Following my article in the November 2025 RadCom, I've had enquiries about conducting antenna pattern measurements using a TinySA spectrum analyser. Yes, it certainly works: with that instrument set in Zero Span mode, it will record received signal level against time, so setting its Sweep time equal to the rotation period will show the antenna pattern directly. But you do need to check that your rotator has a consistent rotation period.
Note that the signal level in that picture is quite noisy, since the RBW filter was set at its widest (600kHz): dropping this to 10 or 3kHz would reduce interfering signals and improve the signal-to-noise ratio.
Now that's in rectangular format, but this same data can be displayed in the familiar circular polar plot format by the excellent free PolarPlot program from G4HFQ, so I've written a Python script to save the sweep from the TinySA as a compatible PLP file.
This is now compiled into a program (updated 22-Oct-2025) TSAtoPolar and here is an example from my garden test-range, measuring a small 70cm beam.
After loading your plot, try clicking the Smooth button once or twice to improve the plot and then Rescale if necessary, to put the peak gain at 0 degrees. The Directivity button will re-calculate the estimated gain. More details of the program can be found in the PLPHelp.chm file in its source folder (usually C:\Program Files (x86)\PolarPlot\).
A 180-degree plot is better for charactersing the main lobe, since it's covered in a continuous sweep (rather than "joining up the ends" of a 360-degree scan). In this case, start the scan 90 degrees off the boresight and continue to 90 degrees past it. When using a 360-degree plot, begin with the antenna on boresight and ensure that the TSA's display captures this peak at both ends of its sweep.
If you want to try a real-time serial port input to PolarPlot, using com0com to create a "virtual null-modem" loopback link, my earlier "TSA2PP" program can be downloaded from here.
You can read more about my antenna measurements in the November 2025 edition of RadCom or on the 23cm NW Forum.
