Hi,

I have placed a coupling slot between two waveguides. In the main waveguide, where the input port is located, the coupling slot is positioned at the guided wavelength (λG​/2) from the short end.

The input port is situated at λG​ from the center of the slot, or (λG/2+λG) from the short end. I have measured the S11 parameters while tuning the length and angle of the slot. The S11 indicates that all power is successfully transferred to the second waveguide. However, when I check Z11 (the input impedance of the waveguide), I observe a large imaginary component. According to transmission line theory, the input impedance should equal ZL​ when the length from the load is approximately λG.

Explanation:

a) The input impedance of a shorted waveguide is typically high, resulting in a poor S11 measurement.

b) When I introduce and tune a slot in the main waveguide to resonate, I expect the input S11 to yield an acceptable value, indicating effective power transmission between the two waveguides and minimal reflection.

However, I find that even when S11 is around -30 dB, the imaginary part of the input impedance for the waveguide containing the slot remains significantly high (approximately 6825ohms)(it is the impedance of the slot regarding the slot theory it can be assumed as a series impedance).

I would like to understand if achieving a good S11 while still having a large imaginary component in the input impedance is possible.