With the massive growth in wireless systems and the internet of things vision, low-cost and rapid antenna measurements in echoic lab environments have attracted significant research, commercial, and educational interest in the last decade. In this paper, we present a low-cost antenna positioning system based on open-source positioning hardware, for in-situ 3-dimensional radiation pattern measurements. The positioner is an off-the-shelf Arduino robotic arm and can perform azimuth and elevation measurements over a hemisphere using different rotation mechanisms. The positioning system is used in conjunction with a commercial Vector Network Analyzer (VNA) and wire λ/2 antennas for the second VNA port. The measured radiation patterns of a flexible microstrip patch antenna using the proposed setup are compared to 3D full-wave simulations as well as anechoic chamber measurements. The measurement setup, excluding the VNA, costs less than $300, and does not require calibrated reference antennas, rotary connectors, or specific PC peripherals. The open-source design library is publicly available at git.soton.ac.uk/mahm1m19/antennameasurement_system/.