Microwave networks with parameters that are periodic in the frequency domain are found in many RF subsystems, e.g., in radar and wireless communication devices. This paper analyses the impulse response (IR) of those microwave networks, obtained by applying the inverse Fourier transform to data limited to the basic passband only. Here, we refer to such an IR as a quasi-impulse response (QIR). We investigate mathematical relations between the QIR and the exact IR. Additionally, we theoretically and experimentally demonstrate the importance of considering the parasitic passbands of these microwave networks when calculating the IR. Our insight is that neglecting the parasitic passbands can lead to unforeseen EMC problems and security vulnerabilities because these passbands may be entrance points for interference. We believe these results to be particularly important when considering intentional electromagnetic attacks.