Why antenna measurements taken too close to the antenna give meaningless results — no matter how precise the equipment is.
Point a measurement probe at an antenna, record a pattern, and it's tempting to trust the number. But the space around every antenna splits into two physically distinct regions, and only one of them behaves the way "radiation pattern" implies. Measure in the wrong one and the data isn't just a little off — it can look like a completely different antenna, and change again if you move the probe a few centimeters closer or farther.