Software Defined Radio (SDR) has become an increasingly credible alternative solution to solid state radio problems. The practical application of SDR in building a spectrum analyser can be complex and expensive; however, the use of lightweight, cost-effective, low-cost SDR and lowpower microprocessors can significantly reduce the cost. This paper investigates ways to build lightweight, cost-effective spectrum analysers and their most effective design and implementation, utilising the RTL-SDR USB device, a software modelling tool (GNU Radio), and a low-power-microprocessor Raspberry Pi. The RTL-SDR USB device, GNU Radio Software and Raspberry Pi are configured together to produce a lightweight SDR-based spectrum analyser. In this particular implementation, the spectrum analyser is used to examine a small section of the electromagnetic spectrum to test the suitability of the configuration for complex Fast Fourier Transform calculations. For comparison purposes, another spectrum analyser is built using a more powerful microprocessor (AMD Turion), and the results recorded and compared. The experimental results and comparative evaluation of the two spectrum analysers demonstrate that the Raspberry Pi-based design performs equally well despite its limited resources and low cost. This shows the success and potential of this SDR and Raspberry Pi-based spectrum analyser as a credible device for a number of intelligence analyses applications in a secure environment.