Question about balun construction - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: June 12, 2023, 10:49:24 pm
content: Many baluns have balanced twisted line used inside the balun. But a coaxial is usually used to connect from and to the balun. Isn't there a mismatch

author: RFDx
author_rank: Regular Contributor
timestamp: June 14, 2023, 06:33:19 pm
content: If all impedances involved (load, bifilar transmission line, coax line) have the correct value than there will be no mismatch.

author: KF6BL
author_rank: Newbie
timestamp: June 17, 2023, 11:54:42 am
content: Don't have an answer but if you want to learn more about baluns, there is a great book called Understanding-Building and Using Baluns and Ununs. Do a google for the book.

author: LM21
author_rank: Regular Contributor
timestamp: June 18, 2023, 10:55:46 am
content: Quote from: RFDx on June 14, 2023, 06:33:19 pmI see. Sudden change from balanced to unbalanced makes me wonder.

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 11:51:04 am
content: I've been wondering the same thing for quite sometime.So there is a balanced antenna. At its feed point, we insert the type of balun with core and parallel wire wound on it. At the output, there is a long line of coax to the shack. I get that parallel wires forms a transmission line, and common current is choked off by virtue of wound around iron core few times. BUT, why does THAT convert balanced input to unbalanced output? (and vise versa) If someone can explain this in layman's terms, it would be great.

author: iMo
author_rank: Super Contributor
timestamp: June 18, 2023, 05:04:28 pm
content: For example:A dipole is 72ohm, afaik, inverted V is 50ohm, let us assume invV dipole.An FT240-43 toroid with 5 turns (an example only) of coax (oranykind of 2 wires -twisted or not) creates those 2 closely coupled coils L1 and L2 each of 25uH inductance.The rules of elmag interaction in a such a coil system (basically a transformer) create from "one unsymmetrical/unbalanced signal" (inner conductor - green, the outer shielding is ground) the "two symmetrical/balanced differential signals" (blue and red) suitable for feeding the symmetrical dipole's legs.Without the common mode choke (aka balun) one of the dipole's legs gets grounded.

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 06:14:51 pm
content: Thanks.So in this simulation, L1 and L2 are transformers; and by virtue of having inductance, input and outputs are isolated, thus one side can be single ended and the other end will be differential signal?

author: coppercone2
author_rank: Super Contributor
timestamp: June 18, 2023, 06:21:18 pm
content: there is coaxial baluns too. and bizzare baluns wound with strange low impedance coaxial cables.I would suggest getting rid of the balun and just seeing what happens when you connect a twisted pair to a coaxial cable.. that might be useful base level analysis for solving your problem. Parts of that might still be useful (the first centimeter can be taken for the total length of the analysis for the area before the core). And then go 1 turn at a time?I don't know the answer BTW.

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 06:34:38 pm
content: In real life implementation, I've actually tried dipoles with and without balun at feed point. It really didn't make much difference. I know it's fundamentally wrong, but I couldn't tell the difference.

author: coppercone2
author_rank: Super Contributor
timestamp: June 18, 2023, 06:40:34 pm
content: I mean just an analysis of the twisted pair connected to a coaxial cable. Then an analysis of a twisted pair attached to a coaxial cable with 1 turn around a core, then 2 turns.

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 07:24:35 pm
content: What would you like to measure? How balanced they are at the output side? (inside being the RF source) I could potentially do that measurement in my lab.

author: A.Z.
author_rank: Super Contributor
timestamp: June 18, 2023, 08:48:52 pm
content: Quote from: tkamiya on June 18, 2023, 06:34:38 pmtry building thishttp://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htmand then use it to measure currents on the outside of the coax feedline, then repeat the measurement with the balun in place

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 09:02:55 pm
content: I would simply terminate each leg to a common ground, then use an ordinary scope to voltage.

author: LM21
author_rank: Regular Contributor
timestamp: June 18, 2023, 09:24:30 pm
content: Quote from: tkamiya on June 18, 2023, 06:40:34 pmA real difference is that without a balun, you can't measure antenna impedance. Atleast I couldn't. I made a balun, choke, or what ever, by putting a lot of ferrite beadfs over coaxial. After that, I got meaningfull results.Note: I modified the quotation.

