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.

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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.

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Branimir Todorov
5 years ago

Thank you so much for helping maintain the availability of this valuable resource! I just started using it yesterday and wanted to ask if you know how to access the “balun” tab in the Component Designer – Coupler. In the help file is shown that there is such a coupler design capability. However, when I open the “Component Designer – Coupler” dialog, there is no Balun tab.
In case the balun tab is not present in this version, do you know what else I can use to represent a balun (50 Ohm unbalanced, to 100 ohm balanced) ? Thank you again for your attention and the useful web-site and articles!

4 years ago

A great program I have used since it became free software. Thanks to Stewart and to you for making it easy to access.

Eric Racicot
Eric Racicot
4 years ago

interresting software.
I have a ploblem, somebody can help me?
how can we zoom in or zoom out in schematic sheet?
Also I cant see left-right scrooling bar.
Thank you.
I dont know if this is a bug… but transformer is not looklike to be floating.

3 years ago

[…] 11. Free RF simulation software – AD5GG […]

Hermann DC4OL
Hermann DC4OL
3 years ago

Dear OM AD5GG,
i search the RFSim99 tutorial in pdf,
Please can you help me?

3 years ago

output graph doesn’t display after running simulation. running win 10.

Reply to  wb0rta
2 years ago

I’m having this exact problem as well…
Not much we can do about screenshots since the simulation window doesn’t even appear.

Last edited 2 years ago by Bren
Reply to  Bren
1 year ago

graphwinstat= wsMinimized
graphwinstat= wsNormal

Reply to  wb0rta
2 years ago

The same problem here: in Win10 output graph window stopped showing. I believe this happened after I minimized output window. After removing RFSim99.cfg file output window shows again.
Also sometimes after operating graph window can not control main window

Reply to  AndreyG
2 years ago

I guess when graph window is minimized there is no way to restore it. This gets recorded in RFsim99.cfg file – last line in the file: graphwinstat= wsMinimized. When this line is changed to graphwinstat= wsNormal graph window is visible.
This is awkward workaround but ok, at least I now know how to restore it. Is there faster way to restore graph window?
I still do not know how to restore lost control of main window. After shutting it down working file gets lost.
Btw none of this happen on my Win’95 PC.

2 years ago

Are you still maintaining this program?
I recently ran into an issue where the simulate button would open a window off screen and theres no way to move it in view… This is on Windows 10.

2 years ago

Many thanks, you saved me of install Win 98 in an older pc just for run this venerable program. (…anyway, I´m thinking to do it, because surfing the web I found almost nothing worth to use for OS after than Win XP.)
Kind regards and 73 de LU4EYW.

2 years ago

If I try to place more than one S2P block sometimes the 2nd block overwrites the first. I say “sometimes” because I have been able to place 4 or 5 S2P blocks before one of them gets overwritten. Using Win95 Comp mode in Windows 7.

I know it is asking a lot but wondered if I was doing something wrong.


6 months ago

Is RFsim99 freeware for commercial use?

2 months ago

is there an open source version of RFSIM99 in order to add additional features

William Highton
1 month ago

I have been using rfsim99 for many years and am still finding new uses for its features. However, I am difficulty using the scaling of the graph to view s2p files from my network analyser because of the bug that cramps the display at the lower part of the graph. I have one computer that I keep for old simulation software and have Windows XP Media Edition 32 bit installed. I have searched the web and downloaded files from several websites but am unable to find a single one that offers a 32 bit version that installs on XP. Do you know where I can find one or could you add one to your website for us dedicated users?

Questions and Comments are welcome!x