Chapter 6 DTF Testing
CellAdvisor JD720C Series User
’s Manual
53
Introducing DTF measurements
While VSWR is an indicator to express the efficiency of the cell site's energy transmission,
DTF is a measurement to identify the fault locations in the antenna line system. Most of the
antenna line system consists of various types of coaxial cables, connectors, and devices
such as dividers and surge arrestors.
Since VSWR is a measurement to verify the impedance discontinuity of the total feed line
system, it is necessary to perform DTF measurement to identify the exact component that is
contributing to the performance degradation of the line system. The DTF measurement
makes it easy to identify the fault location by displaying the relative distance of the signal
reflections or discontinuities from various points of the transmission system.
In DTF measurements, the instrument transmits a test signal along the conductor or
transmission medium. If the conductor is of uniform impedance and properly terminated, the
entire transmitted signal will be absorbed in the far-end termination and no signal will be
reflected toward the instrument. Any impedance discontinuities will cause some of the
incident signal to be sent back towards the source.
Higher impedance creates a reflection that reinforces the original signal whilst lower
impedance creates a reflection that opposes the original signal. The resulting reflected signal
that is measured at the output/input to the instrument is displayed or plotted as a function of
time and, because the speed of signal propagation is relatively constant for a given
transmission medium, it can be read as a function of cable length or distance location.
Because of this sensitivity to impedance variations, the instrument may be used to verify
cable impedance characteristics, splice and connector locations and associated losses, and
estimate cable lengths or faulty location.
Display screen
shows an example of DTF measurement.
Figure 24
DTF measurement screen
