Active sensors are sensing devices which contain active components; usually this solution allows for a more precise measurement with respect to passive components.
In fact, a passive receiving antenna collects energy from the electromagnetic field being measured and makes it available at a RF cable connector. This signal then goes to the spectrum analyzer but the field characteristics can be someway modified by the presence of the cable, especially in near-field conditions.
On the other hand, an effective solution is to transfer on an optical carrier, the electric (or magnetic) field component sensed with an active probe.
The basic components of the system are a receiving electro-optical antenna which is able to transfer, on an optical carrier, the individual electric (or magnetic) field component picked up and to return it in the form of an electrical signal at the output port of an opto-electric converter.
The modulated optical carrier is transferred by means of a fiber-optic link to a converter which extracts the modulating signal and converts it back to an electrical signal.
The electrical signal thus obtained can be then sent to a spectrum analyzer with a 50 Ω common RF cable.