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Distributed capacitance effects Q?
Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency? It would seem that a loop antenna with 100pF of winding capacitance in parallel with a external capacitor of 200pF would resonate at the same frequency as a antenna with no winding capacitance and a external capacitor of 300pF,but apparently that's not the case. The best explanation I got was that winding capacitance represents 'low Q' and a external tuning capacitor represents ' High Q' What is the difference between high and low Q, and why should a loop antenna with no winding capacitance perform any better than one with 50% of the total capacitance in the windings? Where is the energy loss? Thanks, -Bill |
Distributed capacitance effects Q?
On 28 Apr 2007 21:32:18 -0700, Bill Bowden
wrote: Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? Hi Bill, For the usual reasons: Resistance (not capacitance). It would seem that a loop antenna with 100pF of winding capacitance in parallel with a external capacitor of 200pF would resonate at the same frequency as a antenna with no winding capacitance and a external capacitor of 300pF,but apparently that's not the case. It could be the case, your mileage may vary. The best explanation I got was that winding capacitance represents 'low Q' and a external tuning capacitor represents ' High Q' You got bum explanations then. What is the difference between high and low Q, and why should a loop antenna with no winding capacitance perform any better than one with 50% of the total capacitance in the windings? Where is the energy loss? It seems you may be, instead, writing about Unloaded and Loaded Q. Loaded Q would be that found in service (in the actual application, whatever that might be). Unloaded Q would be that found at the bench with no other attachments. The Loaded Q's lower value is due to the R of the "load" ...as it stands to reason. That load will be an antenna's radiation resistance (and any Ohmic loss of the structure). The energy loss is called radiation - if you do it right. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
On 29 abr, 06:32, Bill Bowden wrote:
Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? It would seem that a loop antenna with 100pF of winding capacitance in parallel with a external capacitor of 200pF would resonate at the same frequency as a antenna with no winding capacitance and a external capacitor of 300pF,but apparently that's not the case. The best explanation I got was that winding capacitance represents 'low Q' and a external tuning capacitor represents ' High Q' What is the difference between high and low Q, and why should a loop antenna with no winding capacitance perform any better than one with 50% of the total capacitance in the windings? Where is the energy loss? Thanks, -Bill Hello Bill, I assume that you mean radiation efficiency (ratio between actual radiated power and total electrical input power). I think inter-winding capacitance does not decrease efficiency, it may only change the radiation pattern when the inter-winding capacitance is that much, that the current distribution in the coil is affected. This is almost the case with relative large loops. When you have a loop close to a halve wave, just the own capacitance is sufficient to get resonance (as with, for example, a halve wave dipole). Radiation efficiency may be reduced by losses in the insulation. When windings are close together, the Electric Field strength in the insulation can be that high, that loss becomes significant. This is mostly the case when windings are touching. Another thing can be corona discharge (that may in the end destroy your insulation). Best regards, Wim PA3DJS |
Distributed capacitance effects Q?
"Bill Bowden" wrote in message oups.com... Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? -Bill Hi Bill. I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Anyone care to run with that, or explain it more clearly, or shoot it down. Mike |
Distributed capacitance effects Q?
Bill Bowden wrote:
It would seem that a loop antenna with 100pF of winding capacitance in parallel with a external capacitor of 200pF would resonate at the same frequency as a antenna with no winding capacitance and a external capacitor of 300pF, but apparently that's not the case. The "100pF of winding capacitance" is NOT across the entire coil as is the 200pF external capacitor. When the operating frequency of a coil is more than ~15% of the self-resonant frequency, the lumped circuit model starts to fall apart. In your above example, the operating frequency is ~60% of the self-resonant frequency so you need to use the distributed network model (or Maxwell's equations). Quoting from an IEEE white paper about RF coils at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf "... lumped element circuit theory does not (and cannot) accurately embody a world of second order partial differential equations in space and time." "The concept of coil 'self-capacitance' is an attempt to circumvent transmission line effects on small coils when the current distribution begins to depart from its DC behavior. The notion has been developed by starting with Maxwell's equations and using only the first two terms in the Taylor series expansion for the distributed current to obtain an expression for the self-impedance of a generalized closed circuit. Upon extracting Neumann's formula for the self inductance, the remaining negative component of the reactance permits an expression for the coil self-capacitance. These formulae are valid for a PARALLEL combination of an inductance and a capacitance when the operating frequency is well below 1/SQRT(L*CL). They permit a coil with a SLIGHTLY nonuniform current distribution to be treated AS THOUGH THE CURRENT WERE UNIFORM and the coil was shunted with a lumped element capacitance." The author shows how to estimate the VF and Z0 of a coil that is operated at more than 15% of its self-resonant frequency. It can thus be modeled as a transmission line. The same author shows in his class notes at: http://www.ttr.com/corum/index.htm that the calculated self-resonant frequency of a particular coil based on the measured self-capacitance was in error by 65.2% when the "lumped-element assumption" was used. The calculated self-resonant frequency based on the transmission line distributed network model was within 5% of the measured self-resonant frequency. -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
amdx wrote:
Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) Reminds me of a transmission line distributed network for which a velocity factor can be calculated. Anyone care to run with that, or explain it more clearly, or shoot it down. Please see my other reply where an IEEE white paper agrees with you. -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
Wimpie wrote:
I think inter-winding capacitance does not decrease efficiency, it may only change the radiation pattern when the inter-winding capacitance is that much, that the current distribution in the coil is affected. This is almost the case with relative large loops. This is almost *always* the case with relatively large loops? -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
On Sun, 29 Apr 2007 06:52:17 -0500, "amdx" wrote:
I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Hi Mike, Capacitance does not bring loss. Loss ALWAYS resides in Resistance and nothing else. Between you and Bill, there appears to be a fixation on the loopS (emphasis on there being more than one). If you are going to blame them (that emphasis on there being more than one), and try to tie it to loss (that emphasis being naturally in Resistance, not Capacitance); then it follows it is in the natural increase in conductor Resistance that occurs when wires are spaced closer than 3 or 4 wire diameters to each other. When wires (or loops in this case) are in close proximity, the magnetic field of the near wire (or loop in this case, and each loop in proximity to the next) FORCES the current in that loop to the surface of the wire - INCREASING that conductor's Skin Resistance. Loss thus increases by proximity. Capacitance does too, but that is merely a correlating factor. Remember (and this is good advice, especially suited to Newsgroup rumors you may pick up): Correlation is NOT causality. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
On 29 abr, 15:50, Cecil Moore wrote:
Wimpie wrote: I think inter-winding capacitance does not decrease efficiency, it may only change the radiation pattern when the inter-winding capacitance is that much, that the current distribution in the coil is affected. This is almost the case with relative large loops. This is almost *always* the case with relatively large loops? -- 73, Cecil http://www.w5dxp.com Hello, Cecil, Yes you are right, as soon as electric flux is leaking via inter winding capacitance, the current distribution is no longer uniform. Maybe Bill can find more info when searching for Tesla coil inductors. I made a small one myself (H-bridge, running at about 700 kHz, [yes, I know it is in the AM broadcast band]). The vertical coil behaves almost as a quarter wave resonator, just a small top capacitor was necessary. Best regards and thanks for the correction. Wim PA3DJS |
Distributed capacitance effects Q?
On Apr 29, 6:47 am, Cecil Moore wrote:
amdx wrote: Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) Reminds me of a transmission line distributed network for which a velocity factor can be calculated. Cecil - I think this will interest you: http://www.rhombus-ind.com/dlcat/app1_pas.pdf 73, ac6xg |
Distributed capacitance effects Q?
"Richard Clark" wrote in message ... On Sun, 29 Apr 2007 06:52:17 -0500, "amdx" wrote: I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Hi Mike, Capacitance does not bring loss. I'm not ready to give on that yet, but I could be convinced. It seems I could add capacitors across turns of a coil and increase circulating currents that would show as a lower Q. But I haven't built a coil to test this. Loss ALWAYS resides in Resistance and nothing else. I agree, X/R=Q Lower Q means more loss. (let's not get into radiation resistance right now) Between you and Bill, there appears to be a fixation on the loopS (emphasis on there being more than one). If you are going to blame them (that emphasis on there being more than one), and try to tie it to loss (that emphasis being naturally in Resistance, not Capacitance); then it follows it is in the natural increase in conductor Resistance that occurs when wires are spaced closer than 3 or 4 wire diameters to each other. When wires (or loops in this case) are in close proximity, the magnetic field of the near wire (or loop in this case, and each loop in proximity to the next) FORCES the current in that loop to the surface of the wire - INCREASING that conductor's Skin Resistance. Loss thus increases by proximity. Capacitance does too, but that is merely a correlating factor. Proximity effect could cause all of the additional losses. Or it might just be part of the additional losses. Why is it that when you get near self resonance of a coil the Q gets lower? Note; to help clearify my question, ( as you get nearer and nearer resonance the capacitor you are using to tune the inductor is getting smaller and smaller, and closer to equalling the self capacitance of the inductor) Remember---Correlation is NOT causality. If you measure reading skills in an elementary school you will find the kids with big feet read better. But then 5th graders usually have bigger feet than kindergarteners. Thanks for the discussion____ Mike |
Distributed capacitance effects Q?
"amdx" wrote in message ... "Richard Clark" wrote in message ... On Sun, 29 Apr 2007 06:52:17 -0500, "amdx" wrote: I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Hi Mike, Capacitance does not bring loss. I'm not ready to give on that yet, but I could be convinced. It seems I could add capacitors across turns of a coil and increase circulating currents that would show as a lower Q. But I haven't built a coil to test this. Loss ALWAYS resides in Resistance and nothing else. I agree, X/R=Q Lower Q means more loss. (let's not get into radiation resistance right now) Between you and Bill, there appears to be a fixation on the loopS (emphasis on there being more than one). If you are going to blame them (that emphasis on there being more than one), and try to tie it to loss (that emphasis being naturally in Resistance, not Capacitance); then it follows it is in the natural increase in conductor Resistance that occurs when wires are spaced closer than 3 or 4 wire diameters to each other. When wires (or loops in this case) are in close proximity, the magnetic field of the near wire (or loop in this case, and each loop in proximity to the next) FORCES the current in that loop to the surface of the wire - INCREASING that conductor's Skin Resistance. Loss thus increases by proximity. Capacitance does too, but that is merely a correlating factor. Proximity effect could cause all of the additional losses. Or it might just be part of the additional losses. Why is it that when you get near self resonance of a coil the Q gets lower? Note; to help clearify my question, ( as you get nearer and nearer resonance the capacitor you are using to tune the inductor is getting smaller and smaller, and closer to equalling the self capacitance of the inductor) Remember---Correlation is NOT causality. If you measure reading skills in an elementary school you will find the kids with big feet read better. But then 5th graders usually have bigger feet than kindergarteners. Thanks for the discussion____ Mike Hi Mike I am curious about how the comment in your post --- It seems I could add capacitors across turns of a coil and increase circulating currents that would show as a lower Q. But I haven't built a coil to test this. I would have thought that, when the circulating current increases when a passive devce is introduced, the Q would have Increased. Jerry |
Distributed capacitance effects Q?
On Sun, 29 Apr 2007 14:30:29 -0500, "amdx" wrote:
Capacitance does not bring loss. I'm not ready to give on that yet, but I could be convinced. It seems I could add capacitors across turns of a coil and increase circulating currents that would show as a lower Q. But I haven't built a coil to test this. Hi Mike, This is then a characteristic of the Capacitor called D (dissipation). Any increase in current tied to loss immediately goes to the bottom line of resistance - it is a square law relationship, after all. Loss ALWAYS resides in Resistance and nothing else. I agree, X/R=Q Lower Q means more loss. (let's not get into radiation resistance right now) Why not? Small loops suffer by comparison, and multi-turn loops even more so. Proximity effect could cause all of the additional losses. Or it might just be part of the additional losses. For wire separations beyond 3 or 4 wire diameters, the increase in skin effect is small. It might be noted that interwinding Capacitance also falls. Why is it that when you get near self resonance of a coil the Q gets lower? Note; to help clearify my question, ( as you get nearer and nearer resonance the capacitor you are using to tune the inductor is getting smaller and smaller, and closer to equalling the self capacitance of the inductor) Again, the answer must reside in Resistance. There are many characteristics (wavelength, solenoid diameter, length, pitch, wire gauge, self-capacitance, distributed capacitance, balance, connections, earth proximity, radiation resistance) being juggled with small Loop antennas and some (even many) choices that can be made to resonate the antenna do not lead to an efficient solution. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
"Richard Clark" wrote in message ... On Sun, 29 Apr 2007 14:30:29 -0500, "amdx" wrote: Capacitance does not bring loss. I'm not ready to give on that yet, but I could be convinced. It seems I could add capacitors across turns of a coil and increase circulating currents that would show as a lower Q. But I haven't built a coil to test this. Hi Mike, This is then a characteristic of the Capacitor called D (dissipation). Any increase in current tied to loss immediately goes to the bottom line of resistance - it is a square law relationship, after all. So your saying yes, the thought experiment would show more loss, but the loss is in the capacitor. The loss in a capacitor would be dielectric and loss in the plates right? Loss ALWAYS resides in Resistance and nothing else. I agree, X/R=Q Lower Q means more loss. (let's not get into radiation resistance right now) Why not? Small loops suffer by comparison, and multi-turn loops even more so. I figure it would only confuse the issue. I was trying to stay away from radiation resistance because my experience of the effect Bill ask about has been with small aircore inductors. But on second thought even those have Rr. Proximity effect could cause all of the additional losses. Or it might just be part of the additional losses. For wire separations beyond 3 or 4 wire diameters, the increase in skin effect is small. It might be noted that interwinding Capacitance also falls. Why is it that when you get near self resonance of a coil the Q gets lower? Note; to help clearify my question, ( as you get nearer and nearer resonance the capacitor you are using to tune the inductor is getting smaller and smaller, and closer to equalling the self capacitance of the inductor) Again, the answer must reside in Resistance. There are many characteristics (wavelength, solenoid diameter, length, pitch, wire gauge, self-capacitance, distributed capacitance, balance, connections, earth proximity, radiation resistance) being juggled with small Loop antennas and some (even many) choices that can be made to resonate the antenna do not lead to an efficient solution. Richard, I don't think anyone would disagree that the losses are resistive. You seem to have answered the question I posted by saying it's increased resistance. Yes I agree, Why does the reistance go up near resonance? Mike |
Distributed capacitance effects Q?
On Sun, 29 Apr 2007 15:36:43 -0500, "amdx" wrote:
This is then a characteristic of the Capacitor called D (dissipation). Any increase in current tied to loss immediately goes to the bottom line of resistance - it is a square law relationship, after all. So your saying yes, the thought experiment would show more loss, but the loss is in the capacitor. The loss in a capacitor would be dielectric and loss in the plates right? Hi Mike, Depending upon construction, most assuredly. However, little loss is found in dielectrics (unless you are using particularly crummy examples). For bad dielectric, you can expects arcs and sparks followed by carbon, and then catastrophic heat accumulation. Most lost is in what is specified in ESR (effective series resistance) which you have already identified as in the plates, but often more in the leads and their connections to the plates. To pack in more capacitance, the trend is for thinner plates for a given package volume. You can guess where the resistance will rise there when the circulating currents are see-sawing in that thin metal. (let's not get into radiation resistance right now) Why not? Small loops suffer by comparison, and multi-turn loops even more so. I figure it would only confuse the issue. I was trying to stay away from radiation resistance because my experience of the effect Bill ask about has been with small aircore inductors. But on second thought even those have Rr. The smaller, the worse. It is not so much about the size of Rr, but its relation (ratio) to Ohmic loss. For instance, a 1 meter loop composed of #40 wire is going to be deaf and dumb at 80M, but you might have a chance with 10cM hollow pipe with tight connections. Both exhibit the same Rr, but the wire's Ohmic loss is clearly deadly in comparison to it, than for the pipe's Ohmic loss. Rr in this band, for this size, runs on the order of 0.0075 Ohms. Why does the reistance go up near resonance? I haven't seen that happen. However, for the same resistance, as you approach resonance, the circulating currents climb, and loss is by the square. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
"Richard Clark" wrote in message ... On Sun, 29 Apr 2007 15:36:43 -0500, "amdx" wrote: This is then a characteristic of the Capacitor called D (dissipation). Any increase in current tied to loss immediately goes to the bottom line of resistance - it is a square law relationship, after all. So your saying yes, the thought experiment would show more loss, but the loss is in the capacitor. The loss in a capacitor would be dielectric and loss in the plates right? Hi Mike, Depending upon construction, most assuredly. However, little loss is found in dielectrics (unless you are using particularly crummy examples). For bad dielectric, you can expects arcs and sparks followed by carbon, and then catastrophic heat accumulation. Most lost is in what is specified in ESR (effective series resistance) which you have already identified as in the plates, but often more in the leads and their connections to the plates. To pack in more capacitance, the trend is for thinner plates for a given package volume. You can guess where the resistance will rise there when the circulating currents are see-sawing in that thin metal. I gave you a little bit of a trick question when I ask, The loss in a capacitor would be dielectric and loss in the plates right? In my inductor the interwinding capacitance is made of a dielectric (some type of insulation and air) and the plates (made by the wire). The wire has more current because of that interwinding capacitance, and as you say "loss is by the square". Is my argument moving you at all? (let's not get into radiation resistance right now) Why not? Small loops suffer by comparison, and multi-turn loops even more so. I figure it would only confuse the issue. I was trying to stay away from radiation resistance because my experience of the effect Bill ask about has been with small aircore inductors. But on second thought even those have Rr. The smaller, the worse. It is not so much about the size of Rr, but its relation (ratio) to Ohmic loss. For instance, a 1 meter loop composed of #40 wire is going to be deaf and dumb at 80M, but you might have a chance with 10cM hollow pipe with tight connections. Both exhibit the same Rr, but the wire's Ohmic loss is clearly deadly in comparison to it, than for the pipe's Ohmic loss. Rr in this band, for this size, runs on the order of 0.0075 Ohms. Why does the resistance go up near resonance? I haven't seen that happen. Try measureing the Q of an aircore coil close to it's self resonance (or worse, at self resonance without an additional capacitor) and then at half that frequency. However, for the same resistance, as you approach resonance, the circulating currents climb, and loss is by the square. I'm defining circulating currents as those that circulate between turns and don't necessarily go through the capacitor used to resonate the coil. Does that fit your definition as used in your paragraph above? Thanks, Mike |
Distributed capacitance effects Q?
Wimpie wrote:
The vertical coil behaves almost as a quarter wave resonator, just a small top capacitor was necessary. Sounds like a 75m mobile bugcatcher antenna. :-) -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
Jim Kelley wrote:
I think this will interest you: http://www.rhombus-ind.com/dlcat/app1_pas.pdf Thanks very much, Jim. -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
On Sun, 29 Apr 2007 18:51:52 -0500, "amdx" wrote:
I gave you a little bit of a trick question when I ask, The loss in a capacitor would be dielectric and loss in the plates right? In my inductor the interwinding capacitance is made of a dielectric (some type of insulation and air) and the plates (made by the wire). The wire has more current because of that interwinding capacitance, and as you say "loss is by the square". Is my argument moving you at all? Hi Mike, I'm afraid that if you have expressed an argument, it was lost on me. Why does the resistance go up near resonance? I haven't seen that happen. Try measureing the Q of an aircore coil close to it's self resonance (or worse, at self resonance without an additional capacitor) and then at half that frequency. You have a moving target. Skin effect is shifting as you double/halve the frequency. What does it mean to compare Q at so disparate frequencies? Are you exploring an intellectual curiosity or trying to remedy a defect in application? However, for the same resistance, as you approach resonance, the circulating currents climb, and loss is by the square. I'm defining circulating currents as those that circulate between turns and don't necessarily go through the capacitor used to resonate the coil. Does that fit your definition as used in your paragraph above? Going between turns can be through a turn-to-turn capacitive coupling, the magnetic coupling has already been discussed in regard to increased skin effect due to proximity. Loss still remains the province of resistance. Your best argument is that Capacitance exacerbates loss, but it does not cause it. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
"Richard Clark" wrote in message ... On Sun, 29 Apr 2007 18:51:52 -0500, "amdx" wrote: I gave you a little bit of a trick question when I ask, The loss in a capacitor would be dielectric and loss in the plates right? In my inductor the interwinding capacitance is made of a dielectric (some type of insulation and air) and the plates (made by the wire). The wire has more current because of that interwinding capacitance, and as you say "loss is by the square". Is my argument moving you at all? Hi Mike, I'm afraid that if you have expressed an argument, it was lost on me. Why does the resistance go up near resonance? I haven't seen that happen. Try measureing the Q of an aircore coil close to it's self resonance (or worse, at self resonance without an additional capacitor) and then at half that frequency. You have a moving target. Skin effect is shifting as you double/halve the frequency. What does it mean to compare Q at so disparate frequencies? I agree that skin effect is just one more charactistic that needs to be added to the mix. Are you exploring an intellectual curiosity or trying to remedy a defect in application? No, I just have experienced the effect that Bill ask about and gave my own pet theory about why it happens. Now I'm looking for a little confirmation or where I went wrong. However, for the same resistance, as you approach resonance, the circulating currents climb, and loss is by the square. I'm defining circulating currents as those that circulate between turns and don't necessarily go through the capacitor used to resonate the coil. Does that fit your definition as used in your paragraph above? Going between turns can be through a turn-to-turn capacitive coupling, the magnetic coupling has already been discussed in regard to increased skin effect due to proximity. Loss still remains the province of resistance. Richard, That's like saying rain has water in it. No matter how many times you say it, I'm still going to agree with you. Your best argument is that Capacitance exacerbates loss. I would rephrase that as "interwinding capacitance exacerbates loss". And with that, you have summed up my argument perfectly. You have helped reduce my argument to 4 words. Now, do you agree that interwinding capacitance will reduce Q? (yes, I know it's the province of resistance) Thanks, Mike |
Distributed capacitance effects Q?
On Mon, 30 Apr 2007 06:37:10 -0500, "amdx" wrote:
Are you exploring an intellectual curiosity or trying to remedy a defect in application? No, I just have experienced the effect that Bill ask about and gave my own pet theory about why it happens. Now I'm looking for a little confirmation or where I went wrong. Hi Mike, Well, that is fine and good, but neither of you have given us any real data, and certainly no Q values to judge if what you both experienced was within the range of "normal" or out in left field. RF measurements are difficult to do to any particularly fine accuracy, and what was observed may have been simple variation due to the measurer's proximity (offering just one of many things that can go wrong). Loss still remains the province of resistance. Richard, That's like saying rain has water in it. No matter how many times you say it, I'm still going to agree with you. Then this diverges from Bill's premise of Capacitance being the source of loss and you and he are separable at this point of your common experience. Your best argument is that Capacitance exacerbates loss. I would rephrase that as "interwinding capacitance exacerbates loss". And with that, you have summed up my argument perfectly. You have helped reduce my argument to 4 words. Now, do you agree that interwinding capacitance will reduce Q? (yes, I know it's the province of resistance) Give me some metrics to show it is not skin effect. The issue at hand is your (both you and Bill, or either of you separately) loops keep changing to fit to the loss rather than to the application. It makes for a rather strained progression of design as loops are added, proximity becomes a greater issue, as coil length collapses, insulation is added, and as frequency shifts to follow these changes. It is as though a good 10M loop is evolving to operate poorly there or, worse, in the 160M band where its resonance has finally come to rest through optimizing for loss. I can imagine there being enough turn-to-turn capacitance to induce large currents, but so many correlating factors would have to ride along with this that they could easily eclipse that contribution of loss. In other words, it seems the goal of your argument is to raise that capacitance, which by ordinary means has you drawing the loops together (insulated or not). This compounds the skin effect and for a constant frequency demands a lower inductance. The lower inductance, in turn, then demands a smaller coil which forces a lower Radiation resistance. A smaller coil (to again follow the demand for more Capacitance) drives closer loops. It seems like this is in an infinite regress. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
"Richard Clark" wrote in message ... On Mon, 30 Apr 2007 06:37:10 -0500, "amdx" wrote: Are you exploring an intellectual curiosity or trying to remedy a defect in application? No, I just have experienced the effect that Bill ask about and gave my own pet theory about why it happens. Now I'm looking for a little confirmation or where I went wrong. Hi Mike, Well, that is fine and good, but neither of you have given us any real data, and certainly no Q values to judge if what you both experienced was within the range of "normal" or out in left field. RF measurements are difficult to do to any particularly fine accuracy, and what was observed may have been simple variation due to the measurer's proximity (offering just one of many things that can go wrong). Yes, RF measurements are difficult to do to any particularly fine accuracy. And I claim no great knowledge of how to minimize errors or even how to recognize where they come from. Loss still remains the province of resistance. Richard, That's like saying rain has water in it. No matter how many times you say it, I'm still going to agree with you. Then this diverges from Bill's premise of Capacitance being the source of loss and you and he are separable at this point of your common experience. Your best argument is that Capacitance exacerbates loss. I would rephrase that as "interwinding capacitance exacerbates loss". And with that, you have summed up my argument perfectly. You have helped reduce my argument to 4 words. Now, do you agree that interwinding capacitance will reduce Q? (yes, I know it's the province of resistance) Give me some metrics to show it is not skin effect. The issue at hand is your (both you and Bill, or either of you separately) loops keep changing to fit to the loss rather than to the application. It makes for a rather strained progression of design as loops are added, proximity becomes a greater issue, as coil length collapses, insulation is added, and as frequency shifts to follow these changes. It is as though a good 10M loop is evolving to operate poorly there or, worse, in the 160M band where its resonance has finally come to rest through optimizing for loss. My experience is limited to winding small inductors rather than loop antennas. I can imagine there being enough turn-to-turn capacitance to induce large currents, but so many correlating factors would have to ride along with this that they could easily eclipse that contribution of loss. In other words, it seems the goal of your argument is to raise that capacitance, which by ordinary means has you drawing the loops together (insulated or not). This compounds the skin effect and for a constant frequency demands a lower inductance. The lower inductance, in turn, then demands a smaller coil which forces a lower Radiation resistance. A smaller coil (to again follow the demand for more Capacitance) drives closer loops. It seems like this is in an infinite regress. I don't understand why you think we want more interwinding capacitance, We want less. I will agree that the mechanics involved in trying to reduce interwinding capacitance will probably reduce proximity effects and so to seperate out any affect from the reduces interwinding capacitance would be difficult. I need to go, Later, thanks Richard |
Distributed capacitance effects Q?
"amdx" wrote in message ... "Richard Clark" wrote in message ... On Mon, 30 Apr 2007 06:37:10 -0500, "amdx" wrote: Are you exploring an intellectual curiosity or trying to remedy a defect in application? No, I just have experienced the effect that Bill ask about and gave my own pet theory about why it happens. Now I'm looking for a little confirmation or where I went wrong. Hi Mike, Well, that is fine and good, but neither of you have given us any real data, and certainly no Q values to judge if what you both experienced was within the range of "normal" or out in left field. RF measurements are difficult to do to any particularly fine accuracy, and what was observed may have been simple variation due to the measurer's proximity (offering just one of many things that can go wrong). Yes, RF measurements are difficult to do to any particularly fine accuracy. And I claim no great knowledge of how to minimize errors or even how to recognize where they come from. Loss still remains the province of resistance. Richard, That's like saying rain has water in it. No matter how many times you say it, I'm still going to agree with you. Then this diverges from Bill's premise of Capacitance being the source of loss and you and he are separable at this point of your common experience. Your best argument is that Capacitance exacerbates loss. I would rephrase that as "interwinding capacitance exacerbates loss". And with that, you have summed up my argument perfectly. You have helped reduce my argument to 4 words. Now, do you agree that interwinding capacitance will reduce Q? (yes, I know it's the province of resistance) Give me some metrics to show it is not skin effect. The issue at hand is your (both you and Bill, or either of you separately) loops keep changing to fit to the loss rather than to the application. It makes for a rather strained progression of design as loops are added, proximity becomes a greater issue, as coil length collapses, insulation is added, and as frequency shifts to follow these changes. It is as though a good 10M loop is evolving to operate poorly there or, worse, in the 160M band where its resonance has finally come to rest through optimizing for loss. My experience is limited to winding small inductors rather than loop antennas. I can imagine there being enough turn-to-turn capacitance to induce large currents, but so many correlating factors would have to ride along with this that they could easily eclipse that contribution of loss. In other words, it seems the goal of your argument is to raise that capacitance, which by ordinary means has you drawing the loops together (insulated or not). This compounds the skin effect and for a constant frequency demands a lower inductance. The lower inductance, in turn, then demands a smaller coil which forces a lower Radiation resistance. A smaller coil (to again follow the demand for more Capacitance) drives closer loops. It seems like this is in an infinite regress. I don't understand why you think we want more interwinding capacitance, We want less. I will agree that the mechanics involved in trying to reduce interwinding capacitance will probably reduce proximity effects and so to seperate out any affect from the reduces interwinding capacitance would be difficult. I need to go, Later, thanks Richard Ok, I'm back. Richard, I was starting to lean towards proximity effect possibly causing all of the affect we have been discussing, so I did some Googling. I kept find the same line " increased capacitance lowers Q" But, I think you agree that as I said above most efforts to reduce capacitance will also reduce proximity effect. I ran across W8JI's page, he's usually pretty exacting in his wording, and he says, "Capacitance across any inductor carrying time-varying current increases circulating currents in the inductor, increasing loss while simultaneously reducing system bandwidth." snip "Anything that increases capacitance will reduce component Q" He never mentions the correlation between interwinding capacitance and proximity effect These line were taken from; http://www.w8ji.com/loading_inductors.htm What do you think? Mike |
Distributed capacitance effects Q?
On Mon, 30 Apr 2007 18:30:40 -0500, "amdx" wrote:
Richard, I was starting to lean towards proximity effect possibly causing all of the affect we have been discussing, so I did some Googling. I kept find the same line " increased capacitance lowers Q" But, I think you agree that as I said above most efforts to reduce capacitance will also reduce proximity effect. Hi Mike, Yup. I ran across W8JI's page, he's usually pretty exacting in his wording, and he says, "Capacitance across any inductor carrying time-varying current increases circulating currents in the inductor, increasing loss while simultaneously reducing system bandwidth." Tom is also given to non-sequiturs. He polishes his page off with a list of them such as "Optimum form factor varies with application." As they used to say, if you want to send a message, call Western Union. snip "Anything that increases capacitance will reduce component Q" He never mentions the correlation between interwinding capacitance and proximity effect There is not much that can be taken to the bank about what is NOT said. The moral of this is standard Engineering practice: start with a goal and design towards it. 73's Richard Clark, KB7QHC |
Distributed capacitance effects Q?
On Apr 29, 4:52 am, "amdx" wrote:
"Bill Bowden" wrote in message oups.com... Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? -Bill Hi Bill. I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Anyone care to run with that, or explain it more clearly, or shoot it down. Mike I think you are right. Good explanation. -Bill |
Distributed capacitance effects Q?
"Bill Bowden" wrote in message ups.com... On Apr 29, 4:52 am, "amdx" wrote: "Bill Bowden" wrote in message oups.com... Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? -Bill Hi Bill. I agree with your assertion that distributed winding capacitance degrades efficiency. My thoughts about this are ; Assume a 10 turn loop, between each turn there is a capacitance, so, you have a complete circuit, (L,C,R) there is current flowing through this circuit that is not flowing through the entire 10 turn loop. (this happens in the other 9 turns also) I think these extra currents flowing that don't make the entire 10 turn circuit increase the losses. Anyone care to run with that, or explain it more clearly, or shoot it down. Mike I think you are right. Good explanation. -Bill Well Bill, That has been the theory I've been thinking with for 8 or 9 years now. However, if as Richard suggests the phenomena is caused by proximity effect, the techniques I used to lower interwinding capacitance and raise Q, would be the same I'd use to reduce proximity efect and raise Q. If there are circuilating currents caused by interwinding capacitance, it seems they would cause the proximity effect to be even stronger and pinch down the current flow area even more and raise losses. A question for all, Does a basketweave winding reduce proximity effect? Mike |
Distributed capacitance effects Q?
Bill Bowden wrote:
"Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency?" I`ll speculate that current to build the magnetic field and the current required to charge the stray capacitance of the inductor occur at different times. The magnetic field is the source of self-inductance of the coil, but the displacemnt current in the stray capactance is gratuitous and only adds loss to the coil. Best regards, Richard Harrison, KB5WZI |
Distributed capacitance effects Q?
Distributed capacitance may affect single-turn coils differently from
multiturn-coils. And those vary as their length to diameter ratio varies. Mike wrote: "Anything that increases capacitance will reduce component Q. I believe he was quoting W8JI. Mike also wrote: "What do you think?" In 1999 Tom Bruhns was experimenting, trying to find the relationship between coil Q and parasitic C. He picked up reports that helical resonators weere superior to short coaxial resonators. Tom also wrote: "Reg (Edwards,RJE) then thinks the internal coil capacitance is just femanding extra extra coil current and loss as the result of its cyclic charge and discharge." Reg seems to have had a nice explanation for coil loss from parasitic capacitance. Best regards, Richard Harrison, KB5WZI |
Distributed capacitance effects Q?
"Richard Harrison" wrote in message ... Distributed capacitance may affect single-turn coils differently from multiturn-coils. And those vary as their length to diameter ratio varies. Mike wrote: "Anything that increases capacitance will reduce component Q. I believe he was quoting W8JI. Mike also wrote: "What do you think?" In 1999 Tom Bruhns was experimenting, trying to find the relationship between coil Q and parasitic C. He picked up reports that helical resonators weere superior to short coaxial resonators. Tom also wrote: "Reg (Edwards,RJE) then thinks the internal coil capacitance is just femanding extra extra coil current and loss as the result of its cyclic charge and discharge." Reg seems to have had a nice explanation for coil loss from parasitic capacitance. Do you know where this explanation might be found? Thanks, Mike |
Distributed capacitance effects Q?
amdx wrote:
Do you know where this explanation might be found? From "Current through coils", March 5, 2006 2:47pm Looks like Reg originated this thread. "Every coil has length. Both L and C are distributed. Therefore the coil behaves as a transmission line. There are standing waves. Current and voltage both vary with length." And on March 9: "The whole thing could be summarised in one short sentence - 'Coils are distributed transmission lines.' The same general equations apply to coils of all dimensions, for any number of turns, at all frequencies, in all applications. There's no need to unnecessarily complicate things by artificially dividing them into lumped and other varieties." -- 73, Cecil http://www.w5dxp.com |
Distributed capacitance effects Q?
Bill Bowden wrote in news:1177821138.653191.285430
@u30g2000hsc.googlegroups.com: Does anyone know why the distributed winding capacitance of a loop antenna, or any inductor, degrades the efficiency? It would seem that a loop antenna with 100pF of winding capacitance in parallel with a external capacitor of 200pF would resonate at the same frequency as a antenna with no winding capacitance and a external capacitor of 300pF,but apparently that's not the case. The best explanation I got was that winding capacitance represents 'low Q' and a external tuning capacitor represents ' High Q' What is the difference between high and low Q, and why should a loop antenna with no winding capacitance perform any better than one with 50% of the total capacitance in the windings? Where is the energy loss? Bill, Some thoughts about inductor loss and self capacitance: Consider and ideal coil (ie lossless, no distributed capacitance) in series with a small ideal resistor to represent its loss, the combination having high Q. Connect it to a constant voltage source at some frequency and observe that the current lags the voltage by almost 90 deg. Now shunt that combination coil+resistor with a small lossless capacitor, and note that the current in the capacitor will be small in magnitude, and leading the applied voltage by 90 degrees. The effect of the capacitor is to reduce the total current, and not change its phase slightly. So the combination of coil & series resistance, & shunt capacitance draws less current and at slightly lower (lagging) phase, so it appears to be a smaller but lossier inductor. The discussion above is about conditions below the self resonance of the total combination. Now, real inductors might be represented by a simple circuit as dealt with above, but it is an approximation only. A better representation of real inductors is more complex and highly dependent on the frequency, geometry and materials. An example of the influence of these factors is that a ferrite cored inductor usually needs less turns (and less capacitance) than an air cored inductor of the same inductance; a bifilar split transformer winding on a toroid increases the self capacitance compared to a normal winding, albeit with higher flux leakage; close spaced windings reduce the number of turns needed, and resistance due to decreased wire length however proximity effect increases the resistance per turn. Design is about finding an optimal solution to these effects for the intended usage. Distributed capacitance is not of itself necessarily lossless, the materials in which the electric field alternates might not be ideal dielectrics, and so a further loss is contributed by dielectric losses. Operation of coils approaching their self resonance increases the loss due to this effect. Owen |
Distributed capacitance effects Q?
Owen Duffy wrote in
: .... Some thoughts about inductor loss and self capacitance: Consider and ideal coil (ie lossless, no distributed capacitance) in series with a small ideal resistor to represent its loss, the combination having high Q. Connect it to a constant voltage source at some frequency and observe that the current lags the voltage by almost 90 deg. Now shunt that combination coil+resistor with a small lossless capacitor, and note that the current in the capacitor will be small in magnitude, and leading the applied voltage by 90 degrees. The effect of the capacitor is to reduce the total current, and not change its phase slightly. So the combination of coil & series resistance, & shunt capacitance draws less current and at slightly lower (lagging) phase, so it appears to be a smaller but lossier inductor. A workup at 10MHz of some numbers for a 10uH inductance in series with 10 ohms loss resistance gives Z=10+j628, Q is 62.8. When this is shunted by a 2pf ideal capacitor, the impedance is now 11.8 +j682, Q is 58, apparent inductance is 10.9uH in series with 11.8 ohms of resistance. The small shunt capacitor has increased the apparent inductance, and decreased the Q. Where has this newfound loss come from? The current in the coil's loss resistance is higher than the current from the source, so whilst the two terminal equivalent has a higher impedance, the higher internal current is generating larger loss from the smaller resistance. This is the "circulating current" people are talking about. Owen Note |
Distributed capacitance effects Q?
In message , Owen Duffy
writes Owen Duffy wrote in : ... Some thoughts about inductor loss and self capacitance: Consider and ideal coil (ie lossless, no distributed capacitance) in series with a small ideal resistor to represent its loss, the combination having high Q. Connect it to a constant voltage source at some frequency and observe that the current lags the voltage by almost 90 deg. Now shunt that combination coil+resistor with a small lossless capacitor, and note that the current in the capacitor will be small in magnitude, and leading the applied voltage by 90 degrees. The effect of the capacitor is to reduce the total current, and not change its phase slightly. So the combination of coil & series resistance, & shunt capacitance draws less current and at slightly lower (lagging) phase, so it appears to be a smaller but lossier inductor. A workup at 10MHz of some numbers for a 10uH inductance in series with 10 ohms loss resistance gives Z=10+j628, Q is 62.8. When this is shunted by a 2pf ideal capacitor, the impedance is now 11.8 +j682, Q is 58, apparent inductance is 10.9uH in series with 11.8 ohms of resistance. The small shunt capacitor has increased the apparent inductance, and decreased the Q. Where has this newfound loss come from? The current in the coil's loss resistance is higher than the current from the source, so whilst the two terminal equivalent has a higher impedance, the higher internal current is generating larger loss from the smaller resistance. This is the "circulating current" people are talking about. Owen Note Just out of interest, if you increased the inductance to 10.9uH by increasing the number of turns, what effect would it have on the Q? Ian. -- |
Distributed capacitance effects Q?
Ian Jackson wrote in
: .... Just out of interest, if you increased the inductance to 10.9uH by increasing the number of turns, what effect would it have on the Q? Ian. Ian, that depends on the type of coil. A very simple view (eg if a toroidal core was used) would be that it would take a (10.9/10)^0.5 increase in turns (4.4%), inductive reactance would increase by 9% and R would increase by 4.4%, Q would increase by 4.4%. I don't really understand the relevance of the questions. Owen |
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