0
\$\begingroup\$

A 1:1 Current or Guanella Balun is formed by winding a transmission line around a core and is used at RF frequencies as a broadband Balun.

enter image description here

Here is an example of a terrifically broadband balun rated to work from 4 MHz to 3 GHz: MABA-007159

Descriptions on the web are scarce and I find most hand wavy at best, but here is what I have gathered so far:

The low frequency operation is easily understood by viewing this as a simple transformer/Common mode choke. Flux coupling between the two coils ensures equal but opposite current. Now if we connect a balanced load at the output, and drive the input with an unbalanced source, we get a balun.

enter image description here

At higher frequencies, the core non idealties means we can no longer explain the workings using standard transformer theory. Also, the winding "parasitics" start behaving like transmission lines so transmission line theory must be employed. This is where I am unsure of its operation.

enter image description here

As I grasp it, at high frequencies we are simply left with a transmission line from source to load. How are we still achieving broadband balun action? If for example the workings are the same a standard microstrip transmission line balun (such as a Marchand Balun) it should be narrow band i.e. quarter wavelengths?

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Does it help to think backwards? Start at the high-frequency end, with a 1/4-wavelength balun. Then add ferrite material to extend the low-frequency end (where the wavelength effect becomes less effective). Well-chosen ferrite extends the low-frequency end considerably. \$\endgroup\$ – glen_geek Sep 27 '17 at 1:34
2
\$\begingroup\$

A transmission line kind of behaves like a transformer in some sense: In two parallel wires, if there is current flowing in the first wire, the coupling between the wires will force an equivalent, but anti-parallel current flowing in the second wire. This holds true for any kind of transmission line, even coaxial ones.

Where transmission lines and transformers differ is, that in transmission lines there is a lower cut-off frequency below which the current coupling between the two wires is so low that you can see them as independent wires.

The basic working principle of all these "coax line and ferrite cores" transformers is to extend this lower cut-off frequency by keeping the magnetic fields close and thus couple more into the "other wire" (ie shield) of the coax..

\$\endgroup\$
  • \$\begingroup\$ Thankyou for answer! This is precisely the conclusion I have also come to after collating various sources of information.Also one last point: the balun action of such a Coax balun occurs mainly due to the inherent balanced currents that flow in a Coax. Is it then right to conclude that the exact length of this TL (and hence 1/4 wavelength) only really dictates the usable bandwidth of the balun as this is where the first resonance occurs? \$\endgroup\$ – Adil Malik Sep 27 '17 at 14:19
  • \$\begingroup\$ Yes, this and limits in physical size. \$\endgroup\$ – Attila Kinali Sep 27 '17 at 14:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.