In article ,
"Nicolae Santean" wrote:

I have a rookie question related to baluns.

In what sense is said that a coax cable is an unbalance line

The outer shield is ground and the inner carries the signal
asymmetrically.

an antenna is a balanced system a twin cable is a balanced line ?

If you have a balanced antenna like a dipole then yes.

Of course a second question would be how does the balun interface a
balanced and an unbalanced system?

By shifting the ground reference, splitting and phasing the signal to be
differential going from unbalanced to balanced and performing the
reverse actions to go the other way.

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Telamon
Ventura, California