Matching wideband antenna with QUCS

Question

I am trying to match a dipole antenna to a 50Ω source. Problem is that for my frequencies (VHF) the complex impedance basically circles the 50Ω point between VSWR 2 and 3 lines. Because of that matching with a transformer or a lumped component seems impossible. To get around it I decided to use some relays to switch in different capacitors and inductors to get a good match for every frequency with some switching.

Now I'm trying to optimise these components with QUCS. I started with four sources with four different matching components. All of them go to separate sinks with the same file name.

I am running default Optimization block to try to optimise these four matching components. The problem is describing the goal for the optimiser. I can't give a formula like "max(S11)", because that would optimise one component to be good at every frequency. But I don't know how to tell QUCS that the goal is to minimise "max(min(S11, S22, S33, S44))". I am able to describe the algorithm in code, but don't know how to write it down in QUCS equations.

Answer 1

Well, Qucs comes with a number of examples (File->Examples). One of them (filter_optimization.sch) shows how to set the optimization block... Please make sure that ASCO is installed (http://asco.sourceforge.net/)

IMHO, depending on the application you'll need to choose a lowpass or highpass LC matching network and then you'll need more or less LC stages depending on the bandwidth and the "jump" between the load impedance and Z0.

Hope it helps