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.
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.
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.
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?