author: tkamiya
author_rank: Super Contributor
timestamp: June 18, 2023, 09:32:52 pm
content: I was doing it the Ham Operator way: Lowest SWR.If you want to measure impedance, it'll have to be done at the feed point (before a balun) or have coax cut in multiple of wavelengths, so that Zin = Zout. Dipoles having 73 ohm is only true if it is hung in free space. If it's lower (to the earth), or too close to nearby structure, 50 ohms is a better match. (at least it was like that experimentally).

author: iMo
author_rank: Super Contributor
timestamp: June 19, 2023, 12:03:07 pm
content: Quote from: tkamiya on June 18, 2023, 06:34:38 pmMany people do that as well. On the other hand many people do not know that 1S change on your S-meter means the opposite station changed its output power 4x (provided there is no QSB at that time)..And what is 1S in real life? Nothing.. Thus you could not tell the difference..

author: LM21
author_rank: Regular Contributor
timestamp: June 19, 2023, 02:40:14 pm
content: Quote from: tkamiya on June 18, 2023, 09:32:52 pmSWR and impedance are related. You can't measure SWR, if you can't get impedance. I don't think any bridge will give you SWR if your feedline is hot.Edit: but we are going off-topic now. I was wondering how you can mix balanced and unbalanced transmission lines at a balun, and they still work.

author: tkamiya
author_rank: Super Contributor
timestamp: June 19, 2023, 03:38:36 pm
content: Here's a link to a book written by experts on this topic:https://archive.org/details/fe_Ham_Radio_Transmission_Line_Transformers_Handbook/mode/2up

author: LM21
author_rank: Regular Contributor
timestamp: June 19, 2023, 05:36:17 pm
content: Thanks. I have read Sevick's books. They seem to work, allthough mine sometimes don't. But they show the feature too, balanced and unbalanced transmission lines are connected together.

author: RFDx
author_rank: Regular Contributor
timestamp: June 19, 2023, 07:30:52 pm
content: Quote from: LM21 on June 19, 2023, 05:36:17 pmWhat makes a transmission line "balanced" or "unbalanced"? Is a transmission line made of two twisted bifilar wires "balanced"? You could replace such a transmission line with one made of coaxial cable. Why does it still work?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 11, 2023, 12:05:51 am
content: Balanced = current flowing in both wires equally but oppisiteUnBalanced = one wire is GROUND. this is the coax shieldBalanced = current flowing in both wires equally but oppisiteUnBalanced = one wire is GROUND. this is the coax shieldBalun = unbalanced to balancedThe "Balun" can also be a transformer changing the impedance of the transmission lineTwisted pair is a transmission line that is balanced for the most part. Twisted pairs are about 100 ohms but vary according to tightness of twist. Used mostly for digital lines, Their specific use in a Balun depends on which signal is going thru them it adds capacitive couplinghttps://www.markimicrowave.com/Assets/appnotes/balun_basics_primer.pdf

author: RFDx
author_rank: Regular Contributor
timestamp: July 11, 2023, 05:53:08 pm
content: Quote from: Wallace Gasiewicz on July 11, 2023, 12:05:51 amFor the most part? Meaning?A transmission line made of twisted bifilar wire isn't by default balanced. It depends how the line is driven respectively connected.Quote from: LM21 on June 12, 2023, 10:49:24 pmTwisted, bifilar transmission lines in current baluns may look balanced but are (usually) used in an unbalanced fashion. There is no "mismatch" connecting an unbalanced coax line to a current balun.Take a 1:4 balun as an example. Both bifilar transmission lines should have an impedance of 100 Ohm. One wire of each transmission line is grounded on both sides. At the input the unbalanced transmission lines are in parallel to get 50 Ohms. At the output the lines (one also used as a phase inverter) are connected in series to get an earth free, balanced output impedance of 200 Ohms.