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Hf Antenna Question - so what's the conclusion?
Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have
discused HF chokes made from flat vs. bunched coils of coaxial cable, but I must have missed some posting(s), for I haven't recognized anything that looks like a CONCLUSION. * Is "bunched" or "flat" better? * Are they basically single-band or wide-band or somewhere in-between? * Are there "rules of thumb" to make them for hams with * No test equipment? * A grid-dip meter? * A noise bridge? * An antenna analyzer? -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor Certified Instructor for the Kansas Concealed-Carry Handgun license |
Hf Antenna Question - so what's the conclusion?
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Hf Antenna Question - so what's the conclusion?
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Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
wrote: Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have discused HF chokes made from flat vs. bunched coils of coaxial cable, but I must have missed some posting(s), for I haven't recognized anything that looks like a CONCLUSION. Sorry, my Internet access is down at the moment, so there's no knowing when this will get out. Ian and I agree .... about quite a lot actually :-) that the ARRL was overly optimistic when they asserted that a coiled coax choke can cover all three octaves of HF. Agreed. That claim is made in the general ARRL Handbook. The ARRL Antenna Book gives more detail, but notably does not include that claim. The requirements for a choke feeding an all-HF-band, ladder-line fed dipole are much higher than the requirements for a choke feeding a resonant fan dipole. * Is "bunched" or "flat" better? "Bunched" and "flat" both mean the same to me - the cable is wound into a flat coil like a length of rope. The actual choice is between that shape and a cylindrical solenoid shape. Bunching disadvantage: Lowers the maximum choking impedance and decreases the effective bandwidth. (Bunching increases the stray capacitance.) Bunching advantage?: Lowers the parallel self-resonant frequency so not as many turns are required for narrow-band operation. The bunched/flat shape has more self-capacitance between turns, so for the same total length of cable it has a lower self-resonant frequency. The bandwidth of either shape is wide enough to cover any amateur band with a very high impedance - way higher than you could get with a ferrite choke - but you do need to get the resonant frequency right. * Are they basically single-band or wide-band or somewhere in-between? Depends upon the minimum required choking impedance. A resonant coiled-cable choke is unbeatable on a single band, but you may not actually *need* a choking impedance of several kohms. If you only need an impedance of a few hundred ohms (though more is always OK of course) then I'd agree with Cecil: .. A 2:1 to 3:1 frequency range might be a good rule of thumb, e.g. one choke can cover a tribander's 2:1 frequency range. That seems realistic for either the bunched/flat or the solenoid shape. * Are there "rules of thumb" to make them for hams with * No test equipment? * A grid-dip meter? * A noise bridge? * An antenna analyzer? How about: Using a 2L pop bottle as the coil form, don't use more turns than the number of meters in a wavelength, e.g. no more than 10 turns on 10 meters? That would probably work for 20m-6m as a rule of thumb. The MFJ-259B will measure impedance up to 650 ohms. One could define a "bandwidth" based on that 650 ohm value and knowledge of a typical impedance waveform between those two points. For instance, my 8 turn, 5.35" diameter choke falls below 650 ohms at 4.63 MHz and 28.1 MHz. It might be useful over a 3:1 frequency range from 40m-15m. You could certainly try Cecil's suggestions, though the resonant frequency can be quite critical if you want to spread the performance across more than one band. A GDO should be OK for measuring the resonant frequency, with the two ends of the choke shorted together. However, maybe we're all trying to measure the wrong thing. The reason why we're using these chokes is to reduce the unwanted common-mode current, so why not cut to the chase, and measure the current itself? Whoever you are, if you don't already have a snap-on RF current meter, then make it your next homebrew project or ham radio purchase. It is THE most useful single tool for RFI investigations, so it's always good to have one in the shack. In this particular application, you could measure the common-mode current on the TX side of the choke - the answer is there "in a snap". Then you can either declare the problem solved, or follow the meter readings as you adjust the choke to reduce the current even further. There are details of HB meters on my website, or the MFJ-854 is a good commercial alternative (but definitely not the MFJ-805). -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question - so what's the conclusion?
