Free RF simulation software

I’ve been using RF sim 99 (aka RFSim99) for over a decade, and it’s proved invaluable again and again during that time, and continues to do so. RFSim99 is a freeware RF simulation software utility. It’s small, fast, and reliable. I should make it clear that it performs basic RF simulation only, and is nothing like Genesys/ADS, Microwave Office, Ansoft Designer, HFSS, or of that ilk, so if you want a full-on EM solver, this is not your program.

Having said that, RFSim99 is handy if you want to design a basic filter, impedance match, resistive attenuator, or create/view 1 or 2 port S-parameter files. There are decent lists of other freeware RF simulation utilities here and here if you’re interested.

The RFSim99 installer package stopped working after Windows XP, and I’ve known people who have installed a Windows XP virtual machine in order to run the program, but you don’t have to do that. I grabbed the files required for RFSim99 to run and created a standalone version which you can download below. Just extract this utility into a folder and run RFSim99.exe. You will need to manually create a windows file association if you want RFSim99 to automatically open the *.cct files when double-clicked.

There is a slight quirk with running RFSim99 in Windows 7, 8, 8.1, and 10. Sometimes some of the menu buttons (Simulate, Tune, and some others) fail to appear. You can get them to appear by hovering your mouse pointer over them, clicking and holding the left button, and dragging the pointer around and over the buttons.

If you don’t want to do this, you can disable desktop composition in the Windows compatibility settings for the program:

Another quirk can be found in the rectangular display. The lower scale limit fields are partially obscured. Generally, they’re set to -40dB, so I generally just highlight the field, delete everything in the field, then type in -40dB (or whatever I want my lower limit to be) and hit enter. See below in the screenshot gallery for an example.

Mild quirks aside, once up and running, RFSim99 is very easy to use, intuitive, and produces fast and accurate results (depending of course, on accurate input data! Garbage in = garbage out!). I can’t tell you how many filters I’ve designed roughly with RFSim99 that have found their way into commercial products after a little bit of hardware tweaking. You can’t beat free!

Check out some screenshots from RFSim99 below:

I was going to do a page on S-parameters, but there’s no point in re-inventing the wheel. Microwaves101 has an excellent page on S-parameters here.

Two other utilities I’ve used a lot in the past are AppCAD (from HP, then Agilent, then Avago!) and TXLine. They’re a couple of my go-to utilities for quick transmission line simulation/dimensions.

Download a standalone version of RFSim99 v1.05 (the last/latest version) here:

Yes, it works on Windows 10!
Yes, it works on Windows 10!
Yes, it works on Windows 11!

Some RFSim99 issues and workarounds, courtesy of ZS6GST

i) RFSim99 has a fuse that blows when setting the scale of Smith charts to ‘0’ ! When this happens the input screen is obsoleted and you have to start afresh. Solution; save often and take care never to set the Smith Chart scale to ‘0’.

ii) When sweeping a transmission line between 50 Ohm ports, the display will sometimes go crazy. Solution; alter one of the impedance of one of the ports by a very small (insignificant) amount.

iii) RFSim99 does not tolerate open ended stubs. Solution; place a high value resistor between the open end and ground. This makes RFSim99 happy.

iv) Another solution to the button anomaly: My problem with RFSim99 was solved by selecting small fonts. To then restrict small fonts to RFSim99, I created a secondary user with ‘small font’ being selected.

Known Bugs

Here is a capture of the original RFSim99 website, detailing some known issues with version 1.05.

RFSim99 – An interview with the author.

The following text was taken from an interview with Stewart Hyde, on the website Practical RF (now gone) back in February 2006.

It can cost large sums of money to get a workstation seat at some of the high end simulation and analysis software suites. But the underlying mathematics that these programs use is not secret, it is in the public domain and there are low cost or shareware packages that will do the same computations. Often they lack the features and the extensive user interfaces, but the internal engines are running the same equations.

For low frequency or time domain simulations there are versions of SPICE freely available. For RF simulations the field is smaller, but one free program that repeatedly crops up in the listings is RFSim99.

RFSim99 (the name tells you how old it is) works with S-parameters. This means that not only will it simulate and analyse circuits built out of its internal library of components, but that it can accommodate any circuit block or stage for which these are known. S-parameters are easily measured with a network analyser, so RFSim99 avoids the problems of many packages by not being bound to third party models. As well as the features one would expect to see, such as schematic drawing and graphical analysis, there are a few really good extras, such as an integrated filter design tool and the ability to run tolerance sweeps.

RFSim99 was written by Stewart Hyde. The embedded link to his web site is no longer valid, but at pRF we were so impressed by the program that we tracked him down.

practicalRF: Stewart, what are you working on now?

Stewart Hyde: I am currently working on RF design and development. I was one of the founders of Cambridge Broadband in 2000. We design point to multipoint broadband wireless access and backhaul systems.

pRF: What’s the background to RFSim?

Stewart: Its very old now. I was working as a contractor for some time and developing tools was a sort of ongoing hobby. Eventually, I decided to draw a line under it and make it available on the web.

pRF: That would be 1999? As it stands it works fine and its got far fewer bugs than packages we’ve paid thousands for. But have you any plans to update it?

Stewart: Its unlikely; as you say it works as a finished product. And I have work and family commitments.

pRF: What is your background?

Stewart: I did an apprenticeship with Thorn EMI in Somerset, England, working on radars, and I have a First Class Honours degree in Electrical and Electronic Engineering from Aston University in Birmingham.

pRF: What design software do you use now?

Stewart: I actually use RFSim still, but with a few personal modifications. I’ve used Touchstone and we use Eagleware at work.

pRF: What do you think of the different approaches to simulation and modelling; is simple analysis too basic or is full 3D EM analysis over the top?

Stewart: You can go an awful long way with S-parameters; they will model almost anything. There is a danger in going to far with modelling, though. You can convince yourself of a reality that doesn’t exist.

Help Files

The old Windows HLP files stopped working after Windows XP. I have converted the original HLP file to the modern Windows CHM help file format, and the popular Adobe PDF format. You can download these below.


AD5GG works in the real world as an RF design engineer. Occasionally, he posts articles on this very site. Sometimes they're even worth reading. Please feel free to comment on posts using the comment system at the end of each post, and use the discussion forum.