W3SZ RTL Sun Noise Page

This is is followup to our discussion at the MWL just past regarding measuring sun noise / moon noise.

The key to getting an accurate measurement is in having sufficient bandwidth of the sampled signal, whether the technique used is an old analog method or a more technologically advanced SDR-based method.

Paul Wade W1GHZ wrote an excellent although now dated article on this subject in the 1990s:

Paul wrote this article before SDR’s had come onto the scene for Amateur Radio use, and so he used a wide-band (several MHz bandwidth) analog system using a 10 GHz transverter, some interdigital filters, and an amplifier feeding an HP 432 power meter.

I was going to duplicate Paul’s system more a decade or so ago when I ran into Al Ward W5LUA at Dayton, and he indicated that I should advance to the current century and use an SDR-IQ ( or maybe it was an SDR-14, I don’t remember ) taking the IF signal from the 10 GHz transverter along with SpectraVue running in the Continuum mode to do this. I already had an SDR-IQ, and so that is what I have done ever since.

VK3UM also wrote a detailed paper on sun noise in 2008. He had not yet embraced the digital age:

Here is a brief paper on using SpectraVue for measuring sun noise. There are no startling revelations in it:

So, basically, my opinion is that using anything other than an SDR to measure sun noise (or moon noise) these days is dinosaur-like behavior.  Below is my cookbook for setting up a system to use a cheap $20 RTL-SDR dongle for measuring sun and moon noise.

What You Need to Do This:

RTL SDR Dongle such as are available on Amazon:


HDSDR Free SDR Software:


ExtIO_RTL2832U.dll dll file:


Zadig Driver Utility:


Virtual Audio Cable or equivalent (this is NOT freeware):


SpectraVue Free Software:


In case you get stuck, the RTL-SDR Quick Start Guide at:



Steps to Do This:

  1.  Plug your RTL-SDR dongle into a USB port.  DO NOT install any of the software that came with the dongle, but let Plug and Play try to install it.  If you had already installed any software drivers that came with the dongle, uninstall them.
  2. Go to http://zadig.akeo.ie/ and download Zadig, if you haven’t already done so.
  3. Run Zadig and go to Options->List All Devices and make sure this option is checked.
  4. Select “Bulk-In, Interface (Interface 0)” from the drop down list. Ensure that WinUSB is selected in the box next to where it says Driver. (Note on some PCs you may see something like RTL2832UHIDIR or RTL2832U instead of the bulk in interface. This is also a valid selection). (Do not select “USB Receiver (Interface 0)” however).
  5. Click “Replace Driver” (or “Install Driver” if the button says that). You might get a warning that the publisher cannot be verified, but just accept it by clicking on Install this driver software anyway. This will install the drivers necessary to run the dongle as a software defined radio. Note that you may need to run zadig.exe again if you move the dongle to another USB port, or want to use two or more dongles together.
  6. Download HDSDR from http://hdsdr.de/, using the download button at the bottom of the page, if you haven’t done so already.
  7. Use the installer you just downloaded to install HDSDR, if you haven’t done so already.
  8. If you haven’t done so already, go to http://hdsdr.de/hardware.html and download the ExtIO_RTL2832U.dll dll file from the table entry “RTLSDR (DVB-T/DAB with RTL2832) USB” (Direct Link).
  9. Copy the ExtIO_RTL2832U.dll file into the HDSDR install folder which is by default set to C:\Program Files (x86)\HDSDR.
  10. Open HDSDR. You might be asked to select a .dll file. Choose the ExtIO_RTL2832U.dll file you just copied over and then click Open. It is okay if you do not see this screen as long as you have copied the ExtIO_RTL2832U.dll file over properly in the last step.
  11. Download and install Virtual Audio Cable from https://software.muzychenko.net/eng/vac.htm  This is not freeware, and the trial version is not suitable because it produces a periodic audio message that will interfere with the test measurements.  If your system is 32 bits, select the 32 bit download.  If it is 64 bits, then select the 64 bit download.  There is a freeware alternative (actually donationware, but you don’t need to donate) VB-Cable, which is available at the following URL.  But use of this is beyond the scope of this discussion.  https://www.vb-audio.com/Cable/VirtualCables.htm  
  12. Open the Virtual Audio Cable Control Panel and set the sampling rate of Cable 1 to 192000 in both “SR” boxes, as is illustrated below.  Then click the “Set” button in the “Cable Parameters” box, then click “Restart Audio Engine” and finally, click “Exit”.    Note that when you start it, Virtual Audio Cable should always start in administrator mode.  If it does not, then you may get an error when you try to “Restart Audio Engine”.  If that happens, exit the program and start it by right clicking on its icon and then left-clicking “Run as administrator” instead of double-left-clicking on the icon.  You can also set the program to always run as administrator by right-clicking on it, then clicking “Properties”, then clicking “Compatibility”, then clicking “Run this program as an administrator”, and then clicking “OK”. 
  13. In HDSDR, click on “Soundcard [F5]” and select “Line 1 (Virtual Audio Cable)”.  Then click “OK”.  
  14. Click on “Bandwidth [F6]” and under “Output” click on 192000.  Then click the “X” in the upper right corner of this window.  Do NOT change the Input Bandwidth!  
  15. Click on “Options [F7]” and then on “Output Channel Mode for Rx” and on “IF as I (Left) / Q (Right)”. 
  16. Set the HDSDR LO frequency to a clear segment in the 2M band and make sure that “AGC Off” is showing in the buttons located beneath the recording controls.  You can fine tune the frequency either by clicking in the RF spectrum, or using the Tune numbers.  Make sure that you are in “FM” mode by clicking on the “FM” button if it is not already illuminated.  After you click the “FM” button, you can use the slider to the right of the buttons below the recording controls to bring the bandwidth to 192000 if it is not already set to that value.
  17.   Pull the red bandpass filter line in the bottom right corner to the right until it disappears off the screen.  You will see the frequency of the bandpass edge increase as you do this.  I pull it all the way to 192000, which is well outside the HDSDR window, but nevertheless the output of HDSDR appears to be limited to a bandwidth of 96 kHz. 
  18. Download SpectraVue from http://rfspace.com/RFSPACE/SpectraVue.html and install it.
  19. Run SpectraVue and set InputDevice to Soundcard. 
  20. Click on “Soundcard IN Setup” and set SoundCard to “Line 1 (Virtual Audio Cable)”.  Also set Sample Rate and BW Limit to 192000 and make sure “Stereo (Complex I=R, Q=L)” is checked.  Then click “OK”.  
  21. In the main SpectraVue window, click on “Continuum” to set the receiver to that mode.  Then make sure that Span is set to 0.192000 MHz and then click “Start-F12”.  Then click “Auto Scale (A)” and that should position the signal trace near the bottom of the screen.  You can click below the arrow at the extreme right of the display to move the signal trace higher on the display.
  22. To measure sun or moon noise, just feed the 144 MHz receive output of your tranverter into the input SMA jack of the RTL-SDR dongle, or use a splitter to put the dongle in parallel with the receive input of your 144 MHz-28MHz transverter.  The signal level of the RTL-SDR dongle (once it has warmed up) is quite stable, as you can see below with the dongle’s input seeing a 50 ohm termination. 
  23. Good Luck!

Please let me know if you have questions or comments!

–W3SZ  10-7-2018