Dave wrote:
wrote: Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have discused HF chokes made from flat vs. bunched coils of coaxial cable, but I must have missed some posting(s), for I haven't recognized anything that looks like a CONCLUSION. * Is "bunched" or "flat" better? * Are they basically single-band or wide-band or somewhere in-between? * Are there "rules of thumb" to make them for hams with * No test equipment? * A grid-dip meter? * A noise bridge? * An antenna analyzer? Conclusions! You want WHAT?? You have recognized a GREAT TRUTH about this group. There NEVER is a conclusion! Oh, there are subthreads, red herring threads, three month continuous running threads, insulting threads, I am better than you are threads, even threads that remain close to the original subject. But, there is NEVER a concluding post to a thread. Actually there was enough information to conclude if you want an all hf band 1:1 balun, abandoning the coiled coax and using ferrite may be the way to go. Wow, what a run on sentence! |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
Cecil Moore wrote: Ian and I agree ... about quite a lot actually :-) Lurkers may get the impression that we disagree a lot. :-) Most of us technical types agree on 99 and 44/100% of technical topics. We often argue about which favorite approach is better when they are equally effective. I just plotted the impedance of a lumped inductor in parallel with a lumped capacitor, the impedance based on a transmission line analysis, and the impedance actually measured. The three graphs overlaid each other until approaching the series resonant point where the transmission line analysis more closely matched the measured results. In particular, the actual measured value was around +j258, the transmission line value was around +j135, and the lumped circuit value was around -j570. "Bunched" and "flat" both mean the same to me - the cable is wound into a flat coil like a length of rope. The actual choice is between that shape and a cylindrical solenoid shape. I don't think that's true, Ian. There are three ways to wind a choke. 1. helical, 2. spiral, 3. random I'm pretty sure "bunched" means "random" and *not* flat. Maybe that is the origin of our disagreement over "bunched". If the bunched coil was really random scatter winding, would you like it as much? There are details of HB meters on my website, or the MFJ-854 is a good commercial alternative (but definitely not the MFJ-805). I've got the MFJ-853. It snaps but doesn't unsnap. :-( -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Christopher Cox wrote:
SNIPPED You have recognized a GREAT TRUTH about this group. There NEVER is a conclusion! Oh, there are subthreads, red herring threads, three month continuous running threads, insulting threads, I am better than you are threads, even threads that remain close to the original subject. But, there is NEVER a concluding post to a thread. /s/ DD ... I wrote the above Actually there was enough information to conclude if you want an all hf band 1:1 balun, abandoning the coiled coax and using ferrite may be the way to go. Wow, what a run on sentence! Which sentence? Mine is totally grammatically correct. A real run on sentence will be found if you study the serious works of some Jesuit Priests. When I was in seminary I had to read/study some Jesuit theology texts. The first sentence took three pages. One chapter took an 8 hour flight from Boston to LA. And, I still didn't finish :-( But, they were grammatically correct ... Does that mean your sentence is too short? :-) Now, hopefully to conclude this thread [well maybe] I agree that a broadband ferrite balun is the way to go for typical HF. /s/ DD |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
"Bunched" and "flat" both mean the same to me - the cable is wound into a flat coil like a length of rope. The actual choice is between that shape and a cylindrical solenoid shape. I don't think that's true, Ian. There are three ways to wind a choke. 1. helical, 2. spiral, 3. random I'm pretty sure "bunched" means "random" and *not* flat. Maybe that is the origin of our disagreement over "bunched". If the bunched coil was really random scatter winding, would you like it as much? Sorry, maybe there's language barrier here. Imagine coiling a rope in your hand like a cowboy, then laying the coil on the ground. I meant a "flat" coil in that sense, as opposed to the other way of winding these chokes on a cylindrical former. There are details of HB meters on my website, or the MFJ-854 is a good commercial alternative (but definitely not the MFJ-805). I've got the MFJ-853. It snaps but doesn't unsnap. :-( The 854 that I reviewed for Radcom was the same. We both got one that had the core assembled upside-down, so instead of showing you how to snap the plastic catch open, the instruction manual shows you how to break it. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question - so what's the conclusion?
On Wed, 25 Oct 2006 16:13:59 +0100, Ian White GM3SEK
wrote: that the ARRL was overly optimistic when they asserted that a coiled coax choke can cover all three octaves of HF. Agreed. Hi Ian, Really? 3 to 30 MHz is more than three octaves: 3 - 6 - 12 - 24 + 1/3 octave more Are we talking about Ham Bands? 160 - 80 - 40 - 20 - 10 Are we restricting from the top of the 80M band through 10M? 4 - 8 - 16 - 32 (actually 88% octave more) From 60M up? 60 - 30 - 15 + 75% octave more A subset of HF? such as 160 - 80 - 40 80 - 40 - 20 40 - 20 - 10 To anyone, Just where did the ARRL come up with this new band plan for HF? Hi Myron, Can you see any conclusion yet? Even with the new 3 Octave specification you still have to pick your application from 3 choices. 73's Richard Clark, KB7QHC |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
Sorry, maybe there's language barrier here. Imagine coiling a rope in your hand like a cowboy, then laying the coil on the ground. I meant a "flat" coil in that sense, as opposed to the other way of winding these chokes on a cylindrical former. A lot of arguments are over semantics. Again, there are three ways of winding the chokes; helical, flat spiral, and bunched like a cowboy rope. The flat spiral is also known as a "pancake" choke or coil. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Richard Clark wrote:
Ian White GM3SEK wrote: that the ARRL was overly optimistic when they asserted that a coiled coax choke can cover all three octaves of HF. Agreed. Really? 3 to 30 MHz is more than three octaves: 3 - 6 - 12 - 24 + 1/3 octave more 3 is not in the ham bands. 3.6 - 7.2 - 14.4 - 28.8 There's the three octaves the ARRL was talking about. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Dave wrote:
Christopher Cox wrote: SNIPPED You have recognized a GREAT TRUTH about this group. There NEVER is a conclusion! Oh, there are subthreads, red herring threads, three month continuous running threads, insulting threads, I am better than you are threads, even threads that remain close to the original subject. But, there is NEVER a concluding post to a thread. /s/ DD ... I wrote the above Actually there was enough information to conclude if you want an all hf band 1:1 balun, abandoning the coiled coax and using ferrite may be the way to go. Wow, what a run on sentence! Which sentence? Mine is totally grammatically correct. A real run on sentence will be found if you study the serious works of some Jesuit Priests. When I was in seminary I had to read/study some Jesuit theology texts. The first sentence took three pages. One chapter took an 8 hour flight from Boston to LA. And, I still didn't finish :-( But, they were grammatically correct ... Does that mean your sentence is too short? :-) Now, hopefully to conclude this thread [well maybe] I agree that a broadband ferrite balun is the way to go for typical HF. /s/ DD Hi Dave, I am notorious for my poor grammar and run on sentence creation. It was a knee jerk reaction of mine to point out my poor grammar before someone else does. But three pages! Wow that beats any sentence I could come up with. Regards, Chris |
Hf Antenna Question - so what's the conclusion?
Dave wrote:
wrote: Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have discused HF chokes made from flat vs. bunched coils of coaxial cable, but I must have missed some posting(s), for I haven't recognized anything that looks like a CONCLUSION. * Is "bunched" or "flat" better? * Are they basically single-band or wide-band or somewhere in-between? * Are there "rules of thumb" to make them for hams with * No test equipment? * A grid-dip meter? * A noise bridge? * An antenna analyzer? Conclusions! You want WHAT?? You have recognized a GREAT TRUTH about this group. There NEVER is a conclusion! Oh, there are subthreads, red herring threads, three month continuous running threads, insulting threads, I am better than you are threads, even threads that remain close to the original subject. But, there is NEVER a concluding post to a thread. You need to do some reading. Try "Conclusions for Dummies", along with Feynman's "Special Relativity and the Dipole" and possibly also "String Theory and Its Applications to Near Field Anomalies". After that, you have my permission to post in this NG. :) |
Hf Antenna Question - so what's the conclusion?
