Henrik Forstén has done amazing job designing a homemade vector analyzer with the goal to fit it’s budget of 200EUR. The design is open source and available on github.
Vector network analyzer (VNA) are used to measure scattering parameters of high frequency circuits. When frequency is high enough the reflections of the waves start to matter and distributed effects need to be taken into account. VNA can be used to analyze reflection and transmission coefficients of circuits at high frequencies.
Cheap homemade 30 MHz – 6 GHz vector network analyzer – [Link]