I'm getting to grips with designing coupled line filters and wanted to implement the band-pass filter described in Example 8.7, p. 435 in David Pozar's excellent "Microwave Engineering" just as a first toe-dip into the discipline. However, the outcome for the insertion loss is very much unlike the one stated in the example. I am missing something very basic, probably.

The filter itself is N=3 (four coupled lines) with a 0.5 dB equal ripple response and a center frequency of 2 GHz. $Z_0=50\Omega$ and the bandwidth is 10%.

The stated odd and even mode impedances are:

+---+------------+------------+
| n | Z_0e (ohm) | Z_0o (ohm) |
+---+------------+------------+
| 1 | 70.61      | 39.24      |
| 2 | 56.64      | 44.77      |
| 3 | 56.64      | 44.77      |
| 4 | 70.61      | 39.24      |
+---+------------+------------+


Using Qucs 0.0.18 and its line calculation tool I make the following schematic, assuming I have a 254 um thick ceramic substrate of sorts and that the couplers are $\lambda/4$ long at 2 GHz:

I now get the following results:

which is significantly worse than I expected. There is no ripple in the passband, as seen in Pozar's results, and I only ever achieve a -3 to -4 dB insertion loss.

If I add some transmission lines for impedance matching, I might gain 1 dB in S21 at the expense of bandwidth.

Obviously it is possible to design a much better functioning filter, like this one in the qucs example folder for 10 GHz: http://sourceforge.net/p/qucs/git/ci/master/tree/examples/bpf_10Ghz.sch

Could this be the microstrip models in qucs adding too much reality to the results? ;-) Any comments are greatly appreciated.

• Did you try taking tand and rho to near zero? – The Photon Oct 26 '15 at 23:41
• Incidentally, your h and t parameters look typical for printed circuit scenarios, but your er is awfully high. What substrate are you trying to model? – The Photon Oct 26 '15 at 23:44
• @ThePhoton, looks like substrate is Alumina ($\epsilon _r$=9.8). – gsills Oct 27 '15 at 2:06
• @ThePhoton Loss tangent and conductivity: Yes, I had the same thought, however, the improvement was marginal. – electroGeek Oct 27 '15 at 5:15
• @ThePhoton Substrate material and thicknesses: Yes, this combination is not the most realistic one. I redid the simulation with qucs' built-in RO4350 substrate parameters and recalculated dimensions. I didn't change the outcome very much. – electroGeek Oct 27 '15 at 5:18

• Good that you got if figured out. If you want a real-world (partial) solution it probably means using a thicker substrate (larger h) so that all your Ws will end up larger. – The Photon Oct 29 '15 at 0:58