wrote: * Is "bunched" or "flat" better? * Are they basically single-band or wide-band or somewhere in-between? * Are there "rules of thumb" to make them for hams with * No test equipment? * A grid-dip meter? * A noise bridge? * An antenna analyzer? Possibly this has been posted previously...but just in case: http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ Measurements of practical baluns of various types using an HP Vector Impedance Meter. It should answer most of your questions above. 73, Bill W4ZV |
Hf Antenna Question - so what's the conclusion?
wrote:
Possibly this has been posted previously...but just in case: http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ Thanks for the URL. There are a couple of points to make. When I suggested that 40 turns might be required for 75m, I was chastised for my statement. Note that the above URL contains data on a 38 turn choke on a 4.5" coil form for 75m operation. The choking impedance is 1300 ohms on 75m. Parallel self-resonance on 75m would require more turns than 38. Much of the "Ugly Balun" information should be ignored. It is not possible for an ugly balun to be "effective" from 1.8 MHz to 30 MHz. Note there are no impedance measurements to back up the ugly balun's claim of being "effective" over that frequency range. -- 73, Cecil, http://www.qsl.net/w5dxp |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
The 854 that I reviewed for Radcom was the same. We both got one that had the core assembled upside-down, so instead of showing you how to snap the plastic catch open, the instruction manual shows you how to break it. I'm trying to figure out how I can make more effective measurements on coiled coax chokes with the equipment that I presently have. I can measure the impedance with my MFJ-259B if it is less than 650 ohms. I also have a GDO. If I get an open circuit dip, would that be the parallel resonant frequency? If I get a short circuit dip, would that be the series resonant frequency? I also have a dual-trace 100 MHz Leader o'scope. I should be able to find the resonant frequencies by watching the voltage drop across a series resistor. I also have the X-Y feature so I could look at the source voltage, voltage across the choke, and voltage across the resistor as the frequency is changed. What do you think? -- 73, Cecil, http://www.qsl.net/w5dxp |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:a couple of points to make.
When I suggested that 40 turns might be required for 75m, I was chastised for my statement. Note that the above URL contains data on a 38 turn choke on a 4.5" coil form for 75m operation. The choking impedance is 1300 ohms on 75m. Parallel self-resonance on 75m would require more turns than 38. 8 turns loose wound in a 6.625" diameter bundle looks like an effective single band balun for 80m (approx 500 ohms). W6TC's voltage balun (Feb 1980 Ham Radio) uses 6 turns loose wound on ~5" diameter (8' total length) but adds a compensating winding in the opposite direction. It has nice characteristics for 40-160m but can only be used with balanced antennas (e.g. dipoles, inverted-Vs, etc). I've used a W6TC balun for many years on a trapped 80/160 inverte-V. 73, Bill W4ZV |
Hf Antenna Question - so what's the conclusion?
wrote:
8 turns loose wound in a 6.625" diameter bundle looks like an effective single band balun for 80m (approx 500 ohms). A ladder-line fed dipole might present an impedance of 8000 ohms to the choke. A 500 ohm choke would have very little effect. However, a self-resonant choke might have 40K ohms of choking impedance and it would need to have a lot more turns than 8. -- 73, Cecil, http://www.qsl.net/w5dxp |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
wrote: 8 turns loose wound in a 6.625" diameter bundle looks like an effective single band balun for 80m (approx 500 ohms). This would only be true for a resonant coax-fed 1/2WL dipole. What about all the other possible configurations? A ladder-line fed dipole might present an impedance of 8000 ohms to the choke. A 500 ohm choke would have very little effect. However, a self-resonant choke might have 40K ohms of choking impedance and it would need to have a lot more turns than 8. -- 73, Cecil, http://www.qsl.net/w5dxp |
Hf Antenna Question - so what's the conclusion?
