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Current in antenna loading coils controversy
Howdy perpetual arguers!
Here is the good one for you on eHam.net http://www.eham.net/articles/6512 between yours truly (right) and Tom, W8JI (wrong). It centers around the argument that the RF current in the antenna loading coils is (or not) equal. Classic case of "theory" (wrongly applied) to real life measurements and facts. There is referenced previous article that started it all and on my budding web site www.K3BU.us full text with pictures at http://www.k3bu.us/loadingcoils.htm It looks like this factor is not properly accommodated in modeling programs using loading inductors in antenna elements. In view of the above, time permitting, I will try to do some work and try to shed some more light on the subject. Any experiences out there, rather than more "reasons" why it ain't so? Proper accommodation in modeling programs can give substantial improvement in loaded elements modeling. Let the games begin :-) Yuri, K3BU |
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. . loading coil current . . . is (or is not) equal to what
????????? ". Please state unambiguously and explicitly what it is you wish to know. --- Reg. Look at the article(s), it refers to current being equal (or not) at both ends of the loading coil in the antenna elemnt. In a nutshell, I (and W9UCW, etc.) found that current diminishes accross the coil. W8JI using Kirchoff and Ohm says it can't. I am curious if anyone else made measurements (never mind "theory") and what they found. Yuri |
Can you post a link to this so I know what it is that you think I said, or what I said that I don't remember saying? Wes Stewart N7WS Without going the route of providing "evidence" I posted the question about the modeling loading coils here, and Roy and you provided suggestions of the described workaround (if my memory doesn't fail me :-) Yuri |
Another stiff dick contest between you and Tom?
"Yuri Blanarovich" wrote in message ... Howdy perpetual arguers! |
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"Yuri Blanarovich" wrote
I (and W9UCW, etc.) found that current diminishes across the coil. W8JI using Kirchoff and Ohm says it can't. I am curious if anyone else made measurements (never mind "theory") and what they found. ============================= If you mean the current distribution along the length of the coil was not uniform but tapering then it is not surprising. The important thing is by how much did it taper and in what direction? Up or down? Please describe clearly the type of instruments used and how you made your measurements. By how much did the current measuring instrument affect the strength of current flowing, for example by detuning the antenna? What was the length and diameter of the coil, and the number of turns? What were the lengths of the antenna above and below the coil? At what frequencies were the measurement made? What was the current in amps in the coil wire at the bottom, at the centre and at the top of the coil? What does the current distribution in the coil affect, AND BY HOW MUCH? ---- Reg |
Reg Edwards wrote: If you mean the current distribution along the length of the coil was not uniform but tapering then it is not surprising. The important thing is by how much did it taper and in what direction? Up or down? Please describe clearly the type of instruments used and how you made your measurements. By how much did the current measuring instrument affect the strength of current flowing, for example by detuning the antenna? What was the length and diameter of the coil, and the number of turns? What were the lengths of the antenna above and below the coil? At what frequencies were the measurement made? What was the current in amps in the coil wire at the bottom, at the centre and at the top of the coil? What does the current distribution in the coil affect, AND BY HOW MUCH? ---- Reg Gee, Reg. Until now I've always gotten the impression that you already learned everything there was to know about this stuff. :-) 73, AC6XG |
Another stiff dick contest between you and Tom? Yep, I post fact of life, Tom pompously chimes in that it can't be so with some added ridicule, and I won't budge to a bully that is barking up the wrong tree. This is about 5th time. Watch him to become expert in few moths "he said it all along" :-) and will become a guru. All in the good spirit of learning :-) Yuri |
What does the current distribution in the coil affect, AND BY HOW MUCH? ---- Reg Reg it is all there in my article and references I posted in my post. Can you look up those links or is there a problem? I hate to go over the stuff again. If you can't look up the links, perhaps I could post the text here, but article has details, measurements, pictures of meters and facts of life. Yuri www.K3BU.us |
Gee, Reg. Until now I've always gotten the impression that you already
learned everything there was to know about this stuff. :-) 73, AC6XG ============================= Jim, if you promise, cross-your-heart, not to tell anybody I'll let you into a long-hidden secret. When it comes to the the distribution of current along a loading coil on a vehicle-mounted whip I am severely handicapped in that I have never been mobile in a motor car with a radio transmitter, never owned a motor car, never even held a driver's licence. Otherwise I am quite a normal person who takes an interest in electric currents flowing along wires, rods, through coils and around the surfaces of such things as vehicle bodies. Normallity extends to world-wide objections against being choked by petrol fumes and the now common practice of financing pirate expeditions to obtain the diminishing raw material from which the poisonous liquid is refined. It seems Yuri is interested in modelling short, coil loaded antennas. He refers to controversy. I can set his mind at rest and assure him there is none. When the length of a loading coil is short in comparison with the overall height of the antenna, certainly in comparison with a wavelength, the current into one end can be assumed, with negligible error, to be equal to that which comes out of the other end as with any other coil in an L,C,R network analysis. Its stray capacitance can be ignored except for investigating its self-resonant frequency. It is a lump of inductance effectively concentrated at its midpoint. For estimating antenna behaviour and performance it is necessary only to add half the length of the coil to the length of the lower portion of the antenna, and to do likewise to the upper length. The antenna's distributed radiation and wire loss resistance can be sufficiently accurately estimated from these dimensions, all being transformed to the feedpoint according to the normal transforming action of the lengths of transmission line (the antenna parts) involved. ================================ When coil length is nearly as long as the antenna, ie., a close-wound helical for the lower frequencies, in which coil loss for a high inductance is minimised by using thick wire rather than an inconvenient, very large diameter coil, the antenna is best considered as a continuously loaded 1/4-wavelength transmission line in which its uniformly-distributed capacitance, loss resistance and radiation resistance per unit length is taken into account. ================================ For practical purpose, these different-proportioned sorts of short vertical antennas all have the same, simple, well known radiation pattern. Any slight differences are overwhelmed by variations, entirely out-of-human-control, in the local environment and along the propagation path. What matters is radiating efficiency. The standard of radiating efficiency is that of a very high 1/2-wave dipole of any orientation and there's no need to be concerned here where the radiation disappears to. There is only one question of consequence. At what height up a short vertical is a coil of given intrinsic Q to be located to maximise radiating efficiency? It is never at or very near the top! As coil height increases the required inductance and number of turns increases rapidly. Coil loss resistance always overtakes the improvement in radiation resistance due to the change in distribution of current along the antenna. A high resistance, self resonant coil of many turns of fine wire right at the top of the antenna eventually fails. ================================ There are various special cases which are dealt with by simple programs available from the website below. There is one program which covers from helicals, via screwdrivers, to lumped coils. The coil can slide up and down the antenna to find the location of maximum efficiency for given coil length and diameter. The number of coil turns and wire gauge are automatically recalculated to maintain the same required antenna resonant frequency. Program name is LOADCOIL.exe Download and run in a few seconds. ---- ======================= Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.g4fgq.com ======================= |
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Reg writes:
It seems Yuri is interested in modelling short, coil loaded antennas. He refers to controversy. I can set his mind at rest and assure him there is none. When the length of a loading coil is short in comparison with the overall height of the antenna, certainly in comparison with a wavelength, the current into one end can be assumed, with negligible error, to be equal to that which comes out of the other end as with any other coil in an L,C,R network analysis. Its stray capacitance can be ignored except for investigating its self-resonant frequency. There is none? You are confirming there is one by your above statements. The point is that W9UCW measured, that difference in "normal" loading coil (not long coils or helicals) is in order of 40 to 60% less at the top of the coil. That is significant in calculating or optimizing the efficiency of loaded antenna. MEASURE it and don't rely on myth perpetuated since 1955 by Belrose till today's ARRL Antenna Book. ON4UN has it right in his book. Resonance is no big deal, efficiency is greatly affected, modeling programs are way off especially if you include more loaded parasitic elements. Did you read my article, facts and measurements? You can try to repeat the measurements to validate the effect. That's what I am looking for, if we are in error, would like to have it pointed out. Not speculations that it "should be" like that. Yuri |
Then what's left to be said here? 73's Richard Clark, KB7QHC See the comments from the flat earth society, even Reg has and "hasn't" have a problem. Significant impact on modeling software. If the stuff is not accomodated properly, then results (mainly efficiency) are way off. Mobile antennas, shortened antennas can be made better if we have handle on the thing. Yuri, K3BU/m |
Yuri Blanarovich wrote:
Any experiences out there, rather than more "reasons" why it ain't so? Proper accommodation in modeling programs can give substantial improvement in loaded elements modeling. Assume a transmission line with an SWR of 10:1. Put a series inductor in series with the transmission line. Assuming negligible losses, the forward current is the same at each end of the coil and the reflected current is the same at each end of the coil. The question is: Do the superposed currents, Ifwd+Iref, remain constant? Of course not, because of phase shifts. With a large enough coil, one could cause a current maximum point on one side of the coil and a current minimum point on the other side. That same principle holds true for standing wave antennas which are antennas with (surprise!) standing waves. The current is NOT the same at each end of the coil (unless a current maximum or current minimum occurs in the middle of the coil). However, for traveling wave antennas, the current at each end of a loading coil would be close to equal. -- 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 a nutshell, I (and W9UCW, etc.) found that current diminishes accross the coil. W8JI using Kirchoff and Ohm says it can't. If you put a loading coil 1/3 of the way up on an end-fed 1/2WL vertical, the net current will increase across the coil. The net current can decrease, or increase, or be the same magnitude for special cases. Think Ifwd+Iref with the coil causing major phase shifts. Open-ended antennas like dipoles are standing wave antennas. The forward current is relatively constant through the coil and the reflected current is relatively constant through the coil. But the phasor sum of those two currents can vary wildly from end to end in the coil because of phase shifts. -- 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! =----- |
Thanks Cecil,
the standing waves do it again! So far the best argument against W8JI's Kirchoffs and Ohms! With your permission I will post this public posting back at the eHam.net. Now see what Reg says, hopefuly after reading the article. Yuri |
Open-ended antennas like dipoles are standing wave antennas. The forward current is relatively constant through the coil and the reflected current is relatively constant through the coil. But the phasor sum of those two currents can vary wildly from end to end in the coil because of phase shifts. -- 73, Cecil http://www.qsl.net/w5dxp Don't we have a case of coil being RF choke to certain extent? Also I think that behaviour of radiator before and after the coil defines the magnitude of the current, no? Yuri, K3BU |
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Yuri Blanarovich wrote:
I post fact of life, Tom pompously chimes in that it can't be so with some added ridicule, and I won't budge to a bully that is barking up the wrong tree. This is about 5th time. Watch him to become expert in few moths "he said it all along" :-) and will become a guru. I don't understand what the fuss is all about. The forward current hits the end of the dipole and is reflected. There are standing current waves on a standing wave antenna. A loading coil shifts the phase between Ifwd and Iref so anything is possible across a coil, increasing currents, decreasing currents, or equal magnitudes of currents. It all depends on the phases involved. For dipoles shorter than 1/2WL, the current decreases across the loading coils. For a 1WL dipole with the loading coils located 1/3 of the way from the feedpoint, the net current through the coils will actually increase. -- 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! =----- |
Reg Edwards wrote:
When the length of a loading coil is short in comparison with the overall height of the antenna, certainly in comparison with a wavelength, the current into one end can be assumed, with negligible error, to be equal to that which comes out of the other end as with any other coil in an L,C,R network analysis. But Reg, why do you think they call it a standing wave antenna? Would you also assert that the current is equal when a coil is installed in a transmission line with reflections? If it weren't for reflections from the open ends of a dipole, the feedpoint impedance would be hundreds of ohms. It's the reflections that reduces the feedpoint impedance to ~70 ohms. -- 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:
The point is that W9UCW measured, that difference in "normal" loading coil (not long coils or helicals) is in order of 40 to 60% less at the top of the coil. All explained by the different phasing of the forward and reflected currents at that point. If you want to blow Tom's mind, measure the current in and out of a coil placed 1/3 of the distance up in a 1/2WL vertical. The current will *INCREASE* from the bottom of the coil to the top of the coil. How many times have we been warned not to use lumped circuit theory on distributed networks? -- 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:
Thanks Cecil, the standing waves do it again! See if you can get Tom to assert that the current into and out of a coil in series with a transmission line with reflections is also constant. :-) Same principles apply. So far the best argument against W8JI's Kirchoffs and Ohms! You can get a ballpark estimate of those currents by comparing a 1/2WL dipole to a loading coil dipole. Assuming the following two dipoles are resonant on the same frequency: -----y----------x-----FP-----x----------y----- -----coil-----FP-----coil----- Assume the feedpoint impedances are the same and the losses in the coils are negligible. The net current into the coil is close to the current at 'x'. The net current out of the coil is close to the current at 'y'. -- 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:
Open-ended antennas like dipoles are standing wave antennas. The forward current is relatively constant through the coil and the reflected current is relatively constant through the coil. But the phasor sum of those two currents can vary wildly from end to end in the coil because of phase shifts. Don't we have a case of coil being RF choke to certain extent? RF chokes are usually high enough impedance to drop virtually all the RF voltage across the choke. Also I think that behaviour of radiator before and after the coil defines the magnitude of the current, no? It can be thought of as a very lossy transmission line where the loss is radiation. Please see my other posting comparing a 1/2WL dipole to a loaded dipole. -- 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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Can any of you guys tell me which of the waves on the antenna does the
radiating - is it the forward or is it the backward wave ? --- Reg |
Richard KB7QHC wrote:
This speaks more of simple Resistive heat loss supported by your own direct observation of: I fried the loading coil with 600W into Hustler resonator, melting heat-shrink tubing and wire at the bottom of the coil. No, it confirms that there is a significant (not negligible) difference in the current at the bottom vs. top of the coil. Yes, Hustler has small (almost resistive) wire on 80m resonator. If you trasmit for short period of time (not enough for heat to equalize) and feel it, or use thermal strips to check temperature, you would see the taper in the current from bottom to top. It is in order of 50%, not negligible. Coils in tests are good quality, not "resistive" wire, current relatively low (100mA) as shown in W9UCW measurements and pictures. The point is, if the current was constant or close to it, you would not see the difference as we see it. Heat rises to the top, if anything the top would be warmer if the current was constant. If the coil is uniform colenoid, same wire, diameter (resistance), spacing and it shows difference in heat produced accross the coil, then we can, using I2R formula, deduct that that current at the bottom is greater than on the top. W9UCW measurements confirm that, Cecil explains. Speculations that Earth must be flat might satisfy those reading the (wrong) books, but will not jive with reality. Simple way to test it, transmit 100W to 80m Hustler resonator, and feel the coil. Even insensitive people can feel the significant difference in temperatures. Put 500W to it for longer period and watch the heatshrink tubing shrivel from the bottom up. This eliminates all the "errors" with meters to prove the point. Yuri, K3BU/m |
NM5K
Dunno, I think it varies. But I sort of agree with Tom, I think it's fairly constant across the coil. MEASURE or FEEL it! Or disprove what W5DXP is saying. It appears that current drop is proportional to the current drop in the section of the antenna that is "missing" - replaced by the coil. You can express it in electrical degrees and it appears to correspond to cosine distribution. Yuri, K3BU |
Reg Edwards wrote:
Can any of you guys tell me which of the waves on the antenna does the radiating - is it the forward or is it the backward wave ? An electron experiences the sum of those two waves and emits a photon when it is energized enough. So the answer is both. -- 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:
MEASURE or FEEL it! Or disprove what W5DXP is saying. This is easy to see using EZNEC. Model a 102' G5RV on 20m and look at the current distribution. There are three current maximums and four current minimums. If you install a loading coil at a current maximum or current minimum, the current magnitude will be the same on both sides of the coil. If you install a loading coil at a point where the slope of the current is negative (decreasing), the current at the bottom of the coil will be greater than the current at the top of the coil. This is the usual case for mobile antennas. If you install a loading coil at a point where the slope of the current is positive (increasing), the current at the bottom of the coil will be less than the current at the top of the coil. Note: 'Top' of coil is the end closest to the the ends of the antenna. 'Bottom' of coil is the end closest to the feedpoint. -- 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! =----- |
Reg Edwards wrote:
"Can any of you guys tell me which of the waves on the antenna does the radiating - is it the forward or is it the backward wave?" I agree with Cecil, "So the answer is both." Think of a traveling wave antenna, the rhombic. When it is properly terminated, there is no backward wave and the radiation pattern is unidirectional. Eliminate the termination resistance and a total reflection occurs at the antenna`s far end. Now the rhombic is a bidirectional antenna. Best regards, Richard Harrison, KB5WZI |
Yuri, K3BU wrote:
"I ordered 19th edition of ARRL Antenna Book and followed chain of references that led to information on page 16-7 and Fig. 9 and 10." Excellent purchase. A series circuit tends to have the same current throughout except when its length is significant with respect to wavelength. A standing wave antenna has a reflected wave which makes impedance a function of location along the antenna. So, a certain power, incident and reflected, combine to produce voltage and current variations on an antenna which are related to those on a transmission line. Since radiation from an antenna occurs, power in each direction is not constant as it tends to be on a transmission line because radiation is taking a toll in each direction, and that`s a good thing. It`s the purpose of the antenna. Coil Q is important to efficiency if any part of the XL/R is loss resistance and not radiation resistance. Radiation resistance is the purpose of the antenna. My ON4UN Figures are the same as Yuri`s but appear as Fig 9-22 on page 9-15. These show the current decline across a loading coil including the obvious case of a solenoid used as an antenna where the entire current distribution is within the continuous loading coil. The ARRL Antenna Book has been exposed to scrutiny for many years. By the 19th edition it`s a safe bet that moat of it is correct (without consideration of "strings", 5 dimensions, or 11 parallel universes). From what I`ve seen of ON4UN`s book, he got it right too. Best regards, Richard Harrison, KB5WZI |
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Incorrect. HEAT does not rise at all, however hot gas or hot liquid will rise in response to gravity. This has nothing to do with a loading coil made from solid parts. -- Bill, W7TI Incorrect, incorrect. If we suppose the loading coil is heating up equally (the flat earth society argument), it heats air immediately surrounding it. Air is heating up, rises up as you say, as it rises it picks up the other air that is heating up and they rise together. As they progress, they heat up rest of the coil on the way up, with the result being that the top should be warmer (hotter) than the bottom. If we have quarter wave whip with loading coil, and the bottom contrary to the above mechanism is warmer, thet ergo ipso there must be more current flowing in the bottom part of the coil, confirming what we say. Yuri, K3BU/m |
You are using a thermocouple which is sensitive to heat, the heat of coil loss will inflate the reading. You have twice offered heat at the bottom of the coils that correlate strongly with inflated current values from a caloric sensor. You have no other thermocouple data supporting the nature of the current distribution, just the isolated section you find attractive. Put simply, your measurements have no reference (readings from the entire length of the radiator). The bottom meter is below the coil, so there is no heat heating up the thermocoupled meter. If you insert the meters some distance away from the coil, you would see the corresponding readings showing the difference between the top and bottom. Arguments that heat or magnetic field affect the thermocouple RF ammeters are just not realistic. You went to some trouble to offer testimonial from reference sources on the nature of that distribution, but you did not measure it confirm your testing. Two readings in isolation do not prove you have 100mA into the bottom when there is only one reading below the coil. I did just rough test with one of my meters (has 8 A), flipping the coil and I can see some deflection at the bottom and none at the top with 100 W into the antenna. W9UCW et al did hundreds of measurements and showed just some examples. If you are not interested in obtaining those remaining readings of that current distribution, then you have a poor case. My "case" is to bring this to attention of those who are still "knowing" that the current in loading coils is the same at both ends. If they doubt, they can do their own measurements and see what it is, or show us where we are wrong. Again, ON4UN in his Low Band DXing book has it right, ARRL Antenna Book has it wrong and is perpetuating 50 year old misconception. Just MEASURE or FEEL it! 73's Richard Clark, KB7QHC Yuri, K3BU/m |
I could buy that to an extent I guess. But say if you had a top loaded vertical, with linear current distribution, the current across the coil should be appx equal no matter where the coil is placed. But if no top loading, maybe so.. MK Top loaded vertical does not have LINEAR current distribution, that is another simplification, fallacy. Current in the radiator has cosine distribution. At the base, there is not much difference, just like in cosine of the angle corresponding to the electrical length of radiator at that point. Again, this subject of current distribution is important in optimizing the antenna design by fine tuning the position of the loading coil in the antenna, combination with top loading etc. Morew current flowing in the radiating part of the antenna - the stronger the field and louder signal. The "linear" current distribution mentioned in ARRL Compendium and Antenna Book is the simplification propagated from Belrose's 1955 QST article. It is close, but not exact and introduces confusion as it is demonstrated by the flat earth society. Yuri, K3BU/m |
Yuri wrote,
Again, this subject of current distribution is important in optimizing the antenna design by fine tuning the position of the loading coil in the antenna, combination with top loading etc. Morew current flowing in the radiating part of the antenna - the stronger the field and louder signal. What is "the radiating part of the antenna," Yuri? 73, Tom Donaly, KA6RUH |
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