QUCS might be a bit more popular than Altium Designer. We know about 1 link to it since March 2021 and only 1 link to Altium Designer. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Check out Altium’s website (altium.com. A lot of free resources that might help. They also have a YouTube channel. Good luck. Source: about 1 year ago
If you can get s parameter model you can use that. There are also generic transformer and coupled line models so long as you've got a way of characterising it you should be able to model it. https://sourceforge.net/projects/qucs/. Source: over 1 year ago
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