....... bunched coils of coaxial cable,
but I must have missed some posting(s), for I haven't recognized anything that looks like a CONCLUSION. * Is "bunched" or "flat" better? You have recognized a GREAT TRUTH about this group. There NEVER is a conclusion! But, there is NEVER a concluding post to a thread. You need to do some reading. Try "Conclusions for Dummies", along with Feynman's "Special Relativity and the Dipole" and possibly also "String Theory and Its Applications to Near Field Anomalies". funny those things are simple, and pretty much all i understand, the other stuff?? thats what really confuses me |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
wrote: 8 turns loose wound in a 6.625" diameter bundle looks like an effective single band balun for 80m (approx 500 ohms). A ladder-line fed dipole might present an impedance of 8000 ohms to the choke. A 500 ohm choke would have very little effect. However, a self-resonant choke might have 40K ohms of choking impedance and it would need to have a lot more turns than 8. There's a disconnect between these two statements: they make very different assumptions about the level of performance that is needed. A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are the same ones we've been discussing for the past week (seems like more :-) http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ However, it's very interesting to read the whole of that web page, which is a much longer discussion involving several other designers and users of feedline chokes (aka choke baluns). In that discussion, there was a largely unspoken agreement that, to merit being called "effective", a choke should have an impedance of at least 10 times the cable Zo, ie at least 500 ohms. If 500 ohms is all you need, a coiled-cable choke of either the "bunched" or the "solenoid" type certainly can cover at least two amateur bands an octave apart in frequency... but a wide range of ferrite chokes can do the same, and these have the advantage of being much more broadband so they need no tuning. In contrast, Cecil is asking for a much higher impedance. If that's what you really need, then nothing can beat a resonant coiled-cable choke, which can give a resonant impedance well above 10kohms and possibly as high as 40kohms [1]. Even the very best ferrite choke can't come anywhere near that... but the resonant choke will always be a single-band device and will always have to be adjusted carefully. [1] Measurements on various kinds of chokes, at: http://w8ji.com/Baluns/balun_test.htm (table about half-way down the page). The performance of a common-mode choke is always going to depend on the exact situation in which it is used, so the measured impedance can never be more than an indication of its *potential* performance. For example, a 500-ohm choke may completely kill the common-mode problems at one station (or on one band) and have almost no effect in a different situation. On balance, it might be better to say that 500 ohms should be considered the *minimum* value of impedance that can offer some promise of being useful in a wide variety of different situations. But maybe Cecil is setting the bar too high by imagining the worst possible case. If an antenna/feedline system has a common-mode problem that needs an almost impossibly high value of choking impedance, then (as I said earlier) the choke isn't really the thing we should be looking at. It's the root cause of that problem - the antenna and/or feedline itself - that needs attention. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
But maybe Cecil is setting the bar too high by imagining the worst possible case. I immediately followed that posting up with another in which I said 500 ohms of choking impedance is sufficient for a resonant 1/2WL coax-fed dipole. But assuming the 1/2WL dipole is resonant on 3.75 MHz, 500 ohms of choking impedance is really not enough at the band edges. There's another consideration. The location of the common- mode nodes and antinodes are not necessarily related to the differential signals because the velocity factor is different for common-mode signals Vs differential-mode signals. One could encounter the "worst possible case" and not even be aware of it. The moral is: Follow Ian's advice and actually make common-mode current measurements before and after installing the choke. I have seen the installation of a choke actually increase the common-mode current at a point 1/4WL away from the choke. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote: In that discussion, there was a largely unspoken agreement that, to merit being called "effective", a choke should have an impedance of at least 10 times the cable Zo, ie at least 500 ohms. Hi Ian, I haven't read to the depth you have, but if your synopsis is accurate, they are wrong. The generality should be 10 times the source Z. If 500 ohms is all you need, a coiled-cable choke of either the "bunched" or the "solenoid" type certainly can cover at least two amateur bands an octave apart in frequency... but a wide range of ferrite chokes can do the same, and these have the advantage of being much more broadband so they need no tuning. And if the antennas exhibit a near-match, 500 Ohms is sufficient as those antennas will be presenting a low (~50 Ohm) source Z. a 500-ohm choke may completely kill the common-mode problems at one station (or on one band) and have almost no effect in a different situation. This would the consequence of a very high source Z, such as a half wave end fed design. On balance, it might be better to say that 500 ohms should be considered the *minimum* value of impedance that can offer some promise of being useful in a wide variety of different situations. Minimum, optimal, best, standard.... All such terms demand a criteria that is trying to be met. No choke is 100% effective. The two most compelling of those criteria are 1. RF shock hazard; 2. Antenna null filling. The shock hazard is often ignored if it is not perceived (can't feel it, and yet instrumentation might be pegged). Null filling, or poorer F/B, or F/S performance may only become a nuisance or a real concern depending upon the individual. My own standard is 5 times the source Z, or at least 3 times the source Z. 5 times is suitable for reasonable measurement accuracy, but filling some other criteria throws us back into the subjective quagmire. 73's Richard Clark, KB7QHC |
Hf Antenna Question - so what's the conclusion?
