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Carrier Sleuth

Display high resolution carrier waterfalls of medium wave radio stations

For Windows and Macintosh

Attention Medium Wave DXers! While you can often only hear one, or perhaps two, stations on a frequency at a time, chances are there are actually many other stations lurking on that frequency, even in the daytime. Perhaps dozens. The problem is that they are too weak to notice using normal listening techniques. There's no audio, and even their carriera are often quite weak. But by performing very high resolution FFTs (Fast Fourier Transforms) on recordings made by SDRs (Software Defined Radios) you can extract these carriers up out of the noise and see them.

Also fortunate for us, most MW stations have small, and often constant, errors in their actual frequency. While these errors are often only a few Hz, given sufficient resolution from the FFT they can be separated.

Carrier Sleuth generates very high resolution waterfalls from I/Q recording files made by several SDR (Software Defined Radio) programs. Namely:

I do not have all of these SDR programs, so I rely on recording files made by others, and therefore cannot guarantee that all work correctly.

(Click on the image to view full size)

These waterfalls can be used to observe the multitude of MW (Medium Wave) stations on a single channel at the same time, even when only one station can be clearly heard. Or even when no station can be heard, such as with the 9 kHz channels from Europe and Asia. Carrier Sleuth does this by performing very long FFTs on the data, up to a 32 million point FFT.

When first launched, you will see the main window, which will be blank other than some controls. You first need to process your I/Q recording files to see the waterfalls. It is assumed at this point you have some, if not, you need to make a few with your SDR software. To get started, I suggest making a few files at a 100 to 250 kHz bandwidth, each of which is 1 or 2 GB in length.

Select Open I/Q Files(s)... from the File menu in Carrier Sleuth.

Next you need to specify the output file, this is where Carrier Sleuth will save the resulting waterfalls. Click the Set Output File button, navigate to where you want to store the file, and enter in the name of the file.

Next you need to specify the characteristics of the waterfalls:

First, the FFT Length. Longer FFT lengths produce higher resolution waterfalls, but they are shorter in length (in pixels), and take longer to generate. You can start with a 512K or 1M length to get a feel for things, then experiment with different lengths

Next the Frequency Width. I like to use 100 Hz most of the time, or 200 Hz if there could be some carriers much further away from the nominal frequency. Or 50 Hz or even less if they are all close in. Again, you can experiment.

Then the Display Width, which is the width of the resulting waterfall in pixels. The default 1024 pixels is a good compromise.

Then pick which MW channels you want to look at. 9 kHz channels are used outside of North and South America, 10 kHz channels inside.

If you would like to use your own set of frequencies/channels, perhaps because you want to examine something other than the MW band, check the Custom Channels box. You will see a new window where you can enter in frequencies, in kHz, separated by spaces or commas. Such as 11900 11925 11940 11975 Then click the Set button, and this window will close. The app will use these frequencies. You can copy and paste onto this window, so you can prepare a list of frequencies in a text editor if you wish, and just paste them in, so you do not have to type them by hand.

Now click the Add File(s) button, and select your SDR recording files. Carrier Sleuth will list them in the left side of the window, sorted by time. It will also display the frequency range covered by the files. Note that it only looks at the first file to determine this, it assumes all of the files have the same bandwidth / sample rate. If they do not, strange things could happen. Carrier Sleuth will generate waterfalls for every MW channel that falls within this range.

If you want to remove the files from the list click Clear Files. And then add some new files.

Finally click the Process button. If all goes well, this window will close, and you will return to the main window. The progress indictors at the of of the window will appear, which show how far along the program is in processing the files. The first is the progress within the current file, the second is the progress through all of the files. It can take a long time to process files, depending on how many, their size, the speed of your computer, FFT length, and so on. So you may want to start with just one or two files to get the hang of things.

When it is done, the waterfall for the first channel will be displayed. It may look ugly, so adjust the Max dB and Min dB sliders as necessary. You should see one or more vertical lines which are the carriers of MW stations on that channel (unless of course you picked a channel without any that were received on your recording). You can scroll up and down, and change the window size. There are time markers drawn every hour on the waterfall. You can also move your cursor on the window and the time and frequency will be displayed at the top of the window.

The Channels menu will be populated with a list of every channel a waterfall was generated for. You can select any channel of interested, and use cmd-D and cmd-U (control-D and control-U on Windows) to step up or down through the channels.

Save Waterfall Image... from the File menu will save the image as a PNG file.

Copy Waterfall Image from the Edit menu will copy the image to your clipboard (pasteboard) so you can paste it into another application.

View Notes from the Edit menu brings up a window where you can enter in some free form text notes for your own use. These are stored within the file. This can be useful to record information such as QTH, equipment used, etc.

If you want to re-load this set of waterfalls, or another previously generated file, use Open FFT File from the File menu. It will load them without having to re-generate them from the I/Q recording files, which is a huge time saver.


Support for other SDR recording file formats is possible, you'll need to work with me by providing sample files and details on the format.

Questions or suggestions? If you're a member of the website, you can post your question to the Software forum there

Download Options:

Carrier Sleuth 1.0.4 for Windows - Windows 10, probably 7 and 8, maybe Vista and XP?

Carrier Sleuth 1.0.4 for Mac OS X - Mac OS X 10.6.8 through 10.13 High Sierra, Intel only

Carrier Sleuth is offered without any warranty at all. Use at your own risk. Guaranteed to blow up your computer, zap your radios, and destroy the ionosphere. Keep away from Al Fansome at all costs.

Email your comments and questions to

Last modified August 4, 2017