Balun's Amplitude Imbalance

Hi all,

I am designing a balun and the output seems to have a huge imbalance (about 5 dB) and cannot identify the origins for such discrepancy. The layout looks like this



which I would say it's pretty symetrical without considering the underpass. Port 1 and 3 correspond to the unbalanced side. Port 1 is connected to signal source and Port 3 is connected to GND. Port 4 and 5 correspond to the balanced output, being Port 4 (+) and Port 5 (-). The tap at Port 5 is connected to GND. The output looks like this



The phase difference is pretty much 180° as it can be seen on the plot. What would be the main reason for this discrepancy?

Thanks a lot.

- - - Updated - - -

Sorry, bad info when typing, Port 1 and Port 2 are the unbalanced side, Port 3 and 4 the balanced output and Port 5 is connected to GND. Hope that helps. Thanx!

megaknaller said:

View attachment 131641... which I would say it's pretty symetrical without considering the underpass. ...

Click to expand...

Right; but what about the connections?

Apart from the right side connections (unbalanced side), the right side and the ground connection seem pretty balanced in my opinion. Would this slightly electrical imbalance at the left side should have such big effect on the output?

Slightly imbalance? I see a massive imbalance when connecting port 5 to ground and port 1 or 2 also to ground. Or does the unbalanced line not involve a ground?

If both grounds are not shorted together, the outer conductor of the unbalanced line is no longer neutral and it's common mode impedance must be taken into consideration.