On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote: .... A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are the same ones we've been discussing for the past week (seems like more :-) http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ However, it's very interesting to read the whole of that web page, which is a much longer discussion involving several other designers and users of feedline chokes (aka choke baluns). In that discussion, there was a largely unspoken agreement that, to merit being called "effective", a choke should have an impedance of at least 10 times the cable Zo, ie at least 500 ohms. If 500 ohms is all you need, a coiled-cable choke of either the "bunched" or the "solenoid" type certainly can cover at least two amateur bands an octave apart in frequency... but a wide range of ferrite chokes can do the same, and these have the advantage of being much more broadband so they need no tuning. Ian, I agree that this criteria is oft cited, but it bears examination. It seems to derive from a bench test of a balun, where the balun shunts the balanced load (on one or both legs) by some impedance, often assumed to be purely inductive reactance (though that is not true from some baluns, eg the common W2DU style), and that if that impedance is 10 or more times the balanced load impedance, then the impact of the shunt reactance is negligible, and the common mode current caused by a single leg shunt is negligible. This might be a reasonable criteria for a balun in a bench situation or equipment room situation (eg between instruments or boxes with a mix of balanced and unbalanced interfaces), it should lead to low insertion VSWR which might be important if one was VSWR focused or obsessed. I suggest that such a criteria is not complete enough in itself to predict the impact of the balun on common mode current or balun loss in an antenna + feedline + transmitter + ground scenario. NEC modelling of some scenarios suggests to me that effect of common mode chokes at different frequencies depends not only on their impedance, but also on their location, and that sometimes more than one choke may be more effective than a single larger choke. Those models also reveal the standing wave nature of the common mode feedline current, and the futility of taking a current probe measurement at a single location to infer any more than the current at that specific location (if that was important). Owen -- |
Hf Antenna Question - so what's the conclusion?
Owen Duffy wrote:
On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK wrote: ... A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are the same ones we've been discussing for the past week (seems like more :-) http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ However, it's very interesting to read the whole of that web page, which is a much longer discussion involving several other designers and users of feedline chokes (aka choke baluns). In that discussion, there was a largely unspoken agreement that, to merit being called "effective", a choke should have an impedance of at least 10 times the cable Zo, ie at least 500 ohms. If 500 ohms is all you need, a coiled-cable choke of either the "bunched" or the "solenoid" type certainly can cover at least two amateur bands an octave apart in frequency... but a wide range of ferrite chokes can do the same, and these have the advantage of being much more broadband so they need no tuning. Ian, I agree that this criteria is oft cited, but it bears examination. It seems to derive from a bench test of a balun, where the balun shunts the balanced load (on one or both legs) by some impedance, often assumed to be purely inductive reactance (though that is not true from some baluns, eg the common W2DU style), and that if that impedance is 10 or more times the balanced load impedance, then the impact of the shunt reactance is negligible, and the common mode current caused by a single leg shunt is negligible. This might be a reasonable criteria for a balun in a bench situation or equipment room situation (eg between instruments or boxes with a mix of balanced and unbalanced interfaces), it should lead to low insertion VSWR which might be important if one was VSWR focused or obsessed. I suggest that such a criteria is not complete enough in itself to predict the impact of the balun on common mode current or balun loss in an antenna + feedline + transmitter + ground scenario. That was very much what I said, farther down the message from which you quote. Impedance isn't everything, but it does show the important difference between "good" chokes that are capable of making a large change, and "poor" chokes that will always have a much smaller effect. The next thing is a different point, which can limit the effectiveness of even the best of chokes. NEC modelling of some scenarios suggests to me that effect of common mode chokes at different frequencies depends not only on their impedance, but also on their location, and that sometimes more than one choke may be more effective than a single larger choke. Those models also reveal the standing wave nature of the common mode feedline current, and the futility of taking a current probe measurement at a single location to infer any more than the current at that specific location (if that was important). Common-mode current is well known to be "standing wave" in nature, in the sense that it has maxima and minima alternating along the line, separated by an electrical quarter-wavelength. If you disrupt this pattern by inserting a common-mode choke at a current maximum, you're going to force a current minimum at that point. (That can only happen if the choke has a sufficiently high impedance, but let's assume it has.) RF currents on the entire antenna-feedline system will then rearrange themselves to take this new factor into account. But something must have caused this tendency to have significant common-mode currents in the first place - most commonly an asymmetrical layout of the antenna and/or the feedline, eg when the feedline to a dipole runs parallel to the antenna underneath one side. A common-mode choke can treat the symptom (the common-mode current) but it cannot remove the root cause (the layout). In such cases, inserting a choke will cause a new common-mode current maximum to pop up, a quarter-wavelength away. Now if you insert another choke at this new current maximum, you have made it extremely difficult for the line to support any common-mode current between those two chokes - but yet another new current maximum will pop up, a further quarter-wave away. So on you go, rather like trying to squeeze down a long balloon... However, you are actually making progress. Every correctly placed choke makes it more difficult for common-mode current to exist on the line, and takes you farther away from the local field of the antenna. The name of the game is to reduce the common-mode current along the whole of the feedline, as much as you can, but above all to reduce the current at the location of the equipment that is being affected (eg the transceiver). Therefore it's always important to monitor the current at the victim equipment, and be aware that it can sometimes increase. In practical terms, the two most important places to try a common-mode choke a * At the end of the coax, where it breaks out to feed the antenna, or into parallel line, because this is where common-mode current can be launched onto the outside of the coax by a direct hard-wired connection. * At the victim equipment in the shack, or at some "gateway" point like the ATU. Because of the "pop-up" nature of the current maxima, there are always exceptions for certain layouts and feeder lengths, but these are two good practical places to start. It can be difficult, and an RF current probe will be your best friend and faithful tracker... but it's never futile. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question - so what's the conclusion?
Owen Duffy wrote:
Those models also reveal the standing wave nature of the common mode feedline current, and the futility of taking a current probe measurement at a single location to infer any more than the current at that specific location (if that was important). In fact, the common-mode currents on the feedline turn the feedline into a standing-wave antenna. I suspect that's how an Isotron antenna works. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
I also have a GDO. If I get an open circuit dip, would that be the parallel resonant frequency? If I get a short circuit dip, would that be the series resonant frequency? I have a 20 turn choke wound on a 5.35" diameter Quaker Oatmeal box at four turns per inch. I used my Grid Dip Oscillator to determine the open-circuit (parallel) self-resonance at 11 MHz. The short-circuit (series) self-resonance is at 23 MHz. This is a Knight Kit G-30 GDO with the original 6C4 still working. I built it from a kit in 1959. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
Cecil Moore wrote: I also have a GDO. If I get an open circuit dip, would that be the parallel resonant frequency? If I get a short circuit dip, would that be the series resonant frequency? I have a 20 turn choke wound on a 5.35" diameter Quaker Oatmeal box at four turns per inch. I used my Grid Dip Oscillator to determine the open-circuit (parallel) self-resonance at 11 MHz. The short-circuit (series) self-resonance is at 23 MHz. This is a Knight Kit G-30 GDO with the original 6C4 still working. I built it from a kit in 1959. What kind of wire, what kind of insulation, and how much gap between turns? It affects the self-capacitance. I'd like to try that on the VNA, but here in the homeland of porridge, oatmeal boxes are rectangular! Also we'll soon have visitors arriving for the rest of the weekend, so if anyone in the land of cylindrical oatmeal boxes can get to it in the next couple of days, please go ahead. -- 73 from Ian GM3SEK |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
What kind of wire, what kind of insulation, and how much gap between turns? It affects the self-capacitance. Unfortunately, the spare (75 ohm) coax that I have to experiment with came free from the local TV cable company and is unmarked. It is ~0.26" in diameter with a ~0.04" outer insulation. All conductors are aluminum. There is an aluminum foil wrapped around the foam insulation under the aluminum braid. The choke is fairly close wound with approximately 3.5 turns per inch. I'd like to try that on the VNA, but here in the homeland of porridge, oatmeal boxes are rectangular! Do you have 2 liter plastic pop bottles available there? I'm going to re-do the experiment with one of those as a coil form. There's an endless supply of those coil forms from the day care center next door. -- 73, Cecil http://www.w5dxp.com |
Hf Antenna Question - so what's the conclusion?
Cecil Moore wrote:
Ian White GM3SEK wrote: What kind of wire, what kind of insulation, and how much gap between turns? It affects the self-capacitance. Unfortunately, the spare (75 ohm) coax that I have to experiment with came free from the local TV cable company and is unmarked. It is ~0.26" in diameter with a ~0.04" outer insulation. All conductors are aluminum. There is an aluminum foil wrapped around the foam insulation under the aluminum braid. The choke is fairly close wound with approximately 3.5 turns per inch. OK, that's close enough to several alternatives such as RG58. I'd like to try that on the VNA, but here in the homeland of porridge, oatmeal boxes are rectangular! Do you have 2 liter plastic pop bottles available there? I'm going to re-do the experiment with one of those as a coil form. There's an endless supply of those coil forms from the day care center next door. Ours spell "litre" differently but I guess they're the same - 95mm or 3.75in diameter? If plastic bottles from the USA are a different size, I'll go fetch one from our local beach... you'd be amazed at what floats across. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hf Antenna Question - so what's the conclusion?
On Fri, 27 Oct 2006 23:52:03 GMT, Cecil Moore wrote:
Owen Duffy wrote: Those models also reveal the standing wave nature of the common mode feedline current, and the futility of taking a current probe measurement at a single location to infer any more than the current at that specific location (if that was important). In fact, the common-mode currents on the feedline turn the feedline into a standing-wave antenna. I suspect that's how an Isotron antenna works. For what it's worth, the following is the method I used in developing the W2DU current balun in 1981, that was published in QST, March 1983. I wanted the balun to cover 80 thru 20m. I considered the worst case situation would be on 80 m with the dipole cut to resonate at mid band, 3.75 MHz. Measured impedance of my dipole centered at 3.75 MHz yielded a terminal impedance of 53 - j122 ohms at 3.50 MHz, for an impedance magnitude of 133 ohms at 66.5 degrees. We must consider that the center conductor of the coax feed line connects to one half of the dipole, and the outer conductor to the other half of the dipole. Therefore, the outer conductor sees only one half of the total terminal impedance of the dipole, 66.5 ohms. I then considered that the choking impedance of the balun should be no less that 10 times the half-dipole impedance over the entire frequency range from 80 through 20m. Fifty No. 73 beads satisfied that requirement with lots of margin to spare, right down to the top end of the 160m band at 2.0 MHz. Reference to Fig 21-3, Chapter 21 in Reflections shows the impedance to common mode current on the coaxial feed line throughout the designated portion of the spectrum. Fig 21-3 can be downloaded from my web page at www.w2du.com by selecting Chapter 21 from the menu. Walt, W2DU |
Hf Antenna Question - so what's the conclusion?
Ian White GM3SEK wrote:
Ours spell "litre" differently but I guess they're the same - 95mm or 3.75in diameter? If plastic bottles from the USA are a different size, Our 2L bottles are very close to 4 inches in diameter with about 6 inches of effective winding length. They are very popular here for these kinds of chokes. I just put 20 turns on one. The GDO says parallel resonance is 14 MHz and series resonance is 28 MHz. 20 turns on 20m on a 2L pop bottle is what my rule-of-thumb says. -- 73, Cecil http://www.w5dxp.com |
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