The two outputs of the balun are equal in amplitude and opposite in phase

The two outputs of the balun are equal in amplitude and opposite in phase:

– In the frequency domain, this means that there is a 180 ° phase shift between the two outputs;

– In the time domain, this means that the voltage of one balanced output is the negative value of the other balanced output.

In addition, the conductor of one of the two lines must be clearly grounded.

For example, a balanced line consists of conductors with equal potential amplitudes and opposite phases. Because microstrip lines and coaxial cables use conductors of different sizes, they can be described as unbalanced lines. The design goal of the balun is to solve the problems caused by such unbalanced lines-the balanced (or differential) transmission line in which the balun can transmit the current in opposite phase and the unbalanced (or single-ended) transmission line where the return current is transmitted underground Convert between.

Inside the coaxial cable, since the electric field generated by the current inside the inner conductor and the shielding layer is limited to the space between the two, the two currents have the same amplitude and opposite phases. At the same time, the skin effect causes another current to flow outside the shielding layer. When this current is large, the coaxial cable as a feeder can be made into an antenna, and an electromagnetic field whose radiation intensity is proportional to the magnitude of the current is radiated outward. Because the coaxial cable has a symmetrical physical structure, and the circuits on the two internal conductors are equal in amplitude and opposite in phase, the radiation generated by the internal structure itself is extremely small. However, certain factors can disrupt the current balance in its two conductors (that is, disrupt the "equal amplitude, opposite phase" state). In this case, the inside of the feeder will also produce a larger current like the current outside the shield. Electromagnetic radiation. This imbalance will cause distortion of the pattern, interference and loss.

Key specifications for determining balun types for specific applications include:

1. Frequency coverage

2. Phase balance

3. Amplitude balance

4. Common mode rejection ratio

5. Impedance ratio / turn ratio

6. Insertion loss and return loss

7. Balanced port isolation

8. DC / earth isolation

9. Group delay flatness

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