Electronics > RF, Microwave, Ham Radio

Balun Inductors

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Foxxz:
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.

mag_therm:
Did you see the blog by G7SYW and the pdf by G3TXQ ?
Sorry, can't do links now.

Wallace Gasiewicz:
Try this group:

https://ham-antennas.groups.io/g/main/messages

You have to join but it is free and Groups.io has lots of Ham oriented groups
Then ask your question

As far as I know most people just use wire around a suitable toroid for the balun/transformer

profdc9:
The transformer here would be a 4:1 Guanella balun

https://vk6ysf.com/balun_guanella_current_1-4.htm

To 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.



bob91343:
I don't like baluns.  They usually contain magnetic cores, which generate harmonics.  I prefer coaxial chokes.

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