
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:
RFSim99 Screenshots
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:
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.
For anyone wondering, this is the error you get trying to install RFSim99 using the original .exe installer file on Windows versions later than Windows XP.
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!
Hi Branimir, thanks for your comments and kind words!
AppCAD from Avago (and formerly Agilent and HP) has a lumped element Balun calculator, but I’m not sure that’s what you’re looking for. Have a look and see what you think: http://www.hp.woodshot.com/
Best of luck, AD5GG
A great program I have used since it became free software. Thanks to Stewart and to you for making it easy to access.
Hi,
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.
Eric
Hi Eric,
As far as I know, there is no zoom function in this software. The only way to make everything larger is to run it in a smaller screen resolution.
If you run RFSIM99 in Windows 95 or Windows 98 Compatibility mode, you will see the left-right scroll bar.
What do you mean about the transformer?
[…] 11. Free RF simulation software – AD5GG […]
Dear OM AD5GG,
i search the RFSim99 tutorial in pdf,
Please can you help me?
Hello Hermann,
I don’t know of any online guides or tutorials for RFSIM99. There are some videos on YouTube, but they don’t go too in-depth.
I could possibly do some RFSim99 tutorial videos in the future, if that’s something that people might be interested in.
Thanks for the comment, and 73 from AD5GG
output graph doesn’t display after running simulation. running win 10.
Can you expand on your findings? Include screenshots etc?
I’m having this exact problem as well…
Not much we can do about screenshots since the simulation window doesn’t even appear.
Hi Bren,
Can you try deleting (or moving to another folder) the RFSim99.cfg file, and restart the application? The window positioning data is held in the RFSim99.cfg file, amongst other stuff.
Cheers,
AD5GG
Open
RFsim99.cfg
Change
graphwinstat= wsMinimized
To
graphwinstat= wsNormal
I renamed the RFSim99.cfg file in the archive so it is ignored when you run RFSim99. The program will save a new config file after first use.
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
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.
I haven’t come across this error, but I don’t really use RFSim99 that much these days. I mostly use LTSpice or QUCS for my simulations at home.
When RFSim99 was developed, I’m sure there was no consideration for multi-monitor systems or large screen resolutions. Also, Windows has changed an awful lot over the years, and I expect more issues to appear like this. I am happy to see people posting about errors and workarounds though!
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.
Hi Bren,
I’m not the developer, so alas, there really is nothing to “maintain”. All I did was repackage the files from the original installer, since it wouldn’t work on anything later than Windows XP.
See my reply above.
Thanks!
AD5GG
EXCELLENT..!
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.
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.
Thanks.
Hi Dave, I have not heard of that happening, but I’ve not tried to have multiple S-parameter blocks.
I’m also getting this error a lot these days… I fear that this program may really need a Win95/98 environment to continue operating in the future.
Update – I no longer see this error. I don’t know what changed, but OK! 🙂