Balun Inductors - Page 1
Forum section: electronics / rf-microwave

author: Foxxz
author_rank: Regular Contributor
timestamp: August 09, 2022, 03:59:01 pm
content: I want to build a balun to go from a 12.5 ohm balanced antenna (cobweb design) to unbalanced 50 ohm coax - a 1:4 ratio for a frequency range of 14-54MHz (20-6m). I've seen "recipes" online for how to build similar baluns but most only cover to 160-10m.I don't see the subject covered very well about what toriod to select and why as far as size and material for a desired frequency range. And the 1:4 balun design I believe is sensitive to the impedance of the windings too. Some designs are just using magnet wire while others are using 25ohm coax.If someone could point me to some material that would help me in figuring out what I need to calculate and select the toroid and design the windings that would be helpful.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 09, 2022, 04:11:30 pm
content: Did you see the blog by G7SYW and the pdf by G3TXQ ?Sorry, can't do links now.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 10, 2022, 12:05:38 am
content: Try this group:https://ham-antennas.groups.io/g/main/messagesYou have to join but it is free and Groups.io has lots of Ham oriented groupsThen ask your questionAs far as I know most people just use wire around a suitable toroid for the balun/transformer

author: profdc9
author_rank: Frequent Contributor
timestamp: August 11, 2022, 03:40:19 am
content: The transformer here would be a 4:1 Guanella balunhttps://vk6ysf.com/balun_guanella_current_1-4.htmTo go from 50 ohms to 12.5 ohms, connect the 50 ohm impedance to the two toroids in series connection, and the 12.5 ohm into the two toroids in parallel connection. Depending on the power, you can probably use two FT240-43 toroids or equivalent Fair-rite.It is best if the windings around the toroids are 25 ohm, but realistically you would need to use microstrip-like line to get that low of an impedance (the above link is for a 50 to 200 ohm transformation and so uses parallel wire transmission line which has about 100 ohms impedance). You can use two 50 ohm coax in parallel. However, if the length of the transmission line around the transformer is less than 1/10 wavelength at the shortest wavelength of use, the impedance of the transmission line is not that big of a deal, and I would just use single 50 ohm coax, something like RG-316 or RG-400 teflon coax for high dielectric strength and small diameter. Not that many turns should be required for 14-54 MHz, for example, I think 6 turns should easily suffice, which is about 0.4 m length, less than a 1/10 of a wavelength at 6 m. Try to match up the lengths of the coax on the two toroids as best you can otherwise.

author: bob91343
author_rank: Super Contributor
timestamp: August 11, 2022, 03:31:31 pm
content: I don't like baluns. They usually contain magnetic cores, which generate harmonics. I prefer coaxial chokes.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 16, 2022, 12:59:27 pm
content: Quote from: bob91343 on August 11, 2022, 03:31:31 pmand you have a design for the frequency range of 14-54MHz?

author: bob91343
author_rank: Super Contributor
timestamp: August 16, 2022, 05:48:30 pm
content: I use coaxial chokes. Several feet of cable wound into a coil works for me on all bands.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 16, 2022, 07:29:28 pm
content: sure, off course..

author: A.Z.
author_rank: Super Contributor
timestamp: August 17, 2022, 01:13:03 pm
content: Quote from: bob91343 on August 16, 2022, 05:48:30 pmon ALL bands ?http://www.karinya.net/g3txq/chokes/oh, well; by the way I'm really curious to know what that "works for me" means for you

author: Hamelec
author_rank: Regular Contributor
timestamp: August 17, 2022, 02:45:28 pm
content: the meaning is it will not work for you and all the others

author: bob91343
author_rank: Super Contributor
timestamp: August 17, 2022, 06:05:22 pm
content: I like the broad band performance, minimal cost, and high efficiency of the coaxial choke. It should work well for most frequencies up at least to VHF, with suitable numbers of turns. The only down side might be the additional weight but it's not much. For fancy phased arrays it might not be the best solution.I use RG-213 or similar. I just have an extra long coaxial cable and wind several turns of about 10-12 inches diameter just before the feed point of the antenna.

author: iMo
author_rank: Super Contributor
timestamp: August 17, 2022, 06:26:51 pm
content: The material 43 is not the best one for the 14-54MHz range, imho.An alternative would be 52 or 61.How many turns? There is a "rule" that the inductive reactance of the winding should be at least 5x of the required impedance at the lowest frequency of interest (an example: for 14-54MHz and 50ohm the inductive reactance (XL = 2*pi*f*L) of the winding should be ~250ohm at 14MHz, thus you would need 2.84uH, the number of turns depends on the AL of the material).

author: A.Z.
author_rank: Super Contributor
timestamp: August 18, 2022, 09:20:11 am
content: Quote from: bob91343 on August 17, 2022, 06:05:22 pmAs for cost nothing to say, but regarding broadband... please, have a look at the results of the tests run by G3TXQ (see link at my previous post); an air wound, coaxial choke will only be effective over a limited range of frequencies, outside them it may even worsen things, again, take the time to read that page since G3TXQ spent quite some time running tests and the results may be an eye opener

author: A.Z.
author_rank: Super Contributor
timestamp: August 18, 2022, 09:23:17 am
content: Quote from: imo on August 17, 2022, 06:26:51 pmas for turns and material, check out thishttps://cromwell-intl.com/radio/9-1-unun/let aside that it deals with a "9:1", the calcs are there