RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 84
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
line lengths LsZ1 and LsZ2, and if needed the line widths WZ1 and W50 to place the
isolation notch exactly at 1 GHz and ensure that the attenuation of S21 is the same as that
of S31 at 1 GHz, as shown in figure 4.31. Comparing figure 4.29 and 4.31, shows that the
frequency dependent losses of the Microstrip lines, result in the isolation and reflection
coefficient (S11, S22 and S31) at 1 GHz being slightly worse for the Microstrip circuit
than the ideal circuit.
Figure 4.31. Microstrip Branchline coupler amplitude response.
In many cases, the transmission lines are folded in order to reduce the printed circuit
board area, as shown in figures 4.32 and 4.33. Like figure 4.9, a M2CLIN coupled line is
used, to include the EM coupling between the folded lines. The length of the coupled
lines is tuned to place the centre frequency at 1 GHz.
Figure 4.32. Folded Microstrip Branchline coupler schematic.
The power split between port 2 and port 3, is determined by the impedance of this folded
line. Ideally, it should be a 50 transmission-line. For an input at port 1, an equal power
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