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#1
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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 |
#2
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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 |
#3
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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 |
#4
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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 |
#5
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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 |
#6
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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 |
#7
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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 |
#8
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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 |
#9
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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 |
#10
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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 |
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