Why An antenna analyzer works by injecting a very low-level signal (often well under a milliwatt) into the antenna and measuring the tiny reflected voltage. The antenna under test is also a receiving antenna, so RF from a nearby transmitter arrives at the analyzer's detector at a level comparable to or greater than its own test signal, and the detector cannot tell the difference. The result is erratic or falsely high SWR and impedance readings. The cure is to test when neighboring stations are quiet, or to check whether readings change when the other station keys up.