Non Constant phase difference between Microstrip lines

Hello Everyone,

I am doing a project which involves Butler Matrix feed. As i was designing the feed i encountered that the slope at which the phase v/s frequency plot changes varied with ports. As such the phase difference between ports was not remaining constant wrt frequency. When i tried with two simple microstrip lines of different lengths (45 deg phase difference) but same width i still got varying phase difference. I cannot figure out why this is happening. I have attached the screenshots for reference.

I am using IE3D v14.Please Help.

This is the expected result for lines of different length.

Thanks for the reply. Are you saying that rate of change of phase v/s frequency depends on length too other than width and height of substrate? The individual phases at output will be different ,true, but phase difference should be constant right?

Yes, of course.

Your "phase difference" is also caused by the line length, and thus the phase difference will scale with frequency. Very simple.

Ok makes sense. is there any way i can have a constant phase difference?