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Rule of Thumb for coax chokes
Reg Edwards wrote:
Cec, I can't run Excel. I need something which will run just by clicking on it. Reg, did you get the BASIC program I sent to you? If not, please send me an email so I can reply. -- 73, Cecil, W5DXP |
Rule of Thumb for coax chokes
John Popelish wrote: Digikey carries a pretty broad line of Steward long form ferrite bead cores. You can check the specifications at http://www.steward.com/ For instance, core HFB143064-300 has an impedance of about 180 ohms at 146 MHz. Type HF is their highest frequency material. Core 28B0562-200 (same size in the lower frequency, 28 material) produces an impedance of about 360 ohms per core. I think both of these would fit over RG8X and are about an inch and an eighth long. By the way, if you want the convenience of the snap around cores in a plastic retainer, part # 28A0593-0A2 provides 450 ohms per core at 146 MHz.. They cost about $2 plus shipping. They are about 2 and 1/4 inches long. |
Rule of Thumb for coax chokes
Reg Edwards wrote:
Let us know when your rule-of-thumb is available from your website. I'm looking forward to seeing the details. The rule of thumb is pretty simple and is for 2L pop bottles. Wrapping at 2 turns per inch (RG-213) around a 4 inch diameter pop bottle, the optimum number of turns for a particular band is equal to the numbers of meters in a wavelength. In other words, use 40 turns on 40m, 20 turns on 20m, and 10 turns on 10m. It's not actually linear - just a rule of thumb. It is most accurate around 20m-17m. I guess there will be the usual collection of over-meticulous nit-pickers. Well, here's some data points that will probably surprise you. I don't have a pop bottle so I am using a Quaker's Oats box for a coil form. It is 5 inches in diameter which makes the diameter of the coil using RG-400 about 5.4 inches. I wrapped 8 turns at ~4 turns per inch. My EXCEL spread sheet indicates that coil is 1/4WL self-resonant at about 22.5 MHz making it a good choke on 15m. Doubling that 1/4WL self-resonant frequency to estimate the 1/2WL self-resonant frequency gives 45 MHz. Now brace yourself. The series impedance of that choke falls below 650 ohms at about 27.2 MHz. 650 ohms is the maximum impedance that my MFJ-259B will display. The choking impedance is never again higher than 650 ohms as frequency is increased. It goes to a minimum of 49 ohms at the 1/2WL self-resonant frequency of 45.6 MHz. Note that is reasonably close to double the 1/4WL self-resonant frequency of 22.5 MHz *calculated* by my EXCEL spreadsheet and by the BASIC program that I sent to you. There is a one-wavelength self-resonant point at ~88 MHz and a 1.5WL self-resonant point at ~122 MHz. These measurements seem to prove that the coiled coax choke acts more like a transmission line than like a lumped inductance. Conclusion: A coiled coax choke designed for 20m doesn't function very well on 10m or at any higher frequency. (A coiled coax choke designed for 10m also doesn't function optimally on 80m.) -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
Cecil,
What makes you think your EXCEL spread sheet (whatever that is) gives the right answers? Have you ever made any measurements of the harmonic resonant frequencies? If so, how did you do it? ---- Reg. |
Rule of Thumb for coax chokes
Reg Edwards wrote:
What makes you think your EXCEL spread sheet (whatever that is) gives the right answers? Because it agrees within 3% of my actual measurements. It's the same formula covered by the BASIC program that I emailed to you. Have you ever made any measurements of the harmonic resonant frequencies? The posting to which you are responding has three of those measured harmonic resonant frequencies. If so, how did you do it? I put the coaxial choke across my MFJ-259B terminals and looked for low impedances. They occurred at 45.6 MHz, 88 MHz, and 122 MHz corresponding to 1/2WL, 1WL, and 1.5WL. That proves that the VF given by Corum's equation is correct because it predicted the 1/4WL point at 22.5 MHz, within 3% of the measured results. And if the Corum equation is valid for coaxial chokes, it is probably also valid for mobile antenna loading coils. -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
Cecil,
You have convinced me Corum's formula is in the right ball park. I have not found time to study how it has been derived. It doesn't appear to be particularly useful. I will now tell you how to obtain ALL resonant frequencies, both 1/4-wave and 1/2-wave as you call them. Place a single turn link winding around the CENTRE of the coil under test. Between the link winding and the MFJ-259B connect a loosely twisted pair (or a short length of speaker cable). The whole caboodle can be made from a single length of thin, insulated, stranded wire. I've a feeling that the length of the connection should not be too long. But the 259-B should not be very near to the coil to keep the meter outside the field of the coil. 6" or 10" seems about right depending on the size of the coil. Ideally, length should be much less than 1/4 wavelength at the test frequency. Begin at a low frequency and search for the first high impedance on the moving coil meter on the 259B. The first high impedance resonance corresponds to the self-resonant frequency of the coil. Increase frequency to find a low impedance resonance. Continue to find the next high impedance resonance, etc. The resonant frequencies may not be closely harmonically related. You may not find very close agreement with the results obtained by connecting the 259B directly across the coil. But both sets of results are equally valid (or invalid). I leave it to you to draw conclusions from the sequence of high-Z and low-Z resonant frequencies. Greatest accuracy is obtained by using the link coupling at the high impedance resonances because the coil is then more isolated from its environment. Its environment includes the input impedance of the 259-B itself. The lower the self-resonant frequency, the greater the accuracy. I found a coil in the junk box, 2.7" diameter, 4.0" long, 44 turns, which has a self-resonant frequency of 13.6 MHz. I would have liked it to be as low as 2 MHz. I agree, a coil at sufficiently high frequencies begins to behave something like a transmission line with a very low velocity factor. To investigate what is really happening requires an instrument capable of measuring impedance versus frequency from HF to VHF. It probably doesn't exist. When a coil is used to load a short HF vertical, it operates at a frequency much lower than its self-resonant frequency and transmission line effects don't matter two hoots. ----- Reg, G4FGQ. |
Rule of Thumb for coax chokes
On Sat, 12 Aug 2006 14:30:34 GMT, Cecil Moore wrote:
Reg Edwards wrote: I agree, a coil at sufficiently high frequencies begins to behave something like a transmission line with a very low velocity factor. Just below its self-resonant frequency, it behaves somewhat like a transmission line of less than 90 degrees. snip But maybe one hoot. :-) My 75m bugcatcher coil is operated relatively close to its measured self-resonant frequency of 6.6 MHz. If I wound a bugcatcher coil to be self-resonant on 4 MHz and then used 2/3 of that coil for a loading coil on 4 MHz, its VF would not change and its electrical length would be 60 degrees accompanied by the appropriate 60 degree delay through the coil. Hi Cecil & Reg Sometime during the '70s I measured the self-resonant frequency of the 80m Hustler loading coil, 6MHz. The series resistance of that coil was 31 ohms at 4 MHz. That is why they claimed 'lower swr than with othe brands'. What a fraud. On the other hand, I also measured the Webster KW-80, self-resonant at 14.0 MHz, with a series resistance of 8 ohms at 4 MHz. I reported this on Page 6-12 in Reflections. So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking just to heat the coil instead of radiating the energy into space? Walt, W2DU |
Rule of Thumb for coax chokes
Walter Maxwell wrote:
So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking just to heat the coil instead of radiating the energy into space? Sorry I wasn't explicit, Walt. I use only 2/3 of the coil and chop the other 1/3 off and discard it. That ensures that the VF of the coil of 2/3 length is the same as the VF of the whole coil at the frequency of operation. The alternate approach would be to extend the windings on a 75m bugcatcher coil until self-resonance was reached at 4 MHz. The VF could then be calculated and the extra windings removed. The purpose of the two above exercises is to determine the VF of the coil *at the frequency of operation*. The VF of large real-world loading coils changes with frequency. Knowing the self-resonant frequency of a 75m bugcatcher coil is 6.6 MHz doesn't (necessarily) yield the correct VF at 4 MHz. -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
On Sat, 12 Aug 2006 15:54:24 GMT, Cecil Moore wrote:
Walter Maxwell wrote: So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking just to heat the coil instead of radiating the energy into space? Sorry I wasn't explicit, Walt. I use only 2/3 of the coil and chop the other 1/3 off and discard it. That ensures that the VF of the coil of 2/3 length is the same as the VF of the whole coil at the frequency of operation. The alternate approach would be to extend the windings on a 75m bugcatcher coil until self-resonance was reached at 4 MHz. The VF could then be calculated and the extra windings removed. The purpose of the two above exercises is to determine the VF of the coil *at the frequency of operation*. The VF of large real-world loading coils changes with frequency. Knowing the self-resonant frequency of a 75m bugcatcher coil is 6.6 MHz doesn't (necessarily) yield the correct VF at 4 MHz. But Cecil, I thought this thread was about chokes to prevent common-mode currents from flowing on the feed line. Now yer talking about loading coils for mobile whip antennas. As I understand the issue, one is 180° from the other. For the choke you want a high resistance, which is what you get at the self-resonant frequency. But for the loading coil you want the lowest series resistance possible, which you don't get when anywhere near the self-resonant frequency. Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms. Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means yer coil is going to give you a nice low swr, but yer losing half of yer power in the coil because of the high series resistance you can't avoid. Yer also losing yer mind. Walt |
Rule of Thumb for coax chokes
Walter Maxwell wrote:
But Cecil, I thought this thread was about chokes to prevent common-mode currents from flowing on the feed line. Now yer talking about loading coils for mobile whip antennas. Yes, carrying the subject over from an earlier thread on loading coils. There is a master's thesis paper authored by the Corum brothers, K1AON and KB1EUD, and sponsored by the IEEE at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf which deals with RF coils. Although aimed at Tesla coils, it contains lots of useful information for hams. In particular, it predicts the VF for large real-world coils which is very useful for me. It essentially shoots down the argument that the current through a real-world loading coil is the same at both ends of the coil, i.e. the delay through the coil approaches zero as presented by the lumped circuit model. As I understand the issue, one is 180° from the other. For the choke you want a high resistance, which is what you get at the self-resonant frequency. But for the loading coil you want the lowest series resistance possible, which you don't get when anywhere near the self-resonant frequency. My point is that the same laws of physics apply to loading coils and coaxial coil chokes even if the applications are different. And we do, quite often, operate our 75m loading coils fairly near their self-resonant frequencies - like your Hustler example. Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms. Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means yer coil is going to give you a nice low swr, but yer losing half of yer power in the coil because of the high series resistance you can't avoid. Yer also losing yer mind. Well, that is the measured self-resonant frequency of my often glorified 75m Texas Bugcatcher coil supposed to be one of the highest-Q coils available. -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
On Sat, 12 Aug 2006 18:53:06 GMT, Cecil Moore wrote:
Walter Maxwell wrote: But Cecil, I thought this thread was about chokes to prevent common-mode currents from flowing on the feed line. Now yer talking about loading coils for mobile whip antennas. Yes, carrying the subject over from an earlier thread on loading coils. There is a master's thesis paper authored by the Corum brothers, K1AON and KB1EUD, and sponsored by the IEEE at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf which deals with RF coils. Although aimed at Tesla coils, it contains lots of useful information for hams. In particular, it predicts the VF for large real-world coils which is very useful for me. It essentially shoots down the argument that the current through a real-world loading coil is the same at both ends of the coil, i.e. the delay through the coil approaches zero as presented by the lumped circuit model. As I understand the issue, one is 180° from the other. For the choke you want a high resistance, which is what you get at the self-resonant frequency. But for the loading coil you want the lowest series resistance possible, which you don't get when anywhere near the self-resonant frequency. My point is that the same laws of physics apply to loading coils and coaxial coil chokes even if the applications are different. And we do, quite often, operate our 75m loading coils fairly near their self-resonant frequencies - like your Hustler example. Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms. Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means yer coil is going to give you a nice low swr, but yer losing half of yer power in the coil because of the high series resistance you can't avoid. Yer also losing yer mind. Well, that is the measured self-resonant frequency of my often glorified 75m Texas Bugcatcher coil supposed to be one of the highest-Q coils available. Yeah, but Cecil, have you ever actually MEASURED the series resistance? The slope of the resonance curve that peaks at 6.6MHz ain't gonna be low enough at 4.0. MHz to make an efficient mobile antenna. The Hustler coils, with 31 ohms series resistance was a hoax on the average ham who didn't know the real reason the Hustler gave them a low swr, which is what they mistakenly thought was paradise. When it comes to efficiency in an antenna with a loading coil, the best efficiency comes with the highest swr in absence of any attempt to match the terminal impedance to 50 ohms. IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz was a high-Q coil at 4 MHz. Walt |
Rule of Thumb for coax chokes
On Sat, 12 Aug 2006 15:14:10 -0400, Walter Maxwell wrote:
On Sat, 12 Aug 2006 18:53:06 GMT, Cecil Moore wrote: Walter Maxwell wrote: IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz was a high-Q coil at 4 MHz. Walt What I mean't to say, Cecil, is that you've been hoaxed if you thought a coil self-resonant at 6.6 MHz intended for use at 4.0 MHz was a high-Q coil. Walt |
Rule of Thumb for coax chokes
Walter Maxwell wrote:
IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz was a high-Q coil at 4 MHz. All I know is that Texas Bugcatcher coils tend to be near the top of the the 75m mobile shootout results. Using base loading on a GMC pickup, it is resonant on 3.8 MHz with a six foot stinger. Remove the stinger and it is self-resonant at 6.6 MHz. My 75m Texas Bugcatcher coil was a gift from K7JEB. It is 26.5 turns at 4 tpi on a 6" air core form. Calculates out to be about 70 uH. I don't know how to make it higher Q. -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
On Sat, 12 Aug 2006 23:32:09 GMT, Cecil Moore wrote:
Walter Maxwell wrote: IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz was a high-Q coil at 4 MHz. All I know is that Texas Bugcatcher coils tend to be near the top of the the 75m mobile shootout results. Using base loading on a GMC pickup, it is resonant on 3.8 MHz with a six foot stinger. Remove the stinger and it is self-resonant at 6.6 MHz. My 75m Texas Bugcatcher coil was a gift from K7JEB. It is 26.5 turns at 4 tpi on a 6" air core form. Calculates out to be about 70 uH. I don't know how to make it higher Q. I'm guessing #12 wire in the coil, which leaves lots of spacing between turns , which should make the distributed capacitance very small. I haven't made any calculations, but a seat of the pants estimate would say the self-resonant frequency would be much, much higher than 6.6 MHz. (As I remember, the Hustler used #14 close spaced on about 2" diameter to obtain a self-resonant frequency at 6MHz.) Cecil, I suggest you re-measure the self-resonant frequency of the coil by itself, and if you have the means to do it, also measure the series resistance at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a large difference in the resistances, and that the self-resonant frequency will be much greater than 6.6 MHz. Walt |
Rule of Thumb for coax chokes
The self-capacitance of a multi-turn solenoid has little to do with
spacing between turns. Self-c depends on coil length and diameter. The self-capacitances between adjacent turns are all in series with each other. Resulting capacitance across coil is negligible. ---------------------------------------------------------------------- A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at 3.8 MHz, and self-resonant frequency = 9 MHz. ---- Reg. |
Rule of Thumb for coax chokes
Walter Maxwell wrote:
Cecil, I suggest you re-measure the self-resonant frequency of the coil by itself, and if you have the means to do it, also measure the series resistance at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a large difference in the resistances, and that the self-resonant frequency will be much greater than 6.6 MHz. I'm not sure how to measure the 1/4WL self-resonant frequency with an MFJ-259B without a ground plane. I suppose it could be done using a 1/4WL counterpoise in free space. Let me just report what the MFJ-259B readings are with the isolated 75m Texas Bugcatcher coil across the MFJ-259B terminals. The first dip in impedance is at 14.7 MHz where the MFJ reads 620+j0 ohms. The second dip in impedance is at 24.4 MHz where the MFJ reads 88+j0 ohms. Is the first dip the 1/4WL self- resonant point and the second dip the 1/2WL self-resonant point? I want to make it clear that the previously reported 6.6 MHz self-resonant measurement was made with the base-loaded coil mounted a few inches away from my GMC pickup ground plane. The ground plane was no doubt in the field of the coil at the bottom end so the coil was certainly not isolated as it is in the above reported measurements. -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
Reg Edwards wrote:
A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at 3.8 MHz, and self-resonant frequency = 9 MHz. Wouldn't mounting the coil four inches above a GMC pickup ground plane reduce the Q and the self-resonant frequency? -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
"Cecil Moore" wrote Reg Edwards wrote: A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at 3.8 MHz, and self-resonant frequency = 9 MHz. Wouldn't mounting the coil four inches above a GMC pickup ground plane reduce the Q and the self-resonant frequency? -- ====================================== Cec, Very likely. But not very much. It would not be the self-resonant frequency any more. The srf never changes. And neither does the intrinsic coil Q. We must be careful with our descriptions. Slackness leads to misunderstandings, arguments and fights. ---- Reg |
Rule of Thumb for coax chokes
Reg Edwards wrote:
It would not be the self-resonant frequency any more. The srf never changes. And neither does the intrinsic coil Q. So what would you call the frequency at which a coil alone is resonant when mounted as a base-loading coil over a ground plane? -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
On Sun, 13 Aug 2006 15:02:03 +0100, "Reg Edwards"
wrote: "Cecil Moore" wrote Reg Edwards wrote: A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at 3.8 MHz, and self-resonant frequency = 9 MHz. Wouldn't mounting the coil four inches above a GMC pickup ground plane reduce the Q and the self-resonant frequency? -- ====================================== Cec, Very likely. But not very much. It would not be the self-resonant frequency any more. The srf never changes. And neither does the intrinsic coil Q. We must be careful with our descriptions. Slackness leads to misunderstandings, arguments and fights. ---- Reg Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? Please educate me. Walt |
Rule of Thumb for coax chokes
On Sun, 13 Aug 2006 12:34:35 GMT, Cecil Moore wrote:
Walter Maxwell wrote: Cecil, I suggest you re-measure the self-resonant frequency of the coil by itself, and if you have the means to do it, also measure the series resistance at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a large difference in the resistances, and that the self-resonant frequency will be much greater than 6.6 MHz. I'm not sure how to measure the 1/4WL self-resonant frequency with an MFJ-259B without a ground plane. I suppose it could be done using a 1/4WL counterpoise in free space. Let me just report what the MFJ-259B readings are with the isolated 75m Texas Bugcatcher coil across the MFJ-259B terminals. The first dip in impedance is at 14.7 MHz where the MFJ reads 620+j0 ohms. The second dip in impedance is at 24.4 MHz where the MFJ reads 88+j0 ohms. Is the first dip the 1/4WL self- resonant point and the second dip the 1/2WL self-resonant point? I want to make it clear that the previously reported 6.6 MHz self-resonant measurement was made with the base-loaded coil mounted a few inches away from my GMC pickup ground plane. The ground plane was no doubt in the field of the coil at the bottom end so the coil was certainly not isolated as it is in the above reported measurements. Cecil, I measured the self-resonant frequency of the loading coils with a Measurements 59 grid dip osc with the coil mounted away from all metallic objects. The Webster KW-80 coil that measured 14 MHz for the self-resonant frequency, and 8 ohms resistance at 4.0 MHz, as I remember it from several years ago, was about 3" in diameter and around 7 to 8" long. I don't recall now how I measured the Q, but it was close to 400. I measured the resistance with a GR-1606A RF bridge. If you have a GDO I suggest you remeasure the self-resonant frequency, and then measure the resistance at that frequency with the MFJ 259, and then again at 4.0 MHz. From that data you'll be able to determine the actual Q. Seems like it should be around 500, as Reg calculated. But like I said earlier, I believe the self-resonant frequency of your 6" bugcatcher will be greater than 9 MHz. Walt, W2DU |
Rule of Thumb for coax chokes
"Cecil Moore" wrote So what would you call the frequency at which a coil alone is resonant when mounted as a base-loading coil over a ground plane? ================================= Cec, I would call it the frequency at which the coil alone is resonant when mounted as a base-loading coil over a ground plane. It would depend on whether the ground plane was a bicycle or the deck of a super-tanker. ----- Reg. |
Rule of Thumb for coax chokes
"Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Coi Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. |
Rule of Thumb for coax chokes
On Sun, 13 Aug 2006 22:15:38 +0100, "Reg Edwards"
wrote: "Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Coi Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt |
Rule of Thumb for coax chokes
Is Cecil still beating that same old dead horse that only he rides?
Walter Maxwell wrote: On Sat, 12 Aug 2006 14:30:34 GMT, Cecil Moore wrote: Reg Edwards wrote: I agree, a coil at sufficiently high frequencies begins to behave something like a transmission line with a very low velocity factor. Just below its self-resonant frequency, it behaves somewhat like a transmission line of less than 90 degrees. snip But maybe one hoot. :-) My 75m bugcatcher coil is operated relatively close to its measured self-resonant frequency of 6.6 MHz. If I wound a bugcatcher coil to be self-resonant on 4 MHz and then used 2/3 of that coil for a loading coil on 4 MHz, its VF would not change and its electrical length would be 60 degrees accompanied by the appropriate 60 degree delay through the coil. Hi Cecil & Reg Sometime during the '70s I measured the self-resonant frequency of the 80m Hustler loading coil, 6MHz. The series resistance of that coil was 31 ohms at 4 MHz. That is why they claimed 'lower swr than with othe brands'. What a fraud. On the other hand, I also measured the Webster KW-80, self-resonant at 14.0 MHz, with a series resistance of 8 ohms at 4 MHz. I reported this on Page 6-12 in Reflections. So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking just to heat the coil instead of radiating the energy into space? Walt, W2DU |
Rule of Thumb for coax chokes
"Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. ========================================= Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt ========================================= Walt, Yes, there is only one G4FGQ. Although I confess I don't spend much time on the air these days. Poor health! When it comes to antennas, one reason why I don't publicise the source code of my programs is that they are full of proven little tricks like the foregoing which give answers in the right american ball-park. There is always the danger that unjustified, unqualified criticism would spoil the confidence and integrity to be placed in them by novices. If for any reason you don't like a program you can always have your money back! One day I might list my 60 years of engineering experience but it may be construed as bragging. And a compliment from you, Walt, is a compliment indeed! ---- Reg. |
Rule of Thumb for coax chokes
On Mon, 14 Aug 2006 15:05:50 +0100, "Reg Edwards"
wrote: "Walter Maxwell" wrote ========================================= Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt = snip ======================================== Walt, Yes, there is only one G4FGQ. Although I confess I don't spend much time on the air these days. Poor health! When it comes to antennas, one reason why I don't publicise the source code of my programs is that they are full of proven little tricks like the foregoing which give answers in the right american ball-park. There is always the danger that unjustified, unqualified criticism would spoil the confidence and integrity to be placed in them by novices. If for any reason you don't like a program you can always have your money back! One day I might list my 60 years of engineering experience but it may be construed as bragging. And a compliment from you, Walt, is a compliment indeed! ---- Reg. Reg, with your experience, along with your wonderful assistance to others through your myriad of useful programs made available at no cost to others, but with much cost to you in terms of time spent creating them, you've earned your bragging rights many times over. Apparently, we have a mutual complimentary relationship. Walt |
Rule of Thumb for coax chokes
Reg Edwards wrote:
I would call it the frequency at which the coil alone is resonant when mounted as a base-loading coil over a ground plane. That's the self-resonant frequency "in situ". -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
Walt,
I'm a selfish person just like everybody else. I do it purely for self-satisfaction. ---- Reg. |
Rule of Thumb for coax chokes
"Walter Maxwell" wrote in message ... On Sat, 12 Aug 2006 18:53:06 GMT, Cecil Moore wrote: Walter Maxwell wrote: But Cecil, I thought this thread was about chokes to prevent common-mode currents from flowing on the feed line. Now yer talking about loading coils for mobile whip antennas. Yes, carrying the subject over from an earlier thread on loading coils. There is a master's thesis paper authored by the Corum brothers, K1AON and KB1EUD, and sponsored by the IEEE at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf which deals with RF coils. Although aimed at Tesla coils, it contains lots of useful information for hams. In particular, it predicts the VF for large real-world coils which is very useful for me. It essentially shoots down the argument that the current through a real-world loading coil is the same at both ends of the coil, i.e. the delay through the coil approaches zero as presented by the lumped circuit model. As I understand the issue, one is 180° from the other. For the choke you want a high resistance, which is what you get at the self-resonant frequency. But for the loading coil you want the lowest series resistance possible, which you don't get when anywhere near the self-resonant frequency. My point is that the same laws of physics apply to loading coils and coaxial coil chokes even if the applications are different. And we do, quite often, operate our 75m loading coils fairly near their self-resonant frequencies - like your Hustler example. Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms. Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means yer coil is going to give you a nice low swr, but yer losing half of yer power in the coil because of the high series resistance you can't avoid. Yer also losing yer mind. Well, that is the measured self-resonant frequency of my often glorified 75m Texas Bugcatcher coil supposed to be one of the highest-Q coils available. Yeah, but Cecil, have you ever actually MEASURED the series resistance? The slope of the resonance curve that peaks at 6.6MHz ain't gonna be low enough at 4.0. MHz to make an efficient mobile antenna. The Hustler coils, with 31 ohms series resistance was a hoax on the average ham who didn't know the real reason the Hustler gave them a low swr, which is what they mistakenly thought was paradise. When it comes to efficiency in an antenna with a loading coil, the best efficiency comes with the highest swr in absence of any attempt to match the terminal impedance to 50 ohms. IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz was a high-Q coil at 4 MHz. Walt If the Hustler isn't bad enough you can always get a hamstick. 73 H. NQ5H |
Rule of Thumb for coax chokes
"Reg Edwards" wrote in message ... "Cecil Moore" wrote So what would you call the frequency at which a coil alone is resonant when mounted as a base-loading coil over a ground plane? ================================= Cec, I would call it the frequency at which the coil alone is resonant when mounted as a base-loading coil over a ground plane. It would depend on whether the ground plane was a bicycle or the deck of a super-tanker. ----- Reg. Wouldn't that just be a coiled-up whip? H. |
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