The most important instruments for EMC investigations are an oscilloscope and a spectrum analyzer. While demands on sensitivity and precision are moderate, portability, i.e. battery operation, is an important issue for measurements in the field. Also a notebook (for running DOOCS or other measurement software) and a cell phone (for communication with the control room or other remote operators) are extremely useful in that situation.
The following links offer some introductory and some more advanced material about how spectrum analyzers work and how to use them most efficiently: