It has been discovered that a trisection bandpass filter with mixed cross-coupling <inline-formula> <tex-math notation="LaTeX">$K_{13}$ </tex-math></inline-formula> has flat group delay if the condition <inline-formula> <tex-math notation="LaTeX">$K_{13} = 0$ </tex-math></inline-formula> is satisfied, which occurs when magnetic <inline-formula> <tex-math notation="LaTeX">$K_{m}$ </tex-math></inline-formula> and electric <inline-formula> <tex-math notation="LaTeX">$\vert K_{\mathrm {e}}\vert $ </tex-math></inline-formula> components of the coupling are equal. It has been proven that the group delay of the filter increases as the values <inline-formula> <tex-math notation="LaTeX">$K_{m} = \vert K_{\mathrm {e}}\vert $ </tex-math></inline-formula> increase. The microstrip construction of the trisection delay line filter is proposed. The analysis of this construction has been carried out, which allows controlling the values <inline-formula> <tex-math notation="LaTeX">$K_{m} = \vert K_{\mathrm {e}}\vert $ </tex-math></inline-formula> at the zero of cross-coupling. The results of the measurements and the simulation are presented.
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