S820E UG PN: 10580-00343 Rev. H 3-9
Cable and Antenna Measurements 3-5 Distance-To-Fault (DTF)
3-5 Distance-To-Fault (DTF)
DTF reveals the precise fault location of components in the transmission line
system. This test helps to identify specific problems in the system, such as
connector transitions, jumpers, kinks in a cable, moisture intrusion, or
mechanical damage.
The first step is to measure the distance of a cable, this measurement can be
made with an open or a short connected at the end of the cable. The peak
indicating the end of the cable should be between 0 dB and 5 dB. An open or
short should not be used when DTF is used for troubleshooting the system
because the open/short reflects most of the RF energy from the Site Master.
The true value of a connector might be misinterpreted, or a good connector
might look like a failing connector.
A 50 Ω load is the best termination for troubleshooting DTF problems
because it will be 50 Ω over the entire frequency range. The antenna can also
be used as a terminating device, but the impedance of the antenna will
change over different frequencies because the antenna is typically designed to
have only 15 dB or better return loss in the passband of the antenna.
DTF measurement is a frequency domain measurement, and the data are
transformed to the time domain. The distance information is obtained by
analyzing how much the phase is changing when the system is swept in the
frequency domain. Frequency selective devices such as TMAs (Tower
Mounted Amplifiers), duplexers, filters, and quarter wave lightning arrestors
change the phase information (distance information) if they are not swept
over the correct frequencies. Care needs to be taken when setting up the
frequency range whenever a TMA is present in the path.
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