RFSIM99

RFSIM99 is a piece of abandonware that was originally for running on Windows 3.1 16 bit. It's a very useful and very easy piece of software to use. I just can't do without it. The software license allows anyone to use and install it, but the originators retain intellectual copyright. No keys, dongles or cracks are required to use it. Later versions of Windows won't install 16 bit software unless it's been made ready to run. That means that if you attempt to use the 16 bit installer or setup.exe then it would fail. So it has to be presented as if it's already been installed on a disk as intended. Simply take the files as is and run them.

You can design filters (it does it FOR you), or you can draw your own circuit, simulate it. Design coils and capacitors and much more. It will run perfectly well with WINE and Linux too. The screenshot below shows a brick wall filter for the 20m band (perfect for RTL dongles etc) and a window showing the design of an air cored inductor.

Hint. If you use it with WINE / Linux then there's one small oddity. To make the menus work fully requires you to simply click on any of the menu items and then cancel. It them runs perfectly OK. Link to files here. If you have issue getting the software to run a simulation then try deleting rfsim99.cfg and then close and open software again.