The s parameters of a balun ( three port) are:

s12 = -s13 = s21 = -s31 and,

s11 = -infinity.

What does this mean in terms of conceptual understanding? The interpretation of these s parameter statements are:

1. A balun is a three port device with a matched input and differential outputs.
2. A balun is a three port power splitter
3. The two outputs from a  balun are equal in amplitude but opposite in phase.
4. The voltage of one balanced output is the negative of the other balanced output.
5. The input is matched to the input transmission line.
6. The usual impedance is 50 Ohms. But it could be something else.
7. A balun is a reciprocal device. Its output and inputs may be exchanged

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Simple baluns ( balanced-unbalanced) at lower frequencies use flux coupled transformers.They are generally limited to lower frequencies below 1 GHz. The reason is that at high frequencies the dipoles in the magnetic core cannot switch quickly enough, causing loss in the balun’s coupling between windings.

Parasitic capacitance between the windings leads to high frequency signals that pass  directly to ground without coupling through the metallic core. Magnetic cores also have a large loss tangent. This causes high signal losses at microwave operation.

As a result of these effects, capacitance coupled transmission line baluns were invented. A transmission line balun is a set of coupled transmission lines with one end grounded.  This type of coupling induces equal and opposite signals in both lines.

As a result of using a transmission line to connect to ground, the transmission line allows differential operation. Common forms of transmission line baluns include the 1:4 impedance ratio Ruthroff balun and 1:4 Guanella balun.

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