As others have noted, it really is almost a case of you get what you pay for with antenna analyzers. Lesser models are prone to interference especially around high power installations and will give you only pieces of information you may need. One of my personal favorites is the TE-1000 impedance analyzer made by TOMCO which measures pretty much everything you could hope for and has a high amount of protection from interference. I know several AM and even television broadcast guys that use this around towers without issues.