Frequency Domain Reflectometry (FDR) is a transmission line/antenna fault-isolation technique. The FDR measurement performs a sweep of frequencies of the transmission line/antenna under test. An inverse fast Fourier transform (IFFT) converts the reflected signals into time domain information. The result of this transformation and calculation is a DTF (Distance to Fault) measurement showing actual return loss (or SWR) vs. distance.
FDR technology is different from traditional time domain reflectometry (TDR) techniques, using swept RF instead of TDR-type DC pulses. FDR locates RF problems as well as DC open- or short-circuit conditions, a capability that is beyond that of TDR. This dual role of predicting future failure conditions and isolating existing problems makes a DTF measurement using FDR technology an important part of service/maintenance planning.
Bird Cable & Antenna instruments that use the FDR technique are shown here: