. Just brief test to see how much deflection I get with 100W on 8A meter. I know a thing or two about measurements, done my years at IBM Test Engineering Dept. I will document my tribulations and if get it, use infrared camera too. If W9UCW lied, deceived us I will be the first one to choke him :-) So far he put figures on what I knew and they jive. Why don't you guys that know pitfalls of measurement do it and report ????? Hello???? Yuri. K3BU.us I am not as dumb as Reg portrays me. Got highest IBM Award - Outstanding Contribution Award signed by then chairman Frank Cary for Design and Development Excellence. BTW this was doing something that "experts" at IBM Endicott lab said it couldn't be done. This coil stuff is trivial in comparison. |
#220
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"Ian White, G3SEK" wrote: Since we have assumed a lumped loading coil, the current at the top of the coil is the same 0.9A. But now the current distribution in the top 22.5deg is very different from the full-size case: it tapers much more sharply, from 0.9A down to zero at the very top. Right. But the discussion is about whether the lumped loading coil model is accurate. Any conclusions based upon inaccurate assumptions would probably also be inaccurate. 73, Jim AC6XG |
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