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.
Using DTF Aid
Because of the nature of the measurement, maximum distance range and distance resolution is dependent upon the frequency range and number of data points. DTF Aid (Freq/Distance > Distance > DTF Aid) shown in Figure: DTF Aid explains how the parameters are related.
DTF Aid
If the cable or waveguide is longer than the Max Usable Distance displayed, then the only way to improve the horizontal range is to reduce the frequency span or to increase the number of data points. Similarly, the distance resolution is inversely proportional to the frequency range and the only way to improve the distance resolution is to widen the frequency span.
Note
When determining the frequency range, consider all in‑line frequency selective devices.
Transmission Line Selection
The Microwave Site Master 820E is capable of measuring either coaxial cable or waveguide feeder lines. Selecting the correct coaxial or waveguide type is critical for accurate DTF measurements. Incorrect propagation velocity (PV) (or Cut Off Frequency in the case of waveguide) values affect the distance accuracy, and inaccurate attenuation values affect the accuracy of the amplitude values.
Selecting the line type or creating a custom type is described in the following sections.
Cable List
The Microwave Site Master S820E is equipped with a built‑in, predefined cable list (Freq/Dist > DTF Setup > Cable List), which includes most of the common cables that are currently in use. After the correct cable has been selected, the instrument updates the propagation velocity and the cable attenuation values to correspond with the cable. For setups with several different cables types, choose the main feeder cable.
Note
If the Cable list button is not displaying, then toggle DUT Line Type to Coax
For cables not on the list, select NONE and manually enter the Prop Velocity and Cable Loss in DTF Aid or the DTF Setup submenu.
Note
Custom cable settings that are entered manually are not saved when the instrument is preset, reset, or turned Off.
Custom Cables can be created and uploaded to the instrument by using Line Sweep Tools (LST). Instructions for using the LST Cable Editor are available in the LST software Help menu. The latest version of LST is available from the Anritsu website: http://www.anritsu.com
The name, propagation velocity, and cable loss of the selected cable is displayed below the graph during distance measurements (Measurement > DTF Return Loss or DTF VSWR) as shown in (Figure: Cable List Selection Displayed Under the Graph).
Waveguide List
The Site Master S820E is equipped with a built‑in, predefined waveguide list (Freq/Dist > DTF Setup > Waveguide List) including most of the common waveguides currently in use.
Note
If the Waveguide list button is not displaying, then toggle DUT Line Type to WG
After the correct waveguide has been selected, the instrument updates the Cutoff Frequency and the waveguide attenuation values to correