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Hf Antenna Question
I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here.... http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf say to roll up about 30 feet of coax into a 12" diameter coil. This they label as a choke. This just seems like a "duct tape" solution to me. Is there a better way to accomplish the same thing? Would a few ferrite beads on the coax do the trick or do I just have no idea what I am talking about? P.S. please check out my new Ham Radio Repeater Database web site. http://hrrdb.com -- Chris W KE5GIX "Protect your digital freedom and privacy, eliminate DRM, learn more at http://www.defectivebydesign.org/what_is_drm" Gift Giving Made Easy Get the gifts you want & give the gifts they want One stop wish list for any gift, from anywhere, for any occasion! http://thewishzone.com |
Hf Antenna Question
Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions for antenna set up, which you can find here.... http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf say to roll up about 30 feet of coax into a 12" diameter coil. This they label as a choke. This just seems like a "duct tape" solution to me. Is there a better way to accomplish the same thing? Would a few ferrite beads on the coax do the trick or do I just have no idea what I am talking about? P.S. please check out my new Ham Radio Repeater Database web site. http://hrrdb.com Chris; Your best choke is about 30 feet of coax rolled up into a 12" coil. This is not a duct tape solution but one that has been used for decades. It is the cheapest and easiest choke you can use. Others are better but not easier. Dave WD9BDZ |
Hf Antenna Question
David G. Nagel wrote:
Chris W wrote: I have been looking at the MFJ-1622 antenna. Part of the instructions for antenna set up, which you can find here.... http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf say to roll up about 30 feet of coax into a 12" diameter coil. This they label as a choke. This just seems like a "duct tape" solution to me. Is there a better way to accomplish the same thing? Would a few ferrite beads on the coax do the trick or do I just have no idea what I am talking about? P.S. please check out my new Ham Radio Repeater Database web site. http://hrrdb.com Chris; Your best choke is about 30 feet of coax rolled up into a 12" coil. This is not a duct tape solution but one that has been used for decades. It is the cheapest and easiest choke you can use. Others are better but not easier. Agreed. The coil only *looks* crude - especially if you actually do use duct tape. But looks can be deceptive. With the right length of coax, number of turns and method of winding, such a simple coil resonates with its own self-capacitance and is unbeatable as a single-band choke. With about the same amount of care, a slightly different coil can be selected to give an adequate common-mode impedance across several bands. The design data has been in the ARRL Antenna Handbook for many years, and there's more data about solenoid-wound chokes on the web. The really crude solution is to use just "a few ferrite beads" without giving any thought to which ferrite beads, or how many, or why. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
"David G. Nagel" wrote in message ... Chris W wrote: I have been looking at the MFJ-1622 antenna. Part of the instructions for antenna set up, which you can find here.... http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf say to roll up about 30 feet of coax into a 12" diameter coil. This they label as a choke. This just seems like a "duct tape" solution to me. Is there a better way to accomplish the same thing? Would a few ferrite beads on the coax do the trick or do I just have no idea what I am talking about? P.S. please check out my new Ham Radio Repeater Database web site. http://hrrdb.com Chris; Your best choke is about 30 feet of coax rolled up into a 12" coil. This is not a duct tape solution but one that has been used for decades. It is the cheapest and easiest choke you can use. Others are better but not easier. Dave WD9BDZ I agree that the coiled up coax is a good way to go. As far as cheapest , with the price of good rg-8 size coax the cost is now about the same as the ready made chokes with the beads on them. If you factor in the cost of a ready made center insulator for a dipole the cost could be more. Also 30 feet of coax can be another heavy weight to support. |
Hf Antenna Question
Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions for antenna set up, which you can find here.... http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf say to roll up about 30 feet of coax into a 12" diameter coil. This they label as a choke. This just seems like a "duct tape" solution to me. Is there a better way to accomplish the same thing? Would a few ferrite beads on the coax do the trick or do I just have no idea what I am talking about? I have an EXCEL calculator which indicates that particular coiled coax choke should have maximum effectiveness around ~8 MHz. So it probably does a reasonable job on 40m-20m. Unfortunately, it will reach 1/2WL self resonance somewhere around ~16 MHz and thus be essentially useless 17m-2m because of self-capacitance shorting out the choking impedance. Anyone who says a particular coiled coax choke will cover 40m-2m is mistaken. However, the stress on the choke is reduced because the MFJ antenna appears to be resonant, i.e. a low feedpoint impedance. Here are some actual measurements. http://www.k1ttt.net/technote/airbalun.html In a nutshell, the coiled coax choke works well over a frequency range of 2:1 (conservative) or 3:1 (liberal). The beads in the above measurements work well over a frequency range of 4:1. It's hard to beat stacked ferrite toroids of differing materials as a foundation for an excellent all-HF-band 1:1 choke-balun. I know of no single balun that can cover 7 MHz to 148 MHz well. 1/2WL self-resonance defeats the choking action when trying to cover 4+ octaves. I recently used an MFJ-259B to measure the choking impedance of 8 turns of RG-400 on a 5.35" form. The choking impedance dropped below 650 ohms at 4.63 MHz on the low end and 28.1 MHz on the high end. 650 ohms is not enough choking impedance for all-HF-band ladder- line fed dipoles. Assuming SWR ranging up to 18:1, which is the case for my all-HF-band 130 ft. dipole, the choke- balun may encounter as much as 8000 ohms looking into the Z0=450 ohm ladder-line. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
David G. Nagel wrote:
Your best choke is about 30 feet of coax rolled up into a 12" coil. Seems to me, the best choke(s) would be two chokes, one designed for 75m-15m and the other designed for 12m-2m. I cannot bring myself to believe that a coiled up piece of coax could ever be an effective choke over a 40:1 frequency range. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Ian White GM3SEK wrote:
With the right length of coax, number of turns and method of winding, such a simple coil resonates with its own self-capacitance and is unbeatable as a single-band choke. Unfortunately, at approximately double that single-band frequency, the choke is 1/2WL self-resonant and essentially useless. Someone on QRZ.com quoted the 2006 ARRL Handbook as saying the following: "A flat coil (like a coil of rope) shows a broad resonance that easily covers three octaves, making it reasonably effective over the entire HF range." Such a coil is certainly NOT "reasonably effective over the entire HF range" when used on a typical ladder-line fed all-HF-band dipole. With a 50 ohm 75m dipole, the SWR on 450 ohm ladder-line will be 9:1. Worst case, the choke will see 9*450 = 4050 ohms. An effective choke of five times that value would be 20K ohms or about 850 uH of inductance. What do you reckon would be the 1/2WL self- resonant frequency of an 850 uH coil of coax? -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
It's hard to beat stacked ferrite
toroids of differing materials as a foundation for an excellent all-HF-band 1:1 choke-balun. I know of no single balun that can cover 7 MHz to 148 MHz well. 1/2WL self-resonance defeats the choking action when trying to cover 4+ octaves. Can you direct me to more good information (as in NOT sales literature) about ferrites and toroids as choke baluns? The ARRL Handbook mentions the use of 1:1 baluns on p. 21.16 (Transmission Lines): For 3.5 to 30 MHz, the recommended choke is 10 ft and 7 turns using RG-8, 58, 59,8X and 213 (I don't think I've even heard of the last two) W2DU has a 50 bead balun over RG-303. As you may recall, I am using a homemade G5RV and I simply connected RG-8 directly to the ladderline portion. I can see this connection right out my window (2nd floor shack). I am unaware of any significant problems as it is. Should I consider using one of these methods and re-do my antenna? Thanks for being one of the experts that doesn't talk down to the "rest of us". John AB8O |
Hf Antenna Question
jawod wrote:
As you may recall, I am using a homemade G5RV and I simply connected RG-8 directly to the ladderline portion. I can see this connection right out my window (2nd floor shack). I am unaware of any significant problems as it is. Should I consider using one of these methods and re-do my antenna? Are you having problems with RF-in-the-shack? If not, your feedline radiation may be doing you more good than harm. I'm a purist. I install a very good balun/choke at every BALanced to UNbalanced junction. Some people are good and some people are lucky. If you have a current maximum common-mode loop in the shack, you may not even notice it. It's those voltage maximums that tend to bite. Thanks for being one of the experts that doesn't talk down to the "rest of us". Heh-heh, I only talk down to the gurus for the purpose of creating a crossed transactional analysis. :-) -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: With the right length of coax, number of turns and method of winding, such a simple coil resonates with its own self-capacitance and is unbeatable as a single-band choke. Cecil makes several different points here. Unfortunately, at approximately double that single-band frequency, the choke is 1/2WL self-resonant and essentially useless. It's a persistent ham myth that an RF choke has specially good properties when the total length of wire is a quarter-wavelength, and specially bad properties at twice that frequency. When the wire is wound into any kind of coil, neither of those claims is true (except maybe by some rare coincidence). The choke acts essentially as a parallel-tuned circuit, with its inductance tuned by its own self-capacitance. There will be a series resonance at some higher frequency, but not at twice the parallel-resonant frequency (except, again, perhaps by a rare coincidence). We don't have any performance data for the particular choke recommended by MFJ (and I'll return to that later) but the ARRL Antenna Book does have some measured data on two chokes, both made from 8 turns of RG213 wound into a coil of 6-5/8in diameter. The first choke is a bunched flat coil, and the second is a solenoid. I took the time to import the data (20th Edition, Table 3) into Excel and analyse it carefully. The bunched choke has a sharp parallel resonance at about 6MHz, with a maximum |Z| value of about 8500 ohms (could be higher because the data are in 1MHz steps). The total winding length at this frequency is about 0.085 wavelengths - a very long way from a quarter-wave. At other frequencies up to about 30MHz, the choke behaves like a classic parallel-tuned circuit: the phase angle of Z is almost purely inductive (+90deg) below the resonant frequency, and almost purely capacitive (-90deg) above it. There is NO series resonance at twice the parallel-resonant frequency - that would be about 12MHz, and nothing at all "special" is happening there. At 18MHz, where the total winding length is 0.25 wavelengths, there is a very small wobble in the data, but nothing more. The series resonance, where the phase angle flips from negative to positive again, is at 31.5MHz, which is totally unrelated to any of the other frequencies above. The winding length is 0.5 wavelengths at 35MHz (where the data runs out) but again nothing "special" is happening there. Thus there is no evidence whatever for the myth of the "resonant length of wire in a choke". Turning now to the solenoid-wound choke, the different method of winding has increased the parallel resonance of the same length of cable from 6MHz to 9MHz. This is consistent with simple L-C behaviour, and with the solenoid having less distributed capacitance than the bunched winding. Once again, this choke behaves almost entirely as a parallel-tuned circuit. There are slightly larger wobbles in the data at the frequencies where the total winding lengths are a quarter-wave and a half-wave, but these "transmission-line" effects are still very minor, and completely dominated by the simple L-C behaviour. The other difference is that the solenoid-wound choke has a much higher parallel-resonant impedance - almost 16,000 ohms, compared with 8500 ohms for the bunched choke. Because of its higher resonant frequency, the solenoid choke would be useful (Z 1000 ohms) from 7MHz up to at least 18MHz, covering at least four amateur bands, while the bunched choke would only hit 7MHz. Someone on QRZ.com quoted the 2006 ARRL Handbook as saying the following: "A flat coil (like a coil of rope) shows a broad resonance that easily covers three octaves, making it reasonably effective over the entire HF range." That's in my 2005 ARRL Handbook also... but the claim of a "broad resonance" is not supported by the more detailed information in the ARRL Antenna Book. On the contrary, the parallel resonance is rather sharp. It would be fair to claim that a carefully proportioned coil balun with a resonance around 10-14MHz can have a usefully high impedance as low as 3.5MHz and as high as 30MHz... but the impedance won't be spectacular at either end of that range, almost certainly less than 500 ohms. So I'd agree that those claims need to be revisited. Such a coil is certainly NOT "reasonably effective over the entire HF range" when used on a typical ladder-line fed all-HF-band dipole. With a 50 ohm 75m dipole, the SWR on 450 ohm ladder-line will be 9:1. Worst case, the choke will see 9*450 = 4050 ohms. An effective choke of five times that value would be 20K ohms or about 850 uH of inductance. What do you reckon would be the 1/2WL self- resonant frequency of an 850 uH coil of coax? As shown above, "1/2wl self resonance" ceases to be a valid concept once a length of wire is wound into a coil.... but I do see the point you're getting at: such a large inductor would have too much self-capacitance to be workable solution. If you were absolutely determined to tackle this extreme problem head-on, the best choke balun would be one that exploits its self-capacitance to give a parallel resonance on the operating frequency. However, a far better solution to this problem would be to avoid the extreme impedance by changing the feedline length. As for the choke recommended for the MFJ-1622, that comes out at 12 turns of 9in diameter, which seems to be aimed at that antenna's lowest operating bands. On the higher bands, the choke will be largely ineffective because it's too big. For a more broadband solution based on coiled coax, I'd agree with Cecil's suggestion of cascading a large coil for the lower bands with a smaller coil optimized for the higher bands. It would also be possible to cascade a large coil with a small ferrite choke. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
jawod wrote: Can you direct me to more good information (as in NOT sales literature) about ferrites and toroids as choke baluns? Here are some actual measurements of coax and ferrite chokes. Tests done with a Hewlett-Packard 4193A vector impedance meter. http://www.k1ttt.net/technote/airbalun.html |
Hf Antenna Question
Ian White GM3SEK wrote:
It's a persistent ham myth that an RF choke has specially good properties when the total length of wire is a quarter-wavelength, and specially bad properties at twice that frequency. When the wire is wound into any kind of coil, neither of those claims is true (except maybe by some rare coincidence). Please don't imply that I said anything about the total length of wire - I didn't. What you say is true and I never said otherwise. Well-designed coils can be modeled as rough approximations to transmission lines. The choke acts essentially as a parallel-tuned circuit, with its inductance tuned by its own self-capacitance. There will be a series resonance at some higher frequency, but not at twice the parallel-resonant frequency (except, again, perhaps by a rare coincidence). I didn't say exactly twice the frequency and I said it was an approximation. The chokes at: http://www.k1ttt.net/technote/airbalun.html average close to double the frequency. We don't have any performance data for the particular choke recommended by MFJ (and I'll return to that later) but the ARRL Antenna Book does have some measured data on two chokes, both made from 8 turns of RG213 wound into a coil of 6-5/8in diameter. The first choke is a bunched flat coil, and the second is a solenoid. I took the time to import the data (20th Edition, Table 3) into Excel and analyse it carefully. The bunched choke has a sharp parallel resonance at about 6MHz, with a maximum |Z| value of about 8500 ohms (could be higher because the data are in 1MHz steps). The total winding length at this frequency is about 0.085 wavelengths - a very long way from a quarter-wave. At other frequencies up to about 30MHz, the choke behaves like a classic parallel-tuned circuit: the phase angle of Z is almost purely inductive (+90deg) below the resonant frequency, and almost purely capacitive (-90deg) above it. No one would expect a bunched coil to be very well behaved. Everything I have said applies to a coax choke wound on some kind of coil form with some care given to its design. There is NO series resonance at twice the parallel-resonant frequency - that would be about 12MHz, and nothing at all "special" is happening there. At 18MHz, where the total winding length is 0.25 wavelengths, there is a very small wobble in the data, but nothing more. The series resonance, where the phase angle flips from negative to positive again, is at 31.5MHz, which is totally unrelated to any of the other frequencies above. The winding length is 0.5 wavelengths at 35MHz (where the data runs out) but again nothing "special" is happening there. Again, no one would expect a bunched coil to be well behaved. Thus there is no evidence whatever for the myth of the "resonant length of wire in a choke". You keep saying that as if I said otherwise. I didn't. The length of the wire is irrelevant to this discussion. Turning now to the solenoid-wound choke, the different method of winding has increased the parallel resonance of the same length of cable from 6MHz to 9MHz. This is consistent with simple L-C behaviour, and with the solenoid having less distributed capacitance than the bunched winding. Once again, this choke behaves almost entirely as a parallel-tuned circuit. There are slightly larger wobbles in the data at the frequencies where the total winding lengths are a quarter-wave and a half-wave, but these "transmission-line" effects are still very minor, and completely dominated by the simple L-C behaviour. The point is that there is a 1/4WL high impedance resonance and a 1/2WL low impedance resonance that are roughly where they should be. The 1/2WL low impedance resonance should be avoided. As shown above, "1/2wl self resonance" ceases to be a valid concept once a length of wire is wound into a coil... The 1/2WL self-resonance has little to do with the length of wire. It is where the phase angle flips at a point of low impedance. The 1/4WL self-resonance is where the phase angle flips at a point of high impedance. The length of wire is irrelevant, a moot point. I don't know why you brought it up in the first place. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: It's a persistent ham myth that an RF choke has specially good properties when the total length of wire is a quarter-wavelength, and specially bad properties at twice that frequency. When the wire is wound into any kind of coil, neither of those claims is true (except maybe by some rare coincidence). Please don't imply that I said anything about the total length of wire - I didn't. In that case, I suggest you stop making constant references to "1/4WL self-resonance" and "1/2WL self-resonance". If you don't mean it literally, it's a very misleading metaphor. What you say is true and I never said otherwise. Well-designed coils can be modeled as rough approximations to transmission lines. The choke acts essentially as a parallel-tuned circuit, with its inductance tuned by its own self-capacitance. There will be a series resonance at some higher frequency, but not at twice the parallel- resonant frequency (except, again, perhaps by a rare coincidence). I didn't say exactly twice the frequency and I said it was an approximation. The chokes at: http://www.k1ttt.net/technote/airbalun.html average close to double the frequency. We're actually looking at exactly the same data (except that the original reference quoted on K1TTT's site also includes a ferrite bead choke for comparison). I have graphed the |Z| data for all the chokes (see link to spreadsheet below) and there is no consistent trend. In the following table, Fmax is the frequency of maximum impedance, and Fmin is the frequency of any minimum observable within the frequency range (the 8t 1 layer choke has two very small minima). Ratio is Fmax/Fmin. Choke Fmax Fmin Ratio 6t 1 layer 24 none - 12t 1 layer 15 31 2.1 4t 1 layer 21 34 1.6 8t 1 layer 12 19 1.6 12 32 2.7 8t bunched 6 36 6.0 Judging from the shapes of the graphs and the table above, I would say that "twice the frequency" is not even valid as an approximation. No one would expect a bunched coil to be very well behaved. Everything I have said applies to a coax choke wound on some kind of coil form with some care given to its design. Across the whole 1-30MHz band, the bunched choke behaves as an almost perfect L-C circuit, free from any unwanted resonances. The only problem with that design is to reproduce the exact parallel-resonant frequency from one example to the next. There is NO series resonance at twice the parallel-resonant frequency - that would be about 12MHz, and nothing at all "special" is happening there. At 18MHz, where the total winding length is 0.25 wavelengths, there is a very small wobble in the data, but nothing more. The series resonance, where the phase angle flips from negative to positive again, is at 31.5MHz, which is totally unrelated to any of the other frequencies above. The winding length is 0.5 wavelengths at 35MHz (where the data runs out) but again nothing "special" is happening there. Again, no one would expect a bunched coil to be well behaved. Thus there is no evidence whatever for the myth of the "resonant length of wire in a choke". You keep saying that as if I said otherwise. I didn't. The length of the wire is irrelevant to this discussion. Turning now to the solenoid-wound choke, the different method of winding has increased the parallel resonance of the same length of cable from 6MHz to 9MHz. This is consistent with simple L-C behaviour, and with the solenoid having less distributed capacitance than the bunched winding. Once again, this choke behaves almost entirely as a parallel-tuned circuit. There are slightly larger wobbles in the data at the frequencies where the total winding lengths are a quarter-wave and a half-wave, but these "transmission-line" effects are still very minor, and completely dominated by the simple L-C behaviour. The point is that there is a 1/4WL high impedance resonance and a 1/2WL low impedance resonance that are roughly where they should be. The 1/2WL low impedance resonance should be avoided. As shown above, "1/2wl self resonance" ceases to be a valid concept once a length of wire is wound into a coil... The 1/2WL self-resonance has little to do with the length of wire. It is where the phase angle flips at a point of low impedance. The 1/4WL self-resonance is where the phase angle flips at a point of high impedance. The length of wire is irrelevant, a moot point. I don't know why you brought it up in the first place. If you say "the length of wire is irrelevant to this discussion" - with which I most strongly agree - why do you persist in using these terms "1/4WL" and "1/2WL" - what dimension of the choke are they referring to? The Excel workbook at www.ifwtech.co.uk/g3sek/misc/chokes.xls contains three spreadsheets. 1. Original data For all the coiled chokes (same data in the ARRL Antenna Book and on K1TT's site) with graphs of |Z|. There are minor dips at higher frequencies, but they are *minor*, and always in a region where the impedance is so low that you wouldn't be using that choke anyway. These graphs simply don't support the assertion of a series resonance at "twice the parallel-resonant frequency" - not even as an approximation. 2. Three chokes compared The solenoid-wound 8-turn choke, the bunched 8-turn choke, and the ferrite choke for comparison. The graphs give details of the Z magnitude and phase. 3. LC model For the 8-turn solenoid choke. The inductance is calculated from the physical dimensions of the choke, using the standard ARRL formula (winding length assumes close-wound RG213). The self-capacitance is calculated from the inductance and the choke's parallel-resonant frequency. The dynamic resistance is the peak value from 12MHz, and is assumed constant at all frequencies. Those simple assumptions - a fixed L, C and R, all connected in parallel - give a very good fit to the measured data at all frequencies (only one point has been forced to fit, namely the peak at 12MHz). This shows that the dominant behaviour of the choke is like a simple LC circuit, damped by some loss resistance. Much of the loss resistance is probably due to losses in the PVC jacket of the RG213. If these losses are actually increasing with frequency (rather than being constant, as assumed) then the fit at all frequencies would be improved. This very simple LCR model predicts almost everything that was measured. However, it cannot predict any series resonance at some higher frequency. If Cecil cares to produce a transmission-line model of the same choke that can do better, I'm sure we'd all be interested to see it. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
Ian White GM3SEK wrote:
For a more broadband solution based on coiled coax, I'd agree with Cecil's suggestion of cascading a large coil for the lower bands with a smaller coil optimized for the higher bands. It would also be possible to cascade a large coil with a small ferrite choke. Ian and Cecil, I think you both should disclose any ties you might have to the cable industry. (hi) I haven't been able to find out the proper placement of ferrite beads. Are they placed ANYWHERE along the cable. Do you calculate the transmission length for voltage peaks? Thanks, John AB8O |
Hf Antenna Question
Ian White GM3SEK wrote:
In that case, I suggest you stop making constant references to "1/4WL self-resonance" and "1/2WL self-resonance". If you don't mean it literally, it's a very misleading metaphor. All of those concepts are explained at: www.ttr.com/TELSIKS2001-MASTER-1.pdf in an IEEE sponsored paper by KB1EUD and K1AON. The first high impedance self-resonance point of a coil, where the phase angle changes sign, is called "quarter-wave resonance". (The self-resonant frequency for a 75m bugcatcher loading coil *IS* the 1/4WL self- resonant point.) Under "III. TRANSMISSION LINE MODELING", it says: "By means of conventional distributed-element theory, a thorny boundary value problem has been reduced to a very simple RF transmission line. In fact, the entire design and tuning exercise ... can now be performed conveniently on a Smith Chart." "There are a great number of formulae for coil self-capacitance. None are of particular value for quarter-wave helical resonators anywhere near the 90 degree point." A coiled coax choke operated at its self-resonant frequency *IS* being operated at the quarter-wave (90 degree) point. I doubt that the IEEE would publish a "very misleading metaphor". I have graphed the |Z| data for all the chokes (see link to spreadsheet below) and there is no consistent trend. In the following table, Fmax is the frequency of maximum impedance, and Fmin is the frequency of any minimum observable within the frequency range (the 8t 1 layer choke has two very small minima). Ratio is Fmax/Fmin. What we are looking for is the phase shift from negative to positive. That would indicate the 1/2WL point. Choke Fmax Fmin Ratio 6t 1 layer 24 none - The 1/4WL self-resonant point is at 24 MHz. 48 MHz data is not given. There is no phase shift from negative to positive in the given data. The 1/2WL resonant point is not contained in the data so this set of data is useless for finding the 1/2WL point. 12t 1 layer 15 31 2.1 2.1 is approximately 2 4t 1 layer 21 34 1.6 8t 1 layer 12 19 1.6 Round the 1.6 to a single digit - that's approximately 2 8t bunched 12 32 2.7 Bunched isn't well behaved enough to count. beaded 6 36 6.0 Beaded isn't a coil so doesn't count. Judging from the shapes of the graphs and the table above, I would say that "twice the frequency" is not even valid as an approximation. Someone needs to explain to mathematicians that rounding 1.6 to an integer isn't equal to 2. If I said it was an extremely rough approximation, would that be better? Across the whole 1-30MHz band, the bunched choke behaves as an almost perfect L-C circuit, free from any unwanted resonances. Which means it is not behaving as a slow-wave coil structure. One might say it is misbehaving and is a very poor design. If you say "the length of wire is irrelevant to this discussion" - with which I most strongly agree - why do you persist in using these terms "1/4WL" and "1/2WL" - what dimension of the choke are they referring to? I'm using them because the IEEE uses them. I keep telling you that they do not refer to a physical dimension! They refer to a measurable condition. The first self-resonance is obviously the 1/4WL point. I did make a mental slip-up in my previous posting. I forgot that the VF of the coil changes with frequency. That would help explain the deviation away from the times two value for 1/2WL resonance. To illustrate the transmission line characteristic of the choke, the frequency needs to remain constant while the number of turns is varied. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
jawod wrote:
I haven't been able to find out the proper placement of ferrite beads. Are they placed ANYWHERE along the cable. Ideally, they should be placed at a common-mode standing- wave current antinode (maximum). Aren't you glad you asked? Most hams install the choke at the most convenient place. However, at the feedpoint of a one wavelength dipole is not a good place. A common place is at a BALanced to UNbalanced junction where the choke can perform the balun function. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
jawod wrote:
Ian White GM3SEK wrote: For a more broadband solution based on coiled coax, I'd agree with Cecil's suggestion of cascading a large coil for the lower bands with a smaller coil optimized for the higher bands. It would also be possible to cascade a large coil with a small ferrite choke. Ian and Cecil, I think you both should disclose any ties you might have to the cable industry. (hi) Somewhere in there is a very bad joke about "cable ties"... Cable is much less expensive over here than large ferrite beads imported from the USA; hence my interest in the coiled cable chokes. I haven't been able to find out the proper placement of ferrite beads. Are they placed ANYWHERE along the cable. Do you calculate the transmission length for voltage peaks? The place to start is where the common-mode current would be launched onto the coax. In an antenna fed with coax all the way, that would be the feedpoint. In an antenna fed partly with coax and then with parallel line, it would be at the transition point. But if the system is physically asymmetrical (eg if the feedline runs back horizontally below the antenna) a single choke may not be enough to solve the problem. In that case, the next place to think about would be a quarter-wave closer to the transmitter. ....Doorbell... Someone stopped by for a long Sunday afternoon chat, and in the meantime Cecil has provided the rest of the answer. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
Ian White GM3SEK wrote:
Across the whole 1-30MHz band, the bunched choke behaves as an almost perfect L-C circuit, free from any unwanted resonances. One additional point. If the above were true, as the frequency is increased, the phase angle of the coiled choke impedance would drop from ~90 degrees to zero at the self-resonant frequency, and then rise back to ~-90 degrees and stay there. But that's not what happens. In every single case, the phase angle rises toward -90 degrees *and then decreases* as the 1/2WL self-resonance point is approached. That is a clear indication of transmission line effects. A lumped circuit simply doesn't act that way. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Cecil Moore wrote: David G. Nagel wrote: Your best choke is about 30 feet of coax rolled up into a 12" coil. Seems to me, the best choke(s) would be two chokes, one designed for 75m-15m and the other designed for 12m-2m. I cannot bring myself to believe that a coiled up piece of coax could ever be an effective choke over a 40:1 frequency range. -- 73, Cecil http://www.w5dxp.com Why worry about VHF though? Most people wouldn't want to use their HF wire for VHF. Or at least, I wouldn't. I always use other antennas for those bands. Seems to me, if you were to use two chokes, one for 160-40, or 80-30, and one for 20-10 would be more practical. A good choke for 20-10 "tribander", is 8 turns of RG8 wound on a 6 inch form. There is no real need to wind in an orderly military manner, although some do. I often just throw them together liking rolling up a rope, and they work fine. I had to place a small choke on my mobile for 2m. I kept having problems tuning a 5/8 wave 2m whip I made a few weeks ago. Tried trimming, adjusting the coil, etc. No joy. Came to the conclusion common mode currents were causing me grief, and this is using a hole mount drilled in the trunk lid. I usually don't have problems with those.. I wound a small coil from rg-58, maybe 4-5 turns on a small appx 2 inch dia, and all my problems were gone. A coax choke will need to be pretty small to work well on VHF. I don't really see one made to work the upper HF bands as being very good. So, in designing one for upper HF, I'd use 30 mhz as the upper limit. I've got some info somewhere, maybe in the ARRL ant book that gives appx dimensions for chokes at various frequencies. 30 ft of coax wound on a 12 inch from would more likely suit the lower bands more than the upper HF bands. So I'd use a 8 turn, six inch dia choke for 20-10, and whatever choke for the low bands. I'd leave VHF out of it, being most HF wire antennas are fairly lame for VHF. Heck, on my 706mk2g, using an HF wire antenna is inviting lots of image problems for receive when using 6 meters. 2 meters, I use ground planes, yagi's, etc.. I use a 3 el yagi on 6m. MK |
Hf Antenna Question
On Sun, 22 Oct 2006 18:01:24 +0100, Ian White GM3SEK
wrote: Cable is much less expensive over here than large ferrite beads imported from the USA; hence my interest in the coiled cable chokes. The cost of cores from imported from the US in Australia is also very high. For those in Australia interested in W2DU style baluns, I roughly measured the impedance of an inexpensive core from Jaycar using a Mighty Fine Junk 259B and plotted the results at http://www.vk1od.net/balun/index.htm . Less than A$20 is probably sufficient for an adequate balun for 80m to 10m.... depending... My own view is that the number of cores required for a W2DU style balun is unrelated to the characteristic impedance (inner to outer conductor) of the coax as is commonly held, but that the number of cores / effectiveness varies with location on the feedline and is highly dependent on the scenario (frequency, topology etc) and the optimum solution may required more than a single choke. Ian, perhaps low cost suppression cores are also available directly in your country, candidates for measurement and reporting! Owen PS Interesting plots, better presentation isn't it, the Y axis label on your phase graph needs a fix. -- |
Hf Antenna Question
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Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: [Snip] Only one part of that posting grabbed my attention: beaded 6 36 6.0 Beaded isn't a coil so doesn't count. The word "beaded" did not exist in my original posting... but there it is in Cecil's reply, complete with the double that attributes it to me, so that Cecil can knock the straw-man down. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: Across the whole 1-30MHz band, the bunched choke behaves as an almost perfect L-C circuit, free from any unwanted resonances. One additional point. If the above were true, as the frequency is increased, the phase angle of the coiled choke impedance would drop from ~90 degrees to zero at the self-resonant frequency, and then rise back to ~-90 degrees and stay there. But that's not what happens. In every single case, the phase angle rises toward -90 degrees *and then decreases* as the 1/2WL self-resonance point is approached. That is a clear indication of transmission line effects. A lumped circuit simply doesn't act that way. The spreadsheet at http://www.ifwtech.co.uk/g3sek/misc/chokes.xls now includes graphs of the phase information for all the chokes. [ Thank you, Owen - the phase axis label is now fixed . For anyone interested who doesn't have Excel, Microsoft's free Excel file viewer is at http://tinyurl.com/cup85 ] Coming all the way back to the original question, the data confirms that even though it may look like something held together with duct tape, a coiled coax choke can be an excellent single-band solution. At its parallel self-resonant frequency, it will have a much higher common-mode impedance than a generic string of ferrite beads. It can probably outperform or at least equal a ferrite choke over two or possibly three adjacent HF bands; but being a resonant device, it cannot deliver extreme broadband performance. In the chokes we're looking at, the low-impedance series resonances of which Cecil complains do not occur below 30MHz. Those resonances exist, but not on the HF frequencies where the chokes would actually be used. Within the practical working frequency range of all of these coiled-coax chokes, the performance can be accurately described as that of a simple parallel tuned LC circuit, which displays no transmission-line behaviour whatever. Cecil complains that One might say it is misbehaving and is a very poor design. That sounds to me like the complaint of someone who has a pet theory to hammer, and is disappointed when he can't find a nail. I think that's it, really. The graphs themselves say the rest. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
On Sun, 22 Oct 2006 23:56:04 +0100, Ian White GM3SEK
wrote: [ Thank you, Owen - the phase axis label is now fixed . For anyone interested who doesn't have Excel, Microsoft's free Excel file viewer is at http://tinyurl.com/cup85 ] Hi Owen, It is far easier to simply use OpenOffice which is an executable that will translate to/from Windows Office (any spread sheet, document, presentation, drawing...). The Open Document format is the mandated standard of the European Community, if I recall correctly. 73's Richard Clark, KB7QHC |
Hf Antenna Question
On Sun, 22 Oct 2006 16:24:33 -0700, Richard Clark
wrote: Hi Owen, Sorry Ian, wrong side of the continental divide here. |
Hf Antenna Question
Ian White GM3SEK wrote:
beaded 6 36 6.0 Beaded isn't a coil so doesn't count. The word "beaded" did not exist in my original posting... but there it is in Cecil's reply, complete with the double that attributes it to me, so that Cecil can knock the straw-man down. Ian, I think I corrected an obvious typo of yours. It appeared that you had gotten off by one row. If that was wrong, I apologize. Here's what you posted: Choke Fmax Fmin Ratio 6t 1 layer 24 none - 12t 1 layer 15 31 2.1 4t 1 layer 21 34 1.6 8t 1 layer 12 19 1.6 12 32 2.7 8t bunched 6 36 6.0 There were six chokes. The fifth was bunched and the sixth was beaded. The bunched ratio is 2.7. It seemed obvious that you had made a typo in your chart which I corrected. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Ian White GM3SEK wrote:
In the chokes we're looking at, the low-impedance series resonances of which Cecil complains do not occur below 30MHz. Those resonances exist, but not on the HF frequencies where the chokes would actually be used. But, Ian, what about 75m and 40m? Those resonances above 26 MHz are transmission line effects. The largest coil is only 12 turns, not nearly enough for 75m operation. The largest 12 turn coil has a choking impedance of only 100 ohms on 75m. Scale the choke to 75m and 40-50 turns are probably required. That would certainly lower the 1/2WL resonant frequencies into the HF ham bands. The 12 turn coil's peak impedance is around 15 MHz and its 1/2WL low impedance response is around 31 MHz. If we scale the peak impedance value to 4 MHz by increasing the turns, the 1/2WL low impedance response would be around 8 MHz. Such a choke would be useless above 10 MHz. Bottom Line: If enough turns are used to achieve a high choking impedance on 75m, the 1/2WL resonant transmission line effects will occur in the middle of HF making it useless on the higher HF frequencies. Within the practical working frequency range of all of these coiled-coax chokes, the performance can be accurately described as that of a simple parallel tuned LC circuit, which displays no transmission-line behaviour whatever. This is obviously a false statement proved by your own graphs. If there were no transmission line effects, the phase graphs would all converge on -90 degrees. All of the phase bumps above 26 MHz are transmission line effects. If the chokes had their maximum impedance at 4 MHz, those 1/2WL transmission line effects would probably be in the range of 10 MHz rendering them useless on 30m-10m. Cecil complains that One might say it is misbehaving and is a very poor design. That sounds to me like the complaint of someone who has a pet theory to hammer, and is disappointed when he can't find a nail. Actually, I was just quoting this web page: http://www.k1ttt.net/technote/airbalun.html "- Don't bunch the turns together. Wind them as a single layer on a form. Bunching the turns kills the choking effect at higher frequencies." I agree with WA2SRQ that the bunched coil choke is a poor design. Do you think that WA2SRQ has a pet theory? I think that's it, really. The graphs themselves say the rest. They certainly do. Everything above 26 MHz is obviously transmission line effects. If the chokes were designed for the lower part of the HF spectrum, the 1/2WL low impedance points, caused by the transmission line effects, would be in the middle of the HF spectrum. Scale the high impedance points for 75m and the low impedance bumps due to transmission line effects will occur in the middle of HF. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
On Sun, 22 Oct 2006 16:24:33 -0700, Richard Clark
wrote: Hi Owen, It is far easier to simply use OpenOffice which is an executable that will translate to/from Windows Office (any spread sheet, document, presentation, drawing...). The Open Document format is the mandated standard of the European Community, if I recall correctly. Be careful. If any VB type macros are included in Excel Open Office will barf. Been there done that. Danny, K6MHE |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: beaded 6 36 6.0 Beaded isn't a coil so doesn't count. The word "beaded" did not exist in my original posting... but there it is in Cecil's reply, complete with the double that attributes it me, so that Cecil can knock the straw-man down. Ian, I think I corrected an obvious typo of yours. It appeared that you had gotten off by one row. If that was wrong, I apologize. Here's what you posted: Choke Fmax Fmin Ratio 6t 1 layer 24 none - 12t 1 layer 15 31 2.1 4t 1 layer 21 34 1.6 8t 1 layer 12 19 1.6 12 32 2.7 8t bunched 6 36 6.0 There were six chokes. The fifth was bunched and the sixth was beaded. The bunched ratio is 2.7. It seemed obvious that you had made a typo in your chart which I corrected. Apology accepted. What you had missed was the statement immediately above the table, that: (the 8t 1 layer choke has two very small minima). Therefore that choke has two lines of data. The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6. A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1 and 2.7 are all approximately equal to 2... so how about 6? -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
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Hash: SHA1 Danny Richardson wrote: On Sun, 22 Oct 2006 16:24:33 -0700, Richard Clark wrote: Hi Owen, It is far easier to simply use OpenOffice which is an executable that will translate to/from Windows Office (any spread sheet, document, presentation, drawing...). The Open Document format is the mandated standard of the European Community, if I recall correctly. Be careful. If any VB type macros are included in Excel Open Office will barf. Been there done that. Danny, K6MHE Opens fine in OpenOffice 2 on SUSE 10 but I did have to reposition the graphs which were over the table. Charlie. - -- www.wymsey.co.uk -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iD8DBQFFPIKufhKqaKFAJ/YRAgnOAJ9ZjDMfv6s+TwcswZd0YbJQJK5e1QCcCskT Dbm7fF6MOISjjCsbzjBLihA= =zEFh -----END PGP SIGNATURE----- |
Hf Antenna Question
charlie wrote:
Danny Richardson wrote: On Sun, 22 Oct 2006 16:24:33 -0700, Richard Clark wrote: Hi Owen, It is far easier to simply use OpenOffice which is an executable that will translate to/from Windows Office (any spread sheet, document, presentation, drawing...). The Open Document format is the mandated standard of the European Community, if I recall correctly. Be careful. If any VB type macros are included in Excel Open Office will barf. Been there done that. Danny, K6MHE Opens fine in OpenOffice 2 on SUSE 10 Thanks for that information, Charlie - it's very useful to know what other systems can see. but I did have to reposition the graphs which were over the table. That's how I saved the graphs, so OpenOffice got those layout details right. Once the graphs have been plotted, it isn't necessary to look at the underlying table very often, because the data can be seen in a popup window by running the mouse pointer along each plotted line. If OpenOffice can do that too, you may find it a convenient feature. Alternatively, you could drag both graphs away to the right, and then use the horizontal scroll bar to move between the table and graphs. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
Ian White GM3SEK wrote:
Apology accepted. What you had missed was the statement immediately above the table, that: (the 8t 1 layer choke has two very small minima). Therefore that choke has two lines of data. I apologize. I jumped to conclusions. The obvious typo wasn't so obvious after all The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6. A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1 and 2.7 are all approximately equal to 2... so how about 6? As I already stated: What I have said applies to well- designed coils that can be modeled as transmission lines. That includes properly wound chokes and bugcatcher loading coils. A bunched choke is known not to be well-designed. Quoting WA2SRQ again: "Don't bunch the turns together. Wind them as a single layer on a form. Bunching the turns kills the choking effect at higher frequencies." Bunching also tends to kill the transmission line effects. How many 75m bugcatcher loading coils have you wound in bunched mode? :-) -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Ian White GM3SEK wrote:
The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6. A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1 and 2.7 are all approximately equal to 2... so how about 6? It is a known fact that the VF of these chokes changes with frequency so they don't behave exactly like a transmission line. Bunching probably changes the VF considerably more than a helical wound choke since the first turn and last turn may be on top of each other. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Cecil Moore wrote: Ian White GM3SEK wrote: The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6. A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1 and 2.7 are all approximately equal to 2... so how about 6? It is a known fact that the VF of these chokes changes with frequency so they don't behave exactly like a transmission line. Bunching probably changes the VF considerably more than a helical wound choke since the first turn and last turn may be on top of each other. -- 73, Cecil http://www.w5dxp.com I've never actually really compared bunching to helical wound. The one they specify to build for my triband yagi is a helical coil. On that antenna, I've used both methods, but not at the same time to really compare. I didn't notice any real noticable difference in operation though. I was just looking at the small choke in my trunk for 2m. It hangs from the trunk lid, and is tie wrapped. It was about 2-3 diameter, and about 4-5 turns or so, bunched together. For the purpose I used it for, it worked great. But it could well be possible what you say about the higher frequency operation. I do know a bunched choke is a whole lot better than no choke at all. :) MK MK |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: In the chokes we're looking at, the low-impedance series resonances of which Cecil complains do not occur below 30MHz. Those resonances exist, but not on the HF frequencies where the chokes would actually be used. But, Ian, what about 75m and 40m? Those resonances above 26 MHz are transmission line effects. The largest coil is only 12 turns, not nearly enough for 75m operation. The largest 12 turn coil has a choking impedance of only 100 ohms on 75m. Scale the choke to 75m and 40-50 turns are probably required. Sorry for the delay in replying, but I just did something completely radical for this type of discussion: hauled out some cable, made some chokes, and measured them. These chokes were intended to be like the ones recommended in the ARRL Handbook for 3.5MHz and for 7MHzz, both using 22ft of RG213. What I had was actually 21ft, and this length was wound into a flat coil having various numbers of turns. The choking impedances and phase angles were measured on an N2PK Vector Network Analyser. Ten turns made a choke that had 1000 ohms impedance from 3.4MHz to 6.4MHz, falling to about 650 ohms by 7.3MHz. This makes a respectable two-band choke, if you judge it by the criterion of needing about 500 ohms minimum impedance, but it's a stretch to cover both bands from edge to edge (9 turns was probably a slightly better compromise). Moreover the maximum impedance of 17k is "wasted" between the amateur bands at 4.6MHz. That confirms our shared suspicion that the Handbook's claim of "a broad resonance that easily (sic) covers three octaves" is over-optimistic. Where the coiled-cable choke really excels is as a single-band device, exploiting its extremely high peak impedance at parallel resonance. Without needing to change the overall length of cable, the peak resonance can be shifted to different frequencies by increasing or decreasing the number of turns, although I couldn't persuade 21ft below 4.6MHz (10 turns) because the bending radius was becoming too small for RG213. However, it's perfectly clear that Cecil's estimate of "40-50 turns" needed for a 75m choke is way off. That would certainly lower the 1/2WL resonant frequencies into the HF ham bands. The 12 turn coil's peak impedance is around 15 MHz and its 1/2WL low impedance response is around 31 MHz. If we scale the peak impedance value to 4 MHz by increasing the turns, the 1/2WL low impedance response would be around 8 MHz. Such a choke would be useless above 10 MHz. Bottom Line: If enough turns are used to achieve a high choking impedance on 75m, the 1/2WL resonant transmission line effects will occur in the middle of HF making it useless on the higher HF frequencies. Right conclusion about the choke being useless a long way above its peak resonance - but not the right reason for it. The true reason is the very simple property of a parallel LC circuit (the inductance of the coil, resonated by its own self-capacitance) which means that far above resonance its parallel impedance drops to a very low value. That's what makes it useless at those frequencies. The 4.6MHz choke does have series resonances at about 23MHz 33MHz ,but those cannot be said to affect the performance of the choke in any practical way, because the choke doesn't have any usable performance at these frequencies anyway. Within the practical working frequency range of all of these coiled-coax chokes, the performance can be accurately described as that of a simple parallel tuned LC circuit, which displays no transmission-line behaviour whatever. This is obviously a false statement proved by your own graphs. If there were no transmission line effects, the phase graphs would all converge on -90 degrees. All of the phase bumps above 26 MHz are transmission line effects. If the chokes had their maximum impedance at 4 MHz, those 1/2WL transmission line effects would probably be in the range of 10 MHz rendering them useless on 30m-10m. Cecil keeps resolutely ignoring the reservation: "within the practical working range of the choke". Within the frequency range where the choke has a practically useful value of impedance, there are no - repeat NO - signs of transmission line effects. Its behaviour is purely LC, its own inductance resonating with its self-capacitance. Cecil complains that One might say it is misbehaving and is a very poor design. That sounds to me like the complaint of someone who has a pet theory to hammer, and is disappointed when he can't find a nail. Actually, I was just quoting this web page: http://www.k1ttt.net/technote/airbalun.html "- Don't bunch the turns together. Wind them as a single layer on a form. Bunching the turns kills the choking effect at higher frequencies." I agree with WA2SRQ that the bunched coil choke is a poor design. Do you think that WA2SRQ has a pet theory? I think WA2SRQ's advice is one-sided, because it fails to recognize the benefit of bunching the cable together - namely that it moves the whole resonance curve downward (due to the increased self-capacitance) without needing to increase the length of cable. For someone who wants a low-band choke, that is a Very Good Thing. The poorer performance at higher frequencies is the obvious natural tradeoff for having moved the whole resonance curve downward - but that doesn't make it a "poor design". I think that's it, really. The graphs themselves say the rest. They certainly do. Everything above 26 MHz is obviously transmission line effects. If the chokes were designed for the lower part of the HF spectrum, the 1/2WL low impedance points, caused by the transmission line effects, would be in the middle of the HF spectrum. Scale the high impedance points for 75m and the low impedance bumps due to transmission line effects will occur in the middle of HF. Hands-on measurements prove otherwise - the spurious resonances stay pretty much where they were, above 20MHz. As stated above, I measured a choke which is resonant at 4.6MHz. It has over 1000 ohms impedance at 3.5MHz, rising to over 2k at 4MHz, so it would be a very good performer on both 75m and 80m. But above the main parallel resonance there are no bumps in the impedance or ripples in the phase response at any frequency below 20MHz - none. It is ridiculous for Cecil to describe such a choke as "misbehaving and a very poor design". On the contrary, bunching the turns to increase the self-capacitance has proved a very good way of moving the main resonance downward to make an effective single-band choke for 75/80m, while leaving all the unwanted resonances parked above 20MHz. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
wrote:
I've never actually really compared bunching to helical wound. The one they specify to build for my triband yagi is a helical coil. On that antenna, I've used both methods, but not at the same time to really compare. I didn't notice any real noticable difference in operation though. You wouldn't notice much difference for a single band and maybe not even for a tribander. The helical has a greater bandwidth than the bunched. For instance, at: http://www.k1ttt.net/technote/airbalun.html there's an 8 turn helical Vs 8 turn bunched. If the target choking impedance is 500 ohms minimum, the helical covers 6-24 MHz, a 4:1 ratio, while the bunched covers 4-10 MHz, a 2.5:1 ratio. The helical also has about two times the maximum choking impedance as the bunched one. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Ian White GM3SEK wrote:
However, it's perfectly clear that Cecil's estimate of "40-50 turns" needed for a 75m choke is way off. Wouldn't you say the number of turns depends upon the diameter of the coil? How many turns would be needed for 75m self-resonance if one were using RG-58 wound on a 3 inch diameter PVC pipe? Using the inductance formula in the ARRL Handbook, 40 turns on a 3 inch diameter form at 4 TPI is about 32 uH or about 800 ohms on 4 MHz. The distributed capacitance would lower it even farther. 40 turns on a 3" form would be an absolute minimum for any high SWR situation on 75m. Right conclusion about the choke being useless a long way above its peak resonance - but not the right reason for it. The true reason is the very simple property of a parallel LC circuit (the inductance of the coil, resonated by its own self-capacitance) which means that far above resonance its parallel impedance drops to a very low value. That's what makes it useless at those frequencies. Ian, your own graphs show transmission line effects. The fact that the phase angle does NOT go to -90 degrees and stay there proves those are transmission line effects. Here's what the IEEE paper says: "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." "There are a great number of formulae for coil self-capacitance. None are of particular value for quarter-wave helical resonators anywhere near the 90 degree point." Your parallel self-resonance *IS* the 90 degree point. I wish you would take time out to realize that if there were no transmission line effects, the phase angle would go to -90 degrees and stay there. Please set up a parallel inductor and capacitor and see for yourself what happens as one increases the frequency past parallel resonance. The phase angle is asymptotic to -90 degrees. The 4.6MHz choke does have series resonances at about 23MHz 33MHz ,but those cannot be said to affect the performance of the choke in any practical way, because the choke doesn't have any usable performance at these frequencies anyway. Those grapes were probably sour anyway. :-) Ian, a lumped circuit inductor and parallel capacitance would NOT have those series resonances. A lumped circuit would go to a phase angle of -90 degrees and stay there while the impedance drops inversely proportional to frequency. Seems to me, you have just admitted that the chokes you wound are exhibiting transmission line effects just as I predicted. (Except for the VF error I made.) THE CHOKE DOESN'T HAVE ANY USABLE PERFORMANCE AT THOSE HIGHER FREQUENCIES *BECAUSE* OF THE TRANSMISSION LINE EFFECTS!!! Cecil keeps resolutely ignoring the reservation: "within the practical working range of the choke". That's circular logic, Ian. I said that series resonant transmission line effects limit the practical working range of a coax choke and you disagreed. Now you have proved I am right with your own measurements on chokes of your own design. If those actually were lumped inductors and capacitors as you continue to assert, THE SERIES RESONANCES THAT YOU MEASURED WOULD NOT AND COULD NOT EXIST!!! The series resonant transmission line effects are the *CAUSE* of the practical working range of the choke being limited. In a parallel LC circuit, as the frequency increases, the capacitive reactance becomes dominant and decreases inversely proportional to frequency. The phase angle would be asymptotic to -90 degrees. None of the measurements look anything like that. All of the measurements exhibit transmission line effects. Within the frequency range where the choke has a practically useful value of impedance, there are no - repeat NO - signs of transmission line effects. Its behaviour is purely LC, its own inductance resonating with its self-capacitance. This will be the forth time I have said this, Ian. If there were no sign of transmission line effects, the phase angle would go to -90 degrees and stay there. If the phase angle doesn't go to -90 degrees and stay there, that is *prima facie evidence* of transmission line effects. Lumped parallel inductors and capacitors don't exhibit the effects measured by you on the chokes that you wound. Scale the high impedance points for 75m and the low impedance bumps due to transmission line effects will occur in the middle of HF. Hands-on measurements prove otherwise - the spurious resonances stay pretty much where they were, above 20MHz. Uhhhhh Ian, 20 MHz *IS* HF! The transmission line 1/2WL series resonant effects are occurring in the HF range! That's why the 75m choke doesn't work on 15m!!! 20-30 MHz is 37% of HF. As stated above, I measured a choke which is resonant at 4.6MHz. It has over 1000 ohms impedance at 3.5MHz, rising to over 2k at 4MHz, so it would be a very good performer on both 75m and 80m. But it wouldn't make a good performer on 15m because of the series resonant transmission line effects. That was my point from the beginning. (The mistake I made was forgetting that the VF of the choke changes with frequency.) It is ridiculous for Cecil to describe such a choke as "misbehaving and a very poor design". This thread has not been about single-band or narrow-band chokes. We were talking about broadband chokes being able to cover three octaves of HF. The bunched coil choke is much more narrow-banded than helical wound chokes thus rendering them virtually useless for broadband operation. They *misbehave* in broadband (all-HF) applications. They are a *very poor design* for broadband (all-HF) applications. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question
Cecil Moore wrote:
Ian White GM3SEK wrote: However, it's perfectly clear that Cecil's estimate of "40-50 turns" needed for a 75m choke is way off. Wouldn't you say the number of turns depends upon the diameter of the coil? How many turns would be needed for 75m self-resonance if one were using RG-58 wound on a 3 inch diameter PVC pipe? Using the inductance formula in the ARRL Handbook, 40 turns on a 3 inch diameter form at 4 TPI is about 32 uH or about 800 ohms on 4 MHz. The distributed capacitance would lower it even farther. 40 turns on a 3" form would be an absolute minimum for any high SWR situation on 75m. I am not your offshore lab service, Cecil. If you want to back up your speculations, do your own work. Right conclusion about the choke being useless a long way above its peak resonance - but not the right reason for it. The true reason is the very simple property of a parallel LC circuit (the inductance of the coil, resonated by its own self-capacitance) which means that far above resonance its parallel impedance drops to a very low value. That's what makes it useless at those frequencies. Ian, your own graphs show transmission line effects. The fact that the phase angle does NOT go to -90 degrees and stay there proves those are transmission line effects. Here's what the IEEE paper says: "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." "There are a great number of formulae for coil self-capacitance. None are of particular value for quarter-wave helical resonators anywhere near the 90 degree point." This discussion doesn't involve anybody's formula for self-capacitance. The coil resonates using whatever value of self-capacitance it has in reality. Your parallel self-resonance *IS* the 90 degree point. I wish you would take time out to realize that if there were no transmission line effects, the phase angle would go to -90 degrees and stay there. Please set up a parallel inductor and capacitor and see for yourself what happens as one increases the frequency past parallel resonance. The phase angle is asymptotic to -90 degrees. I know what a parallel LC circuit does. The point you continue to evade is that, from VLF up to about 20MHz, this coil of cable behaves in exactly the same way. The circuit looks inductive below the resonant frequency and capacitive above it, and passing through resonance the phase angle of the impedance flips from +90deg to -90deg... and then it stays very close to 90deg, clear up to about 20MHz. (The VNA reports angles of -88 to -89deg.) The 4.6MHz choke does have series resonances at about 23MHz 33MHz ,but those cannot be said to affect the performance of the choke in any practical way, because the choke doesn't have any usable performance at these frequencies anyway. Those grapes were probably sour anyway. :-) Ian, a lumped circuit inductor and parallel capacitance would NOT have those series resonances. A lumped circuit would go to a phase angle of -90 degrees and stay there while the impedance drops inversely proportional to frequency. Seems to me, you have just admitted that the chokes you wound are exhibiting transmission line effects just as I predicted. (Except for the VF error I made.) THE CHOKE DOESN'T HAVE ANY USABLE PERFORMANCE AT THOSE HIGHER FREQUENCIES *BECAUSE* OF THE TRANSMISSION LINE EFFECTS!!! Cecil keeps resolutely ignoring the reservation: "within the practical working range of the choke". That's circular logic, Ian. I said that series resonant transmission line effects limit the practical working range of a coax choke and you disagreed. Now you have proved I am right with your own measurements on chokes of your own design. If those actually were lumped inductors and capacitors as you continue to assert, THE SERIES RESONANCES THAT YOU MEASURED WOULD NOT AND COULD NOT EXIST!!! That is the usual mixture of selective quoting and false logic. What you continue to overlook is that the behaviour all the way from DC through the 4.7MHz resonance and onward up to about 20MHz can be accurately represented by nothing more elaborate than a simple LCR circuit. The fall in impedance from the resonant frequency up to about 20MHz is completely accounted for by just those three simple parameters: two reactances and one fixed loss resistance in parallel. Above that frequency there are effects that the simple LCR model cannot account for. I have always said so, and yes, a transmission-line model could account for those. The series resonant transmission line effects are the *CAUSE* of the practical working range of the choke being limited. That is simply not true. The limitation in working range is simply the shunting effect of the self-capacitance, which becomes increasingly important above resonance and causes a long progressive fall in impedance. This effect is very simple and entirely predictable. By 20MHz it has reduced the impedance to a hundred ohms or less, which means that the coil of cable is no use as a feedline choke for that frequency. The series resonances above 20MHz cause further dips in impedance to only a few ohms, but those dips are quite localized in frequency. They are not the cause of the long progressive fall in impedance above the parallel resonance, which is what limits the usable bandwidth of the choke. The challenge is still on the table, Cecil, for YOU to develop a quantified transmission-line model that will predict all the measured properties of a resonant choke over that wider range of frequencies. The rest is snipped, because it's just you continuing your own argument with the straw-man you have manufactured. A self-resonant choke is not a wideband solution capable of cover the whole of HF. I never expected it to be, and never claimed it would be, so stop acting as if I had. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question
Ian White GM3SEK wrote:
I am not your offshore lab service, Cecil. If you want to back up your speculations, do your own work. I posted my measurements but I am handicapped by not being able to measure any impedance above 650 ohms. But I can sure see those series resonant points with my MFJ-259B. You know, the points that your lumped circuit model says do not exist? I know what a parallel LC circuit does. The point you continue to evade is that, from VLF up to about 20MHz, this coil of cable behaves in exactly the same way. And falls apart above 20 MHz because of transmission line effects. Your lumped circuit model is a subset of the distributed network model. Of course, they will give similar results up to the point where the lumped circuit model falls apart. The circuit looks inductive below the resonant frequency and capacitive above it, and passing through resonance the phase angle of the impedance flips from +90deg to -90deg... and then it stays very close to 90deg, clear up to about 20MHz. Yes, both models give similar results up to 20 MHz which is about half way around the Smith Chart. Then your model falls apart. The fact that the phase angle departs radically from -90 degrees in reality when your model predicts that it should stay at -90 degrees is prima facie evidence that your model has fallen apart. Your own phase graphs contradict what you are saying. That is the usual mixture of selective quoting and false logic. What you continue to overlook is that the behaviour all the way from DC through the 4.7MHz resonance and onward up to about 20MHz can be accurately represented by nothing more elaborate than a simple LCR circuit. The fall in impedance from the resonant frequency up to about 20MHz is completely accounted for by just those three simple parameters: two reactances and one fixed loss resistance in parallel. Just proving that the lumped circuit model is a subset of the distributed network model and the two results are expected to be similar. But your model falls apart above 20 MHz where the transmission line effects are obvious. Above that frequency there are effects that the simple LCR model cannot account for. I have always said so, and yes, a transmission-line model could account for those. I certainly don't remember you ever saying that but we are making progress. You are agreeing with me and we seem to have little argument left. What I seem to remember you saying is that it is "ridiculous" to model a parallel self-resonant choke as a transmission line. But my memory is not as good as it once was. That is simply not true. The limitation in working range is simply the shunting effect of the self-capacitance, which becomes increasingly important above resonance and causes a long progressive fall in impedance. This effect is very simple and entirely predictable. By 20MHz it has reduced the impedance to a hundred ohms or less, which means that the coil of cable is no use as a feedline choke for that frequency. Your rose colored glasses are giving you false images. In the earlier example, the 12 turn choke had a maximum choking impedance at 15 MHz. At 23 MHz, the phase angle goes to -88 degrees just as both models predict. At 32 MHz, the phase angle is back to +20.4 degrees. Using your lumped circuit model, how does the phase angel get back to +20.4 degrees with that lumped capacitance dominating??? Ian, IF YOU ANSWER ONLY ONE QUESTION, PLEASE ANSWER THIS ONE. Exactly how does the phase angle get to +20.4 degrees at the exact time that your lumped circuit model is predicting -90 degrees??? (That's a 541% error!) The series resonances above 20MHz cause further dips in impedance to only a few ohms, but those dips are quite localized in frequency. They are not the cause of the long progressive fall in impedance above the parallel resonance, which is what limits the usable bandwidth of the choke. You are attempting to use petitio principii to prove the validity of your model and I think you know that is a no-no. A similar long progressive fall happens with the distributed network model but it accurately predicts the transmission line effects proved by the bumps in the phase graphs that you provided. Back to the previously discussed 12 turn choke. The impedance at 23 MHz is 955 ohms at -88 degrees, almost purely capacitive. Your "long progression" model would predict 686 ohms at -89 degrees for 32 MHz. Yet at 32 MHz, the impedance is measured to be 258 ohms at +20.4 degrees. Your lumped circuit "long progressive fall" model could not be any more wrong. Your impedance is off by 166% and your phase is off by 541%. Please note that if your lumped circuit model were correct, the choke would still be performing pretty well at 32 MHz with a choking impedance of 686 ohms. Your above statement is thus proved false by the measured data. The challenge is still on the table, Cecil, for YOU to develop a quantified transmission-line model that will predict all the measured properties of a resonant choke over that wider range of frequencies. My model, although not perfect, yields more accurate results than your model over that wider range of frequencies. My model predicts the bumps in the phase graphs. Your model predicts zero bumps in the phase graphs. Yet the bumps are obvious on your phase graphs. My model, a superset of yours, sure doesn't produce errors like 541%. -- 73, Cecil, w5dxp.com |
Hf Antenna Question
Ian White GM3SEK wrote:
I think WA2SRQ's advice is one-sided, because it fails to recognize the benefit of bunching the cable together - namely that it moves the whole resonance curve downward (due to the increased self-capacitance) without needing to increase the length of cable. For someone who wants a low-band choke, that is a Very Good Thing. The poorer performance at higher frequencies is the obvious natural tradeoff for having moved the whole resonance curve downward - but that doesn't make it a "poor design". Ian, I forgot to congratulate you on making a purse out of a sow's ear. Or is it a pig in a poke? :-) -- 73, Cecil http://www.w5dxp.com |
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