The factory most likely tests their antennas with a laboratory grade VNA. Here is a photo I just took using a calibrated NanoVNA to check one of my 144-520MHz HT antennas. Not bad at all. I can tell you that the results changed, but insignificantly, between holding the VNA vertical, horizontal, standing it on a table, laying it flat on a wooden table, laying it flat on wooden table with the antenna overhanging the edge of the table, and when touching and not touching the chassis ground of the VNA. When comparing HT antenna performance, the best comparison of antennas is not going to be it’s SWR, but relative difference in signal strength at some given far field distance using some given amount of input power. Michael WRHS965 KE8PLM