This software repository provides some scripts used to measure antenna performance.
See http://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/ for more details.
Requires working rtl-sdr software.
Use ./scanner.sh to generate the scan data. Follow onscreen instructions Use gnuplot richtkoppler.gnu to visualize the results.