One of the most recognizable and popular ghost research tools is the EMF detector. EMF detectors measure electro-magnetic waves and frequency. These probes can be generally considered as antennae and should not interfere with the environmental electromagnetic field, while also preventing coupling and reflection as much as possible in order to obtain precise results. There are two main types of EMF measurements:
broadband measurements: performed using a broadband probe, that is a device which senses any signal across a wide range of frequencies and is usually made with three independent diode detectors;
frequency selective measurements: in which the measurement system consists of a field antenna and a frequency selective receiver or spectrum analyzer allowing to monitor the frequency range of interest.
Most EMF probes respond to fields only on one axis, but there are some which are tri-axial, showing components of the field in 3-D (three directions at once). When amplified, active probes can improve measurement precision and sensitivity but their active components may limit their speed of response.