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Coax choke/balun
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AllassoPraise
I have seen desktop antenna/coax assemblies (such as for an SDR) that
take the (thin) coax cable and wrap it around a small toroid about 3 or 4 times, close to the antenna. I understand this is to keep the shield of the coax from radiating. I have tried to find information on the construction of these, but not much comes up that is specific, making it seem like this isn't an exact science, just "wrap a few turns around a toroid." Also, I have heard them referred to as chokes and baluns, although others say they do not function as a balun, only a choke. Can anyone fill me in on this? And are there considerations to be taken into account regarding the size of the toroid and the number of wraps? Also can this principle be used effectively for larger outdoor antennas with larger coax, such as RG58 size? Kevin |
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Common mode choke, literally thousands of pages
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out there dedicated to the topic. Good start for ham use: On Thu, 24 Dec 2020 at 14:22, AllassoPraise <allassopraise@...> wrote:
I have seen desktop antenna/coax assemblies (such as for an SDR) that |
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A common mode choke made by wrapping coax feeder through a toroid is directly equivalent to a 1:1 Guanella "balun". The term balun is in quotes because, although coaxial cable is clearly an unbalanced feeder, the output of such an arrangement is not so much balanced to ground/earth, as effectively free to float. With a dipole antenna, and an effective balun/choke, then the dipole will inherently tend to be balanced, and the feeder will tend to be unbalanced.
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What makes this possible is the impedance of the inductor formed by the turns through the toroid. What also makes this possible is the impedance of the feeder through the toroid. Since it is the same feeder, and the impedance transformation is 1:1, all is well. This arrangement is Guanella's 1:1 [wideband] transmission line transformer. Depending on just how deep you wish to go, and where you are starting from in relation to technical knowledge, then putting "transmission line transformer" in your favourite search engine would be a good place to start! *Beware*: especially in Radio Amateur circles, the term "balun" is often used very loosely. Sometimes comments are made that are at variance to a clear understanding of basic electrical theory. YMMV, of course. HTH, 73, Stay Safe, Robin, G8DQX On 24/12/2020 13:30, Dragan Milivojevic wrote:
Common mode choke, literally thousands of pages |
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On 12/24/20 5:21 AM, AllassoPraise wrote:
I have seen desktop antenna/coax assemblies (such as for an SDR) thatyes.. See this writeup by Jim K9YC - There are configurations that work better or worse, and core materials that work better or worse at different frequencies. For example the ubiquitous little EMI suppression cores you see on keyboards, cables, etc. are optimized for VHF frequencies. |
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Hi Kevin,
The choke folks are correct. This choke is used to stop/reduce the amount of RF on the shield of the coax traveling down the coax into the shack. Some folks will put a choke at the antenna and the shack end. Do a Google search on common mode chokes, you'll get plenty to read. GL. Stay safe, wear a mask. John VE7KKQ On Thu, Dec 24, 2020 at 5:22 AM AllassoPraise <allassopraise@...> wrote: I have seen desktop antenna/coax assemblies (such as for an SDR) that |
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The best and sound information I've located in a single source can be found
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at the following URL. Open the 'balun' presentation. You will come away with a wealth of good sound engineering knowledge. Dave - W?LEV On Thu, Dec 24, 2020 at 5:00 PM John <ve3kkqve3kkq@...> wrote:
Hi Kevin, --
*Dave - W?LEV* *Just Let Darwin Work* |
The best and sound information I've located in a single sourceAdded to Wiki /g/nanovna-users/wiki/13116 |
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On Thu, 24 Dec 2020 at 17:46, Al Dynarski <adynarski83@...> wrote:
Here is a link to the guru of chokes, Jim K9YC: He is also the guru of self-praise.??? Dave -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom |
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Already in Wiki
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Here is a link to the guru of chokes, Jim K9YC: |
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Mel Farrer
I suggest another with practical experience.??
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Mel, K6KBE Thursday, December 24, 2020, 12:48:42 PM PST, Oristo <ormpoa@...> wrote: Already in Wiki Here is a link to the guru of chokes, Jim K9YC: |
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But if the choke balun is highly inductive and not very resistive, it will form a resonant circuit with the antenna's total capacitance to ground, and other reactances in the antenna/feedline/ground circuit. If this resonance happens to fall near your operating frequency, the balun will make matters worse then they are without a balun!
For that reason many authors suggest to use lossy core materials, including authors already mentioned in this thread. Ferrite has a complex permeability. At low frequency it's almost purely inductive, while at higher frequencies it becomes increasingly resistive. For that reason, at a sufficiently high frequency for the material chosen, a choke balun will oppose little inductance, but a lot of resistance to any common-mode current. This makes it work very well as a balun, without any risk for the mentioned resonances in that frequency range. But its resistance must be high enough to keep the loss negligible, or at least acceptable. This comes down to selecting a core material that has a mostly resistive permeability all over the intended operating frequency range, and winding enough turns on it to keep the flux density at a level at which the loss in the core is low. At the same time there shouldn't be so many turns that inter-turn capacitance becomes a problem. I have found the NanoVNA to be a great tool for measuring the characteristics of ferrite cores! You can take an unknown core, wind one or two turns of wire on it, or several if it's a small core, hook it up to the NanoVNA, and see the resulting series RLC graph over the desired frequency range. This gives you an instant idea of the core material's inductive and resistive permeability curves! By comparing to various manufacturer-provided data, you can make a good guess which material your core is made from. |
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The 'big boys' who design the materials use one turn through the core to
characterize the ferrite material. They measure in a 50-Ohm system. Remember, one turn equates to passing a conductor through the core, NOT one turn *around* the core! I have recently characterized most of my larger cores using this method. It works nicely to compare the measured impedance curves (linear magnitude) against the published data curves for each core material. The measured data matched the published data well enough to determine the core material. I built a fixture to accept the one-turn (based on the single pass through the cores) to accomodate serial testing of my cores. The measured data was made in the transmission mode so a complete cal. is required. I even did isolation, just because I could. The measured data matched the published data curves up to 200 MHzrelatively well. I didn't calibrate or measure above that frequency as I was interested only in HF through 50 MHz. Of course, the fixture was calibrated into the measurement setup. This is also a good method for detecting resonances (to be avoided!) in home brew CMCs and current 'baluns'. Getting to the data I put out earlier today, many designs showed resonances which nixed the design of that specific CMC. That which won consisted of 11 bifilar turns (no twists) of AWG #10 stranded and insulated copper conductor wound on two stacked 3" OD 43 material cores. It measured a nice smooth convex curve from 1 MHz through 50 MHz with no resonances in that range. That is the choke noted as (BRN) in my data compilation as that was the color of the wire (I'll attach the data again just for reference). However, the 31 material still won out on 160-meters showing no resonances in the frequency range of concern but was not as good at mid-HF frequencies. Dave - W?LEV On Thu, Dec 31, 2020 at 3:26 PM Manfred Mornhinweg <manfred@...> wrote: But if the choke balun is highly inductive and not very resistive, it will-- *Dave - W?LEV* *Just Let Darwin Work* |
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Dave,
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Thanks for the chart. Were all of the measurements taken with 1 turn except the BRN which you indicated 11 turns? Mike N2MS On 12/31/2020 6:46 PM David Eckhardt <davearea51a@...> wrote: |
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Once again, he demonstrates that transformers do not make baluns as he
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measures only -3 to -4 dB of CMR of the typical current balun. And......the function of the balun is not necessarily to transform impedances. The sole function of a balun or CMC choke is to map a common mode topology into a differential mode topology - nothing more.....nothing less. That's all in its name: Bal = balanced (differential mode), un = unbalanced (common mode). Get it? Dave - W?LEV On Fri, Jan 1, 2021 at 5:31 AM Fred <fmoeves@...> wrote:
Kevin, --
*Dave - W?LEV* *Just Let Darwin Work* |
Oristo
Dr. David Kirkby, Kirkby Microwave Ltd
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