What effect can strong signals from nearby transmitters have on an antenna analyzer?

Antenna analyzers use a very low-power internal RF generator to measure the impedance and SWR of an antenna system. Because the test signal is so weak, the analyzer relies on reading subtle voltage reflections coming back down the feed line. If there is a strong local RF field, such as an AM broadcast station or another amateur radio operator transmitting nearby, your antenna will act as a receiving antenna. It picks up this external RF energy and sends it down the feed line into the analyzer. This external received power interferes with the analyzer's delicate internal measurements. The instrument cannot easily distinguish between its own reflected test signal and the interfering signal, which usually results in fluctuating, erratic, or highly inaccurate SWR readings.