Thank you to RTL-SDR.COM reader MAKA for writing in and sharing his project with us, an open-source 0.1 - 3 GHz network analyzer using an ADALM PlutoSDR and a $15 RF bridge. The system allows you to measure S21 (insertion loss/gain) and S11 (return loss) with real-time visual feedback. MAKA writes:

Key features include:

• FFT-based FIR filter for precise measurements
• Simple S21 and S11 calibration workflows (LOAD & OPEN)
• Real-time plots with smoothing and raw-data toggles
• Interactive markers for easy dB/frequency annotations
• Automatic export of calibration data to .xlsx for offline analysis
• Fully open-source

The complete open-source code for the project is available on GitHub at  https://github.com/fromconcepttocircuit/pluto-network-analyzer

Over on his YouTube channel, MAKA has also uploaded a video demo and walkthrough.