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Current across the antenna loading coil - from scratch
OK, I have been accused of being wrong, claiming that current across the
antenna loading coil is or can be different at its ends. I and "my camp" say that we are seeing somewhere 40 to 60 % less current at the top of the coil, than at the bottom, in other words, significant or noticeable drop. W8JI and "his camp" are claiming it can't be so, current through the coil has to be the same or almost the same, with no significant drop across the loading coil. Let's start the fresh thread and trace step by step where I went wrong. Just reminder that we are talking typical situations, as for example real 40 m (or 80 m) mobile whip with loading coil about 2/3 up the radiator. We are talking about resonant electrical quarter wave monopole. We are talking about standing wave RF current that can be measured with RF ammeter and is shown and plotted in modeling programs like EZNEC. Here we go: wrote in message Let's focus on one thing at a time. You claim a bug cather coil has "an electrical length at 4MHz of ~60 degrees". That concept is easily proven false, just like the claim a short loaded antenna is "90-degree resonant". Both can be shown to be nonsense pictures of what is happening. Assume I have a 30 degree long antenna. If the loading inductor is 60 electrical degrees long, I could move it anyplace in that antenna and have a 90 degree long antenna. We all know that won't happen, so what is it you are really trying to say? 73 Tom OK lets get me some educating here. I understand that, say quarter wave resonant vertical (say 33 ft at 40m) has 90 electrical degrees. Is that right or wrong? The current distrubution on said (full size) vertical is one quarter of the wave of 360 deg. which would make it 90 degrees. Max current is at the base and then diminishes towards the tip in the cosine function down to zero. Voltage distribution is just opposite, min at the base, feed point and max at the tip. EZNEC modeling shows that to be the case too. Is that right or wrong? If we stick them end to end and turn horizontal, we get dipole, which then would be 180 deg. "long" or "180 degrees resonant". If not, what is the right way? If I insert the coil, say about 2/3 up (at 5 ft. from the bottom) the shortened vertical, I make the coil size, (inductance, phys. dimensions) such that my vertical will shrink in size to 8 ft tall and will resonate at 7.87 MHz. I learned from the good antenna books that this is still 90 electrical "resonant" degrees. Maximum of current is at the feed point, minimum or zero at the tip. If you stick those verticals (resonant) end to end and horizontal, you get shortened dipole, with current distribution equal to 180 degrees or half wave. Max current at the feed point, minima or zero at the tips. (RESONANT radiator) How many electrical degrees would that make? How do you arrive at that? Why is this a nonsense? Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? If I can be enlightened about this, we can go then to the next step. Answers, corrections please. Yuri, K3BU |
Current across the antenna loading coil - from scratch
On Thu, 23 Mar 2006 12:50:32 -0500, "Yuri Blanarovich"
wrote: Let's start the fresh thread and trace step by step where I went wrong. Hi Yuri, Are you then abandoning your web page? You know, it would seem to be better effort to stick with the demonstrables there and to make sense of them, than to wander the intellectual landscape of "theory." OK lets get me some educating here. I understand that, say quarter wave resonant vertical (say 33 ft at 40m) has 90 electrical degrees. Is that right or wrong? If you cannot define your limits of error, then Cecil is bound to do it for you and plug in +/- 50% to make any assertion laughable, such as: Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? You've left too many things out to agree to more than a rather insubstantial maybe. If that's sufficient, then there's really no need to go any further, is there? 73's Richard Clark, KB7QHC |
Current across the antenna loading coil - from scratch
Yuri wrote,
" I and "my camp" say that we are seeing somewhere 40 to 60 % less current at the top of the coil, than at the bottom, in other words, significant or noticeable drop. W8JI and "his camp" are claiming it can't be so, current through the coil has to be the same or almost the same, with no significant drop across the loading coil. " I'm not sure who all you put in W8JI's "camp," but I'm absolutely sure that I've read recent postings by W8JI himself that affirm that there can be significant difference in current between the ends of the loading coil. What I DO see him posting is that if the difference is large, the antenna design is almost certainly suboptimal. If you want to understand how a loading coil with zero capacitance to the outside world can actually work, I suggest you read the Joseph Boyer article from "Ham Radio" magazine some 28 years ago. Ian White gave a more complete reference to that article in one of his postings in the interminable thread. I'm always happy to ship out a copy of that article for the cost of postage. Cheers, Tom |
Current across the antenna loading coil - from scratch
Yuri, you have neglected at least one important unit of measu namely,
the action integral of the 'active antenna' times the current [ampere*degrees]. For simplicity, assume a constant one ampere is flowing in a physical 15 degree antenna. The answer is 15 ampere*degrees. In a true 1/4 wavelength antenna the answer is 90 ampere*degrees. The shortened antenna is [=] a tuned 15 degree antenna NOT a 90 degree antenna!! In a real shortened antenna, the base current may be assumed, to a first approximation, as constant from the base to the loading coil. If that portion of the antenna is 10 degrees, then 10 ampere*degrees is the action integral. The current in the top section can be assumed linear from the value at the top of the coil to the tip where I=0. This is a triangle [or a sin function that is close to linear] that can solve to two possible values. The first is based on one ampere exiting the top of the coil and the solution is 1/2*1 ampere*5 degrees = 2.5 ampere degrees. The second is based on a sinusoidal distribution from the tip to the 5 degree point at the top of the coil where the current is 0.087 ampere. [The sin of 5 degrees is 0.087.] So, the action integral is 1/2*0.087 amperes*5 degrees = 0.218 ampere*degrees. The practical application deals with the efficiency of the antenna. Is that tuned 15 degree long antenna a 12.5 ampere degree antenna; or, is it a 10.218 ampere degree antenna? [That's approximately a difference of 1 dB in antenna performance.] The discussion here for the past three infinities is: What is happening inside the tuning coil? Is there a change in current amplitude? If so, please explain the physics. Is there no change in current amplitude? If so, please explain the physics. The coil is physically less than one degree in length, but contains enough wire to be a significant portion of a wavelength. Interwinding capacitance and distributed inductance can make the coil look like a transmission line. The flux density from each turn in an air core coil construction diverges as one progresses along the coil [the flux density at turn #2 is higher than at turn #60 for example]. Restated, there is a leakage inductance along the coil. The flux density has a propagation time in free space of approximately 0.5E-9 seconds. Is this significant? [I don't think so] In the tuning coil there exists an interwinding capacitance and a capacitance to "structure" [whatever that is]. In 1958, my college days, we were instructed to ignore the coil and solve the antenna as two separate sections with an infinitesimal gap at the junction. I never liked that model then and I don't like it today! [It still allows two solutions][We were instructed that the current is constant]. So, we have two well entrenched positions: the current does not change in the coil, and, the current changes in the coil. Like World War I, it is trench warfare with much bloodshed [reputation] on both sides. Due to leakage inductance, I suspect that the current does change within the air core coil but the change is much less than that implied by the simple sin wave distribution [sin 5 degrees] used above. Below the coil the H field dominates. Above the coil the E field dominates. The transition from E to H occurs across [within] the coil. That leads me to conclude that there is a change of current within the coil. In any event, the 15 degree antenna is still a 15 degree antenna! The question is: what about that 1 dB difference in the modeling analysis? This simple engineer is still unconcerned about one dB difference and it's impact on antenna gain. If the science side of this discussion can't agree, then I'll simply continue to operate mobile and not worry if my signal is one dB stronger or weaker at the receiving end of the path!! It is what it is!! # # # Yuri Blanarovich wrote: OK, I have been accused of being wrong, claiming that current across the antenna loading coil is or can be different at its ends. I and "my camp" say that we are seeing somewhere 40 to 60 % less current at the top of the coil, than at the bottom, in other words, significant or noticeable drop. W8JI and "his camp" are claiming it can't be so, current through the coil has to be the same or almost the same, with no significant drop across the loading coil. Let's start the fresh thread and trace step by step where I went wrong. Just reminder that we are talking typical situations, as for example real 40 m (or 80 m) mobile whip with loading coil about 2/3 up the radiator. We are talking about resonant electrical quarter wave monopole. We are talking about standing wave RF current that can be measured with RF ammeter and is shown and plotted in modeling programs like EZNEC. Here we go: wrote in message Let's focus on one thing at a time. You claim a bug cather coil has "an electrical length at 4MHz of ~60 degrees". That concept is easily proven false, just like the claim a short loaded antenna is "90-degree resonant". Both can be shown to be nonsense pictures of what is happening. Assume I have a 30 degree long antenna. If the loading inductor is 60 electrical degrees long, I could move it anyplace in that antenna and have a 90 degree long antenna. We all know that won't happen, so what is it you are really trying to say? 73 Tom OK lets get me some educating here. I understand that, say quarter wave resonant vertical (say 33 ft at 40m) has 90 electrical degrees. Is that right or wrong? The current distrubution on said (full size) vertical is one quarter of the wave of 360 deg. which would make it 90 degrees. Max current is at the base and then diminishes towards the tip in the cosine function down to zero. Voltage distribution is just opposite, min at the base, feed point and max at the tip. EZNEC modeling shows that to be the case too. Is that right or wrong? If we stick them end to end and turn horizontal, we get dipole, which then would be 180 deg. "long" or "180 degrees resonant". If not, what is the right way? If I insert the coil, say about 2/3 up (at 5 ft. from the bottom) the shortened vertical, I make the coil size, (inductance, phys. dimensions) such that my vertical will shrink in size to 8 ft tall and will resonate at 7.87 MHz. I learned from the good antenna books that this is still 90 electrical "resonant" degrees. Maximum of current is at the feed point, minimum or zero at the tip. If you stick those verticals (resonant) end to end and horizontal, you get shortened dipole, with current distribution equal to 180 degrees or half wave. Max current at the feed point, minima or zero at the tips. (RESONANT radiator) How many electrical degrees would that make? How do you arrive at that? Why is this a nonsense? Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? If I can be enlightened about this, we can go then to the next step. Answers, corrections please. Yuri, K3BU |
Current across the antenna loading coil - from scratch
Dave wrote:
. . . The practical application deals with the efficiency of the antenna. Is that tuned 15 degree long antenna a 12.5 ampere degree antenna; or, is it a 10.218 ampere degree antenna? [That's approximately a difference of 1 dB in antenna performance.] . . . I believe you're comparing the field strengths from two antennas both driven by the same current. If you drive them with the same power, a more fair comparison, you'll find a negligible difference in field strength. Efficiency is another issue, solely related to losses in the antennas. Without knowing what those losses might be, we can't say anything about the relative efficiency. In practice it'll probably be extremely closely the same also. Roy Lewallen, W7EL |
Current across the antenna loading coil - from scratch
wrote in message
ups.com... This thread belongs back in the original place, so it flows in context. Sorry I had to take a break and lost the place in original place, so lets try to continue here, we are trying to go step by step. Yuri Blanarovich wrote: OK, I have been accused of being wrong, claiming that current across the antenna loading coil is or can be different at its ends. No one said that. So what is it then you claiming being equal. I and "my camp" say that we are seeing somewhere 40 to 60 % less current at the top of the coil, than at the bottom, in other words, significant or noticeable drop. Quit trying to make it a gang war. It is antenna theory, not a bar room brawl with a bunch of drunks. No gang wars intended, just trying to underline that there are two major supporting "camps" claiming that the current has to be equal, or is appreciably different. W8JI and "his camp" are claiming it can't be so, current through the coil has to be the same or almost the same, with no significant drop across the loading coil. I have no camp. You are lifting what I say out of context and deleting important things. What I say, over and over again, is I can build an inductor in a short mobile antenna that has essentially equal currents at each end. A compact loading coil of good design has this type of performance. I can do that too and do not deny it. The current taper across the inductor is not tied to the number of "electrical degrees" the inductor "replaces". It is tied to the distributed capaciatnce of the coil to the outside world in comparison to the termination impedance at the upper end of the coil. That too, but that seems to be minor cause. Lets do it step by step. I will skip agreements so far. The current distrubution on said (full size) vertical is one quarter of the wave of 360 deg. which would make it 90 degrees. Max current is at the base and then diminishes towards the tip in the cosine function down to zero. Voltage distribution is just opposite, min at the base, feed point and max at the tip. EZNEC modeling shows that to be the case too. Is that right or wrong? Right. Although the distributed capacitance can change the shape. It can change the amplitude, but not the shape of the current distribution curve, that is the maximum is at the feed point (zero reactance - resonance) and zero at the tips and follows cosine function. If we stick them end to end and turn horizontal, we get dipole, which then would be 180 deg. "long" or "180 degrees resonant". If not, what is the right way? Right. If I insert the coil, say about 2/3 up (at 5 ft. from the bottom) the shortened vertical, I make the coil size, (inductance, phys. dimensions) such that my vertical will shrink in size to 8 ft tall and will resonate at 7.87 MHz. I learned from the good antenna books that this is still 90 electrical "resonant" degrees. Maximum of current is at the feed point, minimum or zero at the tip. What "good book"? It would help to see the context. Say ARRL Antenna Book, 20th edition, page 16-7, Fig 10 Shows lengths h1 and h2 expressed as 15 deg. eaach. None of my engineering books use electrical degrees except to describe overall antenna height or length. But that relates to describing the antenna properties in relation to resonant frequency for that particular radiator. They might say "60 degree top loaded resonant radiator" but they don't say "60 degree tall radiator 90 degree resonant". If you stick the coil at the base in series with radiator and bring it to resonance (zero reactance at the frequency of interest) what "degree resonant" will than radiator become, if not 90? ("Measured" from the feed point, through the coil and then straight radiator.) There might be a correct context, but I can't think of one off hand. So I need an example from a textbook. If you stick those verticals (resonant) end to end and horizontal, you get shortened dipole, with current distribution equal to 180 degrees or half wave. Max current at the feed point, minima or zero at the tips. (RESONANT radiator) The current distribution would not be the same as a half wave, becuase the antenna is not 1/2 wave long. Well, is 180 degrees half wavelength or not? Is the current maximum at the feedpoint (center) and zero at the end, or not? The current distribution is not the same, but is exhibiting properties of resonant half wave dipole with current max at the center and zero at the tips. The shape is not the smooth continuous cosine curve as in straight dipole, but affected by the loading coils (drops) in their place (subject of disagreement). Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? Yes. It works fine for length. It does NOT work for loading inductors, it does not work for short antennas which have anything form a uniform distribution to triangular distribution, or any mix between including curves of various slopes. Why not? What happens to cosine current distribution curve when we insert the loading element (inductance, coil, loading stub, resistance) in the radiator? What formula applies to get the uniform or triangular distribution? Can you show some mathematics? So we have resonant standing wave element, that has current max at the feedpoint and zero at the tip, which gives us 90 degree (or 180 with dipole) or quarter wave distribution from the base to the tip. (reality) We can express the straight pieces of radiator in degrees, but not the coiled up piece that the wave has to go through? The "uniform" and "triangular" distribution was used for approximation or simplification of showing the current distribution in short loaded radiators, while they are in reality segments of the cosine curve belonging to length of the straight portions of the radiator. EZNEC shows that, when you magnify the curve you can see there are no uniforms or triangles but a cosine curve. A 30 degree tall antenna with base loading simply has power factor correction at the base, provided the inductor is not a significant fraction of a wavelength long. It is a 30 degree base loaded radiator, not a 90 degree antenna. And the inductor is not 60 degrees long. We are not talking here about base loaded radiator. No detours please. So how many electrical degrees has the quarter wave resonant radiator that is loaded with loading coil (or stub) about 2/3 way up and is say 30 deg. physical "length" to make it resonant? 73 Tom 73 Yuri, K3BU |
Current across the antenna loading coil - from scratch
On Sun, 2 Apr 2006 22:54:10 -0400, "Yuri Blanarovich"
wrote: They might say "60 degree top loaded resonant radiator" but they don't say "60 degree tall radiator 90 degree resonant". If you stick the coil at the base in series with radiator and bring it to resonance (zero reactance at the frequency of interest) what "degree resonant" will than radiator become, if not 90? ("Measured" from the feed point, through the coil and then straight radiator.) Hi Yuri, This must be a convention that is particular to only a very few Hams. The FCC database describes AM antennas in both electrical and physical height as follows. WGOP 80.00° tall 125.2 meters tall 540 kHz WWCS 63.50° tall 98.8 meters tall 540 kHz WFTD 79.00° tall 64.0 meters tall 1080 kHz KYMN 118.60° tall 92.3 meters tall 1080 kHz WWLV 90.00° tall 47.2 meters tall 1620 kHz WTAW 204.00° tall 106.7 meters tall 1620 kHz There may be some discrepancy, but it certainly looks like antenna specification is by the electrical equivalent of the physical height (and whatever l/d fudging) and with only one happening to be 90°. Further, given most references (for professionals) is aimed at a common specification that is largely driven by this agency, it would seem odd to step out of this expectation to change to calling all antennas 90° simply because they resonate. http://www.fcc.gov/mb/audio/amq.html 73's Richard Clark, KB7QHC |
Current across the antenna loading coil - from scratch
On Sun, 2 Apr 2006 22:54:10 -0400, "Yuri Blanarovich"
wrote: So how many electrical degrees has the quarter wave resonant radiator that is loaded with loading coil (or stub) about 2/3 way up and is say 30 deg. physical "length" to make it resonant? Hi Yuri, This is thin ice. If you are treading along the 1:1 replacement of coil (wire-length/phase/Vf/whatever) for missing radiator length; then Cecil is prepared to prove it to within ±59% error. We could call it Cecil's 1.59:0.41 replacement rule - guaranteed to prove just about anything, and to whiten your teeth. 73's Richard Clark, KB7QHC |
Current across the antenna loading coil - from scratch
Yuri Blanarovich wrote: So what is it then you claiming being equal. In a loading coil with very small distributed capaciatnce to the outside world compared to termination impedance, current has to be equal. Phase shift in current at each end has to be nearly zero. This is true for ANY antenna length or loading coil location. This is what everyone was saying. No gang wars intended, just trying to underline that there are two major supporting "camps" claiming that the current has to be equal, or is appreciably different. One group is saying the distributed capacitance of the inductor to the outside world controls the distribution, another group appears to be saying it is a function of electrical degrees the coil makes up. I agree with the majority of people posting in these threads. It is the capacitance from the coil to the outside world that controls current distribution in the inductor and produces and phase difference in CURRENT at each end. If you look at Reg, Ian, Roy, all the Toms, Gene, Richard Clark, and on and on we are all saying that same thing. Richard Harrison, Yuri, and Cecil seem to argue against that, but that is how ANY inductor behaves. It can be proven to behave the way most people are trying to explain. The coil does NOT represent the "missing degrees". It does NOT have to have current taper (as a matter of fact a good indictor and antenna design won't show any significant current taper). Right. Although the distributed capacitance can change the shape. It can change the amplitude, but not the shape of the current distribution curve, that is the maximum is at the feed point (zero reactance - resonance) and zero at the tips and follows cosine function. The current distribution in an antenna is primarily a function of displacement currents caused by capacitance to the outside world and series impedance. If I have a short small diameter whip of uniform cross section, it has triangular distribution. If I sufficiently end-load the very same antenna with no change in length, the current distribution becomes close to uniform. The thinner the elenment and larger the end loading, the more uniform the current. This is why radiation resistance is someplace around FOUR times greater in an end-loaded antenna when compared to a base loaded antenna of the same height. Radiation resistance is tied to ampere-feet, and ampere-feet is larger with end loading rather than base loading. What "good book"? It would help to see the context. Say ARRL Antenna Book, 20th edition, page 16-7, Fig 10 Shows lengths h1 and h2 expressed as 15 deg. eaach. That does not say 90 degrees. It says 15 degrees. If you stick the coil at the base in series with radiator and bring it to resonance (zero reactance at the frequency of interest) what "degree resonant" will than radiator become, if not 90? ("Measured" from the feed point, through the coil and then straight radiator.) It is resonant. It is 15 degrees tall. It is NOT 90 degree resonant. Degrees of height is a distance measurement, not an electrical parameter. If someone is using it to describe resonance thay are misusing the term. I can have a 180 degree long resonant dipole. I can have a 20 degree long resoannt dipole. I cannot have a 180 degree resonant dipole that is 20 degrees long. I cannot have a 15 degree tall vertical that is 180 degree resonant, or 90 degree resonant. That argues against itself. Well, is 180 degrees half wavelength or not? Yes, 180 degrees is 1/2 wave. Is the current maximum at the feedpoint (center) and zero at the end, or not? Yes. The current distribution is not the same, but is exhibiting properties of resonant half wave dipole with current max at the center and zero at the tips. The shape is not the smooth continuous cosine curve as in straight dipole, but affected by the loading coils (drops) in their place (subject of disagreement). So what? The degrees are a measure of physical length. A 20 degree long coil loaded dipole is a 20 degree long resonant dipole. It is not a 180 degree resonant antenna. Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? Yes. It works fine for length. It does NOT work for loading inductors, it does not work for short antennas which have anything form a uniform distribution to triangular distribution, or any mix between including curves of various slopes. Why not? What happens to cosine current distribution curve when we insert the loading element (inductance, coil, loading stub, resistance) in the radiator? What formula applies to get the uniform or triangular distribution? Can you show some mathematics? This has all been explained over and over again. You can also see it in any engineering book. ON4UN's book initially had it wrong, but it is corrected now. So we have resonant standing wave element, that has current max at the feedpoint and zero at the tip, which gives us 90 degree (or 180 with dipole) or quarter wave distribution from the base to the tip. (reality) No, it does not. 90 degrees when used with antennas is a measure of distance as it relates to frequency. You can't have a 15 degree long antenna that is "180 degree resonant", and the current distribution depends on distributed capacitance and series impedance. We can express the straight pieces of radiator in degrees, but not the coiled up piece that the wave has to go through? Right. You cannot build a "60 degree coil to make my 30 degree antenna 90 degrees long". Think about it. If the coil was 60 degrees long, you could move it anywhere in the antenna and it would be resonant at the same frequency! The "uniform" and "triangular" distribution was used for approximation or simplification of showing the current distribution in short loaded radiators, while they are in reality segments of the cosine curve belonging to length of the straight portions of the radiator. EZNEC shows that, when you magnify the curve you can see there are no uniforms or triangles but a cosine curve. Oh, we are picking nits now. In this case we have to get every engineering book to say "it is such a small portion of a curve it looks straight" instead of using triangular or uniform. After all current might be 1.0001 amperes at one point and 1.0000 amperes ten feet away, so I guess that is actually not uniform if we pick the nits enough. The cosine shape is not true as a rule, however. That shape depends on distributed capacitance being uniform. We are not talking here about base loaded radiator. No detours please. If your theory does not cover base, center , and top loading it is incomplete, If it does not treat a coil as a coil no matter how it is used or set limits, it is incomplete. Base loaded, center loaded. It doesn't matter. So how many electrical degrees has the quarter wave resonant radiator that is loaded with loading coil (or stub) about 2/3 way up and is say 30 deg. physical "length" to make it resonant? That's already been explained. If it is 30 degrees tall, it is a resonant 30-degree tall radiator. It is not a "90 degree resonant 30 degree tall radiator with a 60 degree coil". 73 Tom |
Current across the antenna loading coil - from scratch
"Richard Clark" wrote
This must be a convention that is particular to only a very few Hams. The FCC database describes AM antennas in both electrical and physical height as follows. .... it would seem odd to step out of this expectation to change to calling all antennas 90° simply because they resonate. _____________ The FCC data cited does not include the reduced velocity of propagation along the radiator -- which means that an FCC "90 degree" radiator is not resonant, it has some inductive reactance. A network is used at the radiator feedpoint to transform the complex impedance there to properly match the transmission line. That "90 degree" radiator would need to be shortened by several percent in order to be self-resonant. Kraus (3rd Ed, p 182) shows a feedpoint Z of 73 + j42.5 ohms for a thin-wire, linear dipole that is a physical 1/2-wavelength, and that self-resonance occurs at a length a few percent shorter, when the radiation resistance drops to about 65 ohms. An unloaded 1/4-wave MW broadcast monopole working against the typical broadcast radial ground system has about 1/2 the impedance that Kraus shows for a dipole in free space. RF |
Current across the antenna loading coil - from scratch
"Richard Clark" wrote Hi Yuri, This must be a convention that is particular to only a very few Hams. The FCC database describes AM antennas in both electrical and physical height as follows. WGOP 80.00° tall 125.2 meters tall 540 kHz WWCS 63.50° tall 98.8 meters tall 540 kHz WFTD 79.00° tall 64.0 meters tall 1080 kHz KYMN 118.60° tall 92.3 meters tall 1080 kHz WWLV 90.00° tall 47.2 meters tall 1620 kHz WTAW 204.00° tall 106.7 meters tall 1620 kHz There may be some discrepancy, but it certainly looks like antenna specification is by the electrical equivalent of the physical height (and whatever l/d fudging) and with only one happening to be 90°. Further, given most references (for professionals) is aimed at a common specification that is largely driven by this agency, it would seem odd to step out of this expectation to change to calling all antennas 90° simply because they resonate. http://www.fcc.gov/mb/audio/amq.html 73's Richard Clark, KB7QHC That's fine, no argument there. But do you agree that there are towers of X height in meters and when "naked" having Y electrical degrees, loaded with top hat of size S, not changing the physical height, but adding Z degrees. So the top hat adds some degrees to the tower. Is it such ham radio crime to say that coil can do that too, if it is inserted within the radiator? We use imaginary lumped inductor to understand coils better, but we can not use electrical degrees to 'splain the behavior of coiled antenna wire? I think we are progressing into antenna modeling and design and I see nothing wrong with using degrees to describe electrical properties (resonance) of the loaded radiator. 73 Yuri, K3BU actually WWLV 90.00° tall 47.2 meters tall 1620 kHz should show closer to 92 deg. and assuming that they use fatter tower, even more. |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
wrote: In a loading coil with very small distributed capaciatnce to the outside world compared to termination impedance, current has to be equal. Phase shift in current at each end has to be nearly zero. That is a false statement and is at the root of the misconceptions. Standing wave current does not have to be equal. I have shown how current at the bottom of the coil can be zero while the current at the top of the coil is one amp. Do you think the coil is sucking that one amp sideways from somewhere else through its distributed capacitance? There's no magic involved, just simple, easy to understand, distributed network theory. The current at the top and bottom of a coil depends upon where it is placed in the standing wave environment. Standing wave current doesn't flow. It is the underlying forward current and reflected current that is doing the flowing. Such is obvious from the equations. Hecht, in "Optics", has the best description of standing waves that I have ever read. He says: "[Equation (7.30)] is the equation for a STANDING or STATIONARY WAVE. Its profile does not move through space. ... [Its phase] doesn't rotate at all, and the resultant wave it represents doesn't progress through space - it's a standing wave." Translating into RF language. Func(kx)*Func(wt) is the equation for a STANDING or STATIONARY WAVE, i.e. the standing wave is stationary. Its magnitude does not move through the wire. Its phase doesn't rotate at all, and the resultant standing wave it represents doesn't progress through a wire or through a coil - it's a standing wave. Until everyone takes time to understand the nature of standing waves, people will keep making the same tired mistake over and over. Hecht was talking about two opposing waves of the same phase and amplitude interfering with each other. You can't guarantee, in a real antenna, that the two waves do have the same phase and magnitude. 73, Tom Donaly, KA6RUH |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
wrote: In a loading coil with very small distributed capaciatnce to the outside world compared to termination impedance, current has to be equal. Phase shift in current at each end has to be nearly zero. That is a false statement and is at the root of the misconceptions. Standing wave current does not have to be equal. I assume you are meaning that the RMS current at one physical point must not equal the RMS current at some other point. I have shown how current at the bottom of the coil can be zero while the current at the top of the coil is one amp. Do you think the coil is sucking that one amp sideways from somewhere else through its distributed capacitance? (snip) Of course that is what is happening. It is what happens in any transmission line like device. There is a standing voltage wave, also, and that produces displacement current through any capacitance, just as the antenna does. Aren't you claiming that the coil has transmission line like properties, in that it takes time for a wave to pass through it? Any such device needs two mechanisms for storing energy, one magnetic (inductive) and one electrical (capacitive). Even free space has both. If you eliminate either mechanism (or make one of them insignificant, as would happen to the capacitance if the inductor approaches zero size), you lose the transmission line like properties as the dominant mechanism. |
Current across the antenna loading coil - from scratch
On Mon, 3 Apr 2006 10:36:45 -0400, "Yuri Blanarovich"
wrote: So the top hat adds some degrees to the tower. Hi Yuri, This is simply new wine in an old bottle. The same FCC site contains top loaded antennas too. If you can find an example to support your thesis, you will still have an obscure usage. 73's Richard Clark, KB7QHC |
Current across the antenna loading coil - from scratch
Tom Donaly wrote:
Hecht was talking about two opposing waves of the same phase and amplitude interfering with each other. You can't guarantee, in a real antenna, that the two waves do have the same phase and magnitude. :-) Hecht was talking about two coherent EM waves traveling in opposite directions. We are talking about two coherent EM waves traveling in opposite directions. There is a small traveling wave component but it doesn't affect the standing wave. It is what is left over from the standing wave. This discussion has not been about coils. We need to discuss an unterminated lossless transmission line and then move on to 1/2 wavelength thin-wire standing wave antennas. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
John Popelish wrote:
Cecil Moore wrote: That is a false statement and is at the root of the misconceptions. Standing wave current does not have to be equal. I assume you are meaning that the RMS current at one physical point must not equal the RMS current at some other point. Yes, the RMS value of the standing wave current at the bottom of the coil doesn't have to bear any relationship to the RMS value of the standing wave current at the top of the coil. Aren't you claiming that the coil has transmission line like properties, in that it takes time for a wave to pass through it? Yes Any such device needs two mechanisms for storing energy, one magnetic (inductive) and one electrical (capacitive). Even free space has both. If you eliminate either mechanism (or make one of them insignificant, as would happen to the capacitance if the inductor approaches zero size), you lose the transmission line like properties as the dominant mechanism. There is no net charge carried over from cycle to cycle. There is no net storage of charge even if the steady-state RMS value of the standing wave current is zero at one end of the coil and 2 amps at the other end. The problem here is not how a coil works. The problem is how standing waves work. Forget the coil. Start with a lossless unterminated transmission line and then step up to a 1/2 wavelength thin wire dipole. It is obvious that a number of people just don't understand the nature of a standing wave that doesn't move through a wire along with its phasor that doesn't rotate relative to the source. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
John Popelish wrote: Cecil Moore wrote: That is a false statement and is at the root of the misconceptions. Standing wave current does not have to be equal. I assume you are meaning that the RMS current at one physical point must not equal the RMS current at some other point. Yes, the RMS value of the standing wave current at the bottom of the coil doesn't have to bear any relationship to the RMS value of the standing wave current at the top of the coil. Aren't you claiming that the coil has transmission line like properties, in that it takes time for a wave to pass through it? Yes Any such device needs two mechanisms for storing energy, one magnetic (inductive) and one electrical (capacitive). Even free space has both. If you eliminate either mechanism (or make one of them insignificant, as would happen to the capacitance if the inductor approaches zero size), you lose the transmission line like properties as the dominant mechanism. There is no net charge carried over from cycle to cycle. Of course. no one is talking about the red herring of charge stored over a whole cycle. Everyone (except, possibly you) is talking about charge stored and recovered twice per cycle. There is no net storage of charge even if the steady-state RMS value of the standing wave current is zero at one end of the coil and 2 amps at the other end. And no one but you brings up "net storage". We are all talking about ordinary capacitive charge storage within a cycle. And there are two equal and opposite half cycles of that. If there is Ac voltage and capacitance to the universe, there is charge storage, twice within every cycle, one positive and one negative. The problem here is not how a coil works. The problem is how standing waves work. Standing waves have AC voltage swing. That applied to capacitance causes real charge storage and retrieval. Just as it does with traveling waves. How could the standing AC voltage not charge and discharge, charge the other direction and discharge every cycle, the capacitance between the conductor and the universe? Forget the coil. Start with a lossless unterminated transmission line and then step up to a 1/2 wavelength thin wire dipole. The capacitance in a lossless transmission line is between the two conductors. For the 1/2 wavelength thin wire dipole, the capacitance is to the surroundings. But the charge stored and dumped into that capacitance twice a cycle is very similar, except that in the case of the antenna, some energy leaves in the form of radiation. It is obvious that a number of people just don't understand the nature of a standing wave that doesn't move through a wire along with its phasor that doesn't rotate relative to the source. It is obvious to me that you are one of them. Every point on a line carrying a standing wave (except the node points) has AC voltage on it, and AC current through it. The amplitude and phase of those voltages and currents can be described as a phasor, with respect to some reference phase of the same frequency. As you move along the line, the amplitude changes and when you pass through a node the phase reverses. So the phasor does not rotate with position change, except for a step change of 180 degrees at nodes, rather than smooth rotation with respect to position. For a traveling wave, every point on the line has an AC voltage on it, and an AC current passing through it. The amplitude is constant along the line, but the phasor rotates as you move along the line (the phase is linearly dependent on position). But at any single point on the line, a non rotating phasor describes the amplitude and phase with respect to a reference phase of the same frequency. |
Current across the antenna loading coil - from scratch
Richard Clark wrote:
"This must be a convention that is particular to only a very few hams. The FCC database describes AM antennas in both electrical degrees and physical height as follows." It is the convention to describe AM broadcast towers in electrical degrees. Harold Ennes reprints an RCA resistance chart for heights between 50 and 200 degrees in "AM-FM Broadcast Maintenance". Formula given is: Height in electrical degrees = Height in feet X frequency in kc X 1.016 X 10 to the minus 6 power. Example Towers: 50-degrees self-supporting: R=7. jx=-j100 50-degrees guyed mast: R=8, jx=-j222 90-degrees self-supporting: R=40, jx=+j35 90-degrees guyed mast: R=36, jx=j0 200-degrees self-supporting: R=23, jx=-j50 200-degrees guyed mast: R=80, jx=-400 There are values of R and X for 16 different heights. If you are interested, look at the book. Best regards, Richard Harrison, KB5WZI |
Current across the antenna loading coil - from scratch
John Popelish wrote:
Of course. no one is talking about the red herring of charge stored over a whole cycle. Of course, *everyone* except you and Tom Donaly are talking about charge stored over a whole cycle. That's the entire base of their arguments. The unbalance in the *RMS* current at the bottom of the coil and the *RMS* current at the top of the coil is what the entire discussion is all about. The currents measured by W8JI and W7EL were *RMS* currents. The currents reported by EZNEC are *RMS* currents. And no one but you brings up "net storage". We are all talking about ordinary capacitive charge storage within a cycle. If so, that is completely irrelevant to the discussion since W8JI and W7EL are using *RMS* currents for their measurements and EZNEC is reporting *RMS* currents. Let me summarize it for you. W8JI and W7EL apparently think that the RMS current value of zero at the bottom of the coil Vs the RMS current value of one amp at the top of the coil means energy is being sucked into the coil from some external source. How about assisting in a tutorial on standing waves rather than diverting and obfuscating the issues? -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
Tom Donaly wrote: Hecht was talking about two opposing waves of the same phase and amplitude interfering with each other. You can't guarantee, in a real antenna, that the two waves do have the same phase and magnitude. :-) Hecht was talking about two coherent EM waves traveling in opposite directions. We are talking about two coherent EM waves traveling in opposite directions. There is a small traveling wave component but it doesn't affect the standing wave. It is what is left over from the standing wave. This discussion has not been about coils. We need to discuss an unterminated lossless transmission line and then move on to 1/2 wavelength thin-wire standing wave antennas. Has it ever occurred to you, Cecil, that a half wave dipole with equal current and voltage waves traveling in opposite directions wouldn't accept power? 73, Tom Donaly, KA6RUH |
Current across the antenna loading coil - from scratch
John Popelish wrote:
. . . It is obvious to me that you are one of them. Every point on a line carrying a standing wave (except the node points) has AC voltage on it, and AC current through it. The amplitude and phase of those voltages and currents can be described as a phasor, with respect to some reference phase of the same frequency. As you move along the line, the amplitude changes and when you pass through a node the phase reverses. So the phasor does not rotate with position change, except for a step change of 180 degrees at nodes, rather than smooth rotation with respect to position. For a traveling wave, every point on the line has an AC voltage on it, and an AC current passing through it. The amplitude is constant along the line, but the phasor rotates as you move along the line (the phase is linearly dependent on position). But at any single point on the line, a non rotating phasor describes the amplitude and phase with respect to a reference phase of the same frequency. There's a potential for ambiguity here, and that ambiguity has been used a number of times in this thread to cause confusion. So let me try to clarify things. All phasors "rotate", in that every one contains an implicit term e^jwt. That term describes a rotation of the complex phasor quantity at the rotational frequency w (omega), but no change in amplitude. If a quantity doesn't include this implicit term, it's not a phasor, by definition. We can look at any phasor quantity in a system and compare the phase of its rotation with the phase of a reference, and from this assign a phase angle to it. In steady state, the phase angle doesn't change with time -- it's the phase difference between the w - rotating phasor and the w - rotating reference. Phasors of different rotational rates (that is, of different frequencies) can't be combined in the same analysis, unless the implicit term is made explicit, in which case they're no longer phasors. The use of "rotation" in John's posting is talking about a change of phase with physical position. This usage has been confused with the time rotation of the phasor which comes from the implicit e^jwt term. I'd prefer to use the term "phase", which doesn't change with time in a steady state system, directly rather than "rotation" to describe a change in phase with position. With that convention, we see that the phase of a pure traveling wave changes linearly with position. But when we sum forward and reverse traveling waves together to get a total current (or voltage), the phase of the total current (or voltage) is no longer a linear function of position. In the special case of an open or short circuited transmission line, where the forward and reverse traveling waves are equal in amplitude, the phase doesn't change with position at all (except for a periodic reversal in current and voltage direction, which can be interpreted as a 180 degree phase change). But the phasor representing total voltage or current (which Cecil refers to as "standing wave current") at any point, which is the sum of two phasors representing forward and reverse traveling waves, does indeed rotate at w (omega) radians/second rate, just like its constituent phasors. The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. This isn't revolutionary or controversial -- you can find phasors discussed in any elementary circuit analysis text.[*] And it's not difficult to do the summation of forward and reverse traveling waves to see the result, but if you'd like to see how someone else did it, one of the clearest discussions I've found is in Chipman's _Transmission Lines_, a Schaum's Outline book. [*] You have to be a little careful, though. In most introductions to phasors, the author introduces the e^jwt term early on, and quickly drops it from the phasor notation as is customary. So it's easy to forget it's there. But remembering that it is there is vital to understanding this topic, and to keep from being misled by misdirection which takes advantage of confusion and abbreviated notation. Roy Lewallen, W7EL |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
John Popelish wrote: Of course. no one is talking about the red herring of charge stored over a whole cycle. Of course, *everyone* except you and Tom Donaly are talking about charge stored over a whole cycle. Bull. That's the entire base of their arguments. The unbalance in the *RMS* current at the bottom of the coil and the *RMS* current at the top of the coil is what the entire discussion is all about. The currents measured by W8JI and W7EL were *RMS* currents. The currents reported by EZNEC are *RMS* currents. And the capacitive currents can also be measured in RMS terms. So what? And no one but you brings up "net storage". We are all talking about ordinary capacitive charge storage within a cycle. If so, that is completely irrelevant to the discussion since W8JI and W7EL are using *RMS* currents for their measurements and EZNEC is reporting *RMS* currents. Let me summarize it for you. W8JI and W7EL apparently think that the RMS current value of zero at the bottom of the coil Vs the RMS current value of one amp at the top of the coil means energy is being sucked into the coil from some external source. I don't read their responses that way. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. The path through the wire to the other end is not the only path for current. How about assisting in a tutorial on standing waves rather than diverting and obfuscating the issues? I'm trying. |
Current across the antenna loading coil - from scratch
Roy Lewallen wrote:
John Popelish wrote: . . . It is obvious to me that you are one of them. Every point on a line carrying a standing wave (except the node points) has AC voltage on it, and AC current through it. The amplitude and phase of those voltages and currents can be described as a phasor, with respect to some reference phase of the same frequency. As you move along the line, the amplitude changes and when you pass through a node the phase reverses. So the phasor does not rotate with position change, except for a step change of 180 degrees at nodes, rather than smooth rotation with respect to position. For a traveling wave, every point on the line has an AC voltage on it, and an AC current passing through it. The amplitude is constant along the line, but the phasor rotates as you move along the line (the phase is linearly dependent on position). But at any single point on the line, a non rotating phasor describes the amplitude and phase with respect to a reference phase of the same frequency. There's a potential for ambiguity here, and that ambiguity has been used a number of times in this thread to cause confusion. So let me try to clarify things. All phasors "rotate", in that every one contains an implicit term e^jwt. That term describes a rotation of the complex phasor quantity at the rotational frequency w (omega), but no change in amplitude. If a quantity doesn't include this implicit term, it's not a phasor, by definition. We can look at any phasor quantity in a system and compare the phase of its rotation with the phase of a reference, and from this assign a phase angle to it. In steady state, the phase angle doesn't change with time -- it's the phase difference between the w - rotating phasor and the w - rotating reference. Phasors of different rotational rates (that is, of different frequencies) can't be combined in the same analysis, unless the implicit term is made explicit, in which case they're no longer phasors. The use of "rotation" in John's posting is talking about a change of phase with physical position. This usage has been confused with the time rotation of the phasor which comes from the implicit e^jwt term. I'd prefer to use the term "phase", which doesn't change with time in a steady state system, directly rather than "rotation" to describe a change in phase with position. With that convention, we see that the phase of a pure traveling wave changes linearly with position. But when we sum forward and reverse traveling waves together to get a total current (or voltage), the phase of the total current (or voltage) is no longer a linear function of position. In the special case of an open or short circuited transmission line, where the forward and reverse traveling waves are equal in amplitude, the phase doesn't change with position at all (except for a periodic reversal in current and voltage direction, which can be interpreted as a 180 degree phase change). But the phasor representing total voltage or current (which Cecil refers to as "standing wave current") at any point, which is the sum of two phasors representing forward and reverse traveling waves, does indeed rotate at w (omega) radians/second rate, just like its constituent phasors. The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. This isn't revolutionary or controversial -- you can find phasors discussed in any elementary circuit analysis text.[*] And it's not difficult to do the summation of forward and reverse traveling waves to see the result, but if you'd like to see how someone else did it, one of the clearest discussions I've found is in Chipman's _Transmission Lines_, a Schaum's Outline book. [*] You have to be a little careful, though. In most introductions to phasors, the author introduces the e^jwt term early on, and quickly drops it from the phasor notation as is customary. So it's easy to forget it's there. But remembering that it is there is vital to understanding this topic, and to keep from being misled by misdirection which takes advantage of confusion and abbreviated notation. Excellent! |
Current across the antenna loading coil - from scratch
Cecil Moore wrote: Let me summarize it for you. W8JI and W7EL apparently think that the RMS current value of zero at the bottom of the coil Vs the RMS current value of one amp at the top of the coil means energy is being sucked into the coil from some external source. John Popelish wrote: I don't read their responses that way. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. The path through the wire to the other end is not the only path for current. You read what I wrote and what Roy wrote correctly John. Cecil changes what other people write to suit his own needs. He changes what other people say, and then points out why the creatively edited text he invented is wrong. That's his debating style. Watch out for it! 73 Tom |
Current across the antenna loading coil - from scratch
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Current across the antenna loading coil - from scratch
Tom Donaly wrote:
Has it ever occurred to you, Cecil, that a half wave dipole with equal current and voltage waves traveling in opposite directions wouldn't accept power? It is an approximation, Tom, like a lossless line. For real world dipoles, the voltage and current decay by about 10% between the forward wave and the arrival of the reflected wave. Kraus and Terman both use that approximation in their examples. We aren't saying anything about the traveling wave part of the waves. The discussion is about the standing wave portion of the wave which, by definition, requires equal magnitudes. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
Roy Lewallen wrote:
The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. Yes, when I said standing wave current phase doesn't rotate, I meant with respect to the source current phase. At any instant in time, the phase of the standing wave current is unchanging up and down the line. Assume the standing wave current all up and down the dipole is of constant phase with no variation with 'x'. Roy, you used that current to try to measure the delay through a coil. How did you plan to measure that delay with a signal known to be the same phase not only at both ends of the coil but all up and down the antenna? -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
John Popelish wrote:
Cecil Moore wrote: Of course, *everyone* except you and Tom Donaly are talking about charge stored over a whole cycle. Bull. If that's what you think and you can find someone to discuss energy exchange within a cycle, be my guest. As far as I know, Tom Donaly introduced the subject as a diversion. I don't read their responses that way. I couldn't believe it either but after years of arguing with them, it is apparent that many of the gurus here on r.r.a.a are simply ignorant of the nature of standing waves. I really expected them to shout, "April Fool, we have been pulling your leg!" But, sad to say, they are serious about standing wave current "flowing" into the bottom of the coil and out the top. They apparently haven't read "Optics", by Hecht where he says: "E(x,t) = 2Eo*sin(kx)*cos(wt) This is the equation for a standing wave, as opposed to a traveling wave. Its profile does not move through space. ... [The standing wave] phasor doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." If standing waves of light don't move through space, standing waves of RF don't move through a wire. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. That is a secondary effect. The primary effect is the phasor addition of the forward current and reflected current which you provided. Compared to zero amps of standing wave current when the forward current phasor and the reflected current phasor are 180 degrees out of phase, just how much effect can capacitance have? -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
wrote:
Cecil changes what other people write to suit his own needs. He changes what other people say, and then points out why the creatively edited text he invented is wrong. That's his debating style. Watch out for it! Tom, do you or do you not believe that standing wave current flows into the bottom of the coil and out the top? It's a simple yes/no question. You have earlier stated that "current is current" and that, in spite of the different equations for for the two currents, standing wave current moves like traveling wave current in spite of what Hecht had to say in "Optics". "E(x,t) = 2*Eo*sin(kx)*cos(wt) This is the equation for a STANDING or STATIONARY WAVE, as opposed to a traveling wave. Its profile does not move through space; it is clearly not of the form Func(x +/- vt). .... [Standing wave phase] doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." If standing waves of light don't move through space, standing waves of RF don't move through wires. Time to stop the ad hominem attacks and address this technical issue. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
Roy Lewallen wrote:
Absolutely true. Cecil complains that people won't engage in a technical discussion with him. Many have tried, and all we get in response is evasion, misquotes, diversion, and brushing off of any evidence contrary to his preconceived notions. Roy, I'll tell you the same thing I told W8JI. It's time to stop the ad hominem attacks and discuss the technical issues. You tried to use standing wave current, containing no phase, to measure the delay through a coil. You have said previously that standing wave current flows just like traveling wave current. You said that in spite of what Hecht says in "Optics". Hecht had to say in "Optics": "E(x,t) = 2*Eo*sin(kx)*cos(wt) This is the equation for a STANDING or STATIONARY WAVE, as opposed to a traveling wave. Its profile does not move through space; it is clearly not of the form Func(x +/- vt). .... [Standing wave phase] doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." If standing wave light doesn't move through space, then standing wave RF doesn't move through a wire. Do you disagree with Hecht? -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
(snip) Compared to zero amps of standing wave current when the forward current phasor and the reflected current phasor are 180 degrees out of phase, just how much effect can capacitance have? A standing wave voltage passes exactly as much (AC RMS) current through a capacitance as a traveling wave voltage does. If there is voltage at the ends of the coil, then there is capacitive current driven by those voltages, regardless of whether the voltage is from a single traveling wave or the superposition of two of them. |
Current across the antenna loading coil - from scratch
Cecil wrote, "It's time to stop
the ad hominem attacks and discuss the technical issues." Ready when you are, lad. Suggest you start by establishing just how it is that an antenna wire supports waves. Gauss's theorem and Faraday's law may come in handy. Please don't spare anything. The reason you need to do this for me to even begin to believe you have any idea what you are talking about is that you have rejected out of hand some very fundamental concepts that I've put numbers on for you. You ask for something, and then you reject the answer but give no valid reason why. I've tried to give you a way to SUPPORT what you are saying, and you can't even recognize that, apparently. Now YOU go back to the real fundamentals and give it to us straight, with full math treatment. Until you do, as far as I'm concerned, you don't have a leg to stand on. If you can do a credible job starting with Maxwell's equations, I might begin to believe you have some understanding of the subject. And I don't want it parroted from someone else's writing, I want it done from the ground up by you. If you have some trouble doing that with an antenna wire, just try it with an ideal coaxial TEM line. It's easy there; I've done it out of idle curiosity one evening, and it was quite enlightening to see how nicely it all agreed with what I already knew about propagation along a line. Lay it on us, Cecil. Start with the fundamentals. And don't be dragging out that tired old travelling-waves/standing-waves stuff till you've established that you actually can even have waves, and just what it is that governs their behaviour. Cheers, Tom |
Current across the antenna loading coil - from scratch
John Popelish wrote:
Cecil Moore wrote: Compared to zero amps of standing wave current when the forward current phasor and the reflected current phasor are 180 degrees out of phase, just how much effect can capacitance have? A standing wave voltage passes exactly as much (AC RMS) current through a capacitance as a traveling wave voltage does. But the two waves are different as can be seen from their equations. A traveling wave transfers net energy along a transmission line or antenna wire. A standing wave transfers zero net energy along a transmission line or antenna wire. From "Fields and Waves in Modern Radio", by Ramo & Whinnery, 2nd edition, page 43: "The total energy in any length of line a multiple of a quarterwavelength long is constant, merely interchanging between energy in the electric field of the voltages and energy in the magnetic field of the currents." Hecht says it best in "Optics" concerning standing waves: "The composite disturbance is then: E = Eo[sin(kx+wt) + sin(kx-wt)] Applying the identity: sin A + sin B = 2 sin 1/2(A+B)*cos 1/2(A-B) yields: E(x,t) = 2*Eo*sin(kx)*cos(wt)" "This is the equation for a STANDING or STATIONARY WAVE, as opposed to a traveling wave. Its profile does not move through space; it is clearly not of the form Func(x +/- vt)." [Standing wave phase] "doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." Speaking of "... net transfer of energy, for the pure standing wave there is none." -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
K7ITM wrote:
The reason you need to do this for me to even begin to believe you have any idea what you are talking about is that you have rejected out of hand some very fundamental concepts that I've put numbers on for you. I've rejected your obvious attempts at logical diversions. Lay it on us, Cecil. Start with the fundamentals. And don't be dragging out that tired old travelling-waves/standing-waves stuff till you've established that you actually can even have waves, and just what it is that governs their behaviour. :-) Just one more attempt at a logical diversion. I think we can all assume that EM waves exist and are capable of propagating along a transmission line, or antenna wire, or even in free space, e.g. light. What we cannot assume is that standing waves move or progress through space (or wire). Eugene Hecht says they don't. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
but we can not
use electrical degrees to 'splain the behavior of coiled antenna wire? I can see how problems could arise going by the length of coil wire length in degrees only. Lets say you run a coil 1 foot from the base. Lets say that coil uses 25 turns to tune a particular frequency. Now, move the coil up 2 ft higher, and see if that same 25 turns will tune the same frequency. It won't. You will have to add a few more turns. So just going by the total mast plus coil wire length in degrees could vary all over the map just by changing the position of the coil. As you raise the coil, you will have to add more and more of "degrees" of wire to tune the same frequency. :/ Dunno...There may well be some variation of current from the bottom vs the top of the coil, but overall, I still view the operation of a loading coil as a "lumped" mechanism overall. Even if you all decide that the current changes, or it doesn't , it ain't gonna make a hoot's worth of difference in the design of mobile whips. I think it's an argument that has no real value to me as far as mobile whips go. The performance of all the various coil heights, and configs have been well known for years. Coil current taper or not. I just don't see the facination with arguing about something that even if decided one way or the other, still won't make any difference in the final antenna design. Oh well...Continue the tail chasing excercise.... I'm outa this one... One post is all I will waste on this subject.. I couldn't mount my coil much higher if I wanted to... Current taper or not. :/ MK |
Current across the antenna loading coil - from scratch
Cecil Moore wrote:
John Popelish wrote: Cecil Moore wrote: Compared to zero amps of standing wave current when the forward current phasor and the reflected current phasor are 180 degrees out of phase, just how much effect can capacitance have? A standing wave voltage passes exactly as much (AC RMS) current through a capacitance as a traveling wave voltage does. But the two waves are different as can be seen from their equations. A traveling wave transfers net energy along a transmission line or antenna wire. A standing wave transfers zero net energy along a transmission line or antenna wire. From "Fields and Waves in Modern Radio", by Ramo & Whinnery, 2nd edition, page 43: "The total energy in any length of line a multiple of a quarterwavelength long is constant, merely interchanging between energy in the electric field of the voltages and energy in the magnetic field of the currents." Hecht says it best in "Optics" concerning standing waves: "The composite disturbance is then: E = Eo[sin(kx+wt) + sin(kx-wt)] Applying the identity: sin A + sin B = 2 sin 1/2(A+B)*cos 1/2(A-B) yields: E(x,t) = 2*Eo*sin(kx)*cos(wt)" "This is the equation for a STANDING or STATIONARY WAVE, as opposed to a traveling wave. Its profile does not move through space; it is clearly not of the form Func(x +/- vt)." [Standing wave phase] "doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." Speaking of "... net transfer of energy, for the pure standing wave there is none." Cecil, how can you quote Hecht when you don't have the foggiest notion what he's talking about? Here's a more general equation for you Cecil: (A1-A2)*Cos(wt-kx) + 2*A2*Cos(kx+d/2)*Cos(wt+d/2). Do you have any idea what it should represent? Does it satisfy the wave equation? Does it represent anything real? Sit and think about it before you get hysterical. 73, Tom Donaly, KA6RUH |
Current across the antenna loading coil - from scratch
Cecil Moore wrote: Time to stop the ad hominem attacks and address this technical issue. -- 73, Cecil http://www.qsl.net/w5dxp I'm glad to hear that. When you show a track record of being honest and you stop those attacks and your constant distortions of what other people say, I'm sure people will start talking to you again. 73 Tom |
Current across the antenna loading coil - from scratch
wrote:
I just don't see the facination with arguing about something that even if decided one way or the other, still won't make any difference in the final antenna design. The sun still rises every morning whether it is caused by the earth's rotation or by the Sun God riding his chariot across the sky. The facination is called technical correctness. -- 73, Cecil http://www.qsl.net/w5dxp |
Current across the antenna loading coil - from scratch
"Richard Harrison" wrote:
It is the convention to describe AM broadcast towers in electrical degrees. Harold Ennes reprints an RCA resistance chart for heights between 50 and 200 degrees in "AM-FM Broadcast Maintenance". Formula given is: Height in electrical degrees = Height in feet X frequency in kc X 1.016 X 10 to the minus 6 power. _______________ If electrical length is defined as the physical condition where feedpoint reactance is zero (e.g., resonance), then the true electrical length of an AM broadcast radiator on a given frequency is a function of the physical length AND physical width of that radiator. This was proven experimentally, and documented by George Brown of RCA Labs in his paper "Experimentally Determined Impedance Characteristics of Cylindrical Antennas" published in the Proceedings of the I.R.E. in April, 1945. It also has been proven in thousands of independent measurements of AM broadcast radiators ever since. The curves in Figure 3 of Brown's paper show the feedpoint reactance terms of the base impedance of an unloaded monopole of various lengths and widths, working against a nearly perfect ground plane. Those values cross the zero reactance axis at physical heights ranging from about 80 degrees (for the widest radiator) to about 86 degrees for the most narrow. Brown calculated height in degrees as (Physical Height in feet x Frequency in kHz ) / 2725 . Brown's equation, the one in the Harold Ennes quote above, and the one that the FCC uses in their published data all define only the relationship of the physical length of the radiator to its free-space wavelength in degrees at that frequency. But clearly these lengths in degrees do not define the self-resonant length of that radiator. The self-resonant length, invariably, will be shorter by several percent. This fact is easily confirmed by simple NEC models, for those who want to probe into George Brown's data. Tables relating a single value of base impedance as typical for towers of various electrical heights (only) must be read with an understanding of the above realities. For example, Ennes' list shows a tower of 90 electrical degrees to have zero reactance. But Brown's 1945 paper and a great amount of later field experience shows that this is incorrect, for the conventional use of this term. RF |
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