group delay of waveguides

Hello, as straight waveguides should have linear dependency between transmission S21 phase and frequency, and group delay is the derivative of transmission phase with respect to frequency, why is group delay over frequency of a straight waveguide not constant. Thank you for your help, -e

Because, waveguides are also frequency dependent in their own nature. When u design a waveguide or a transmission line, it actually works perfect for a single frequency. So, if your aim is to get a constant group delay over a band of frequencies; you can look up for True Time Delay modules.

one common example regarding that: https://ieeexplore.ieee.org/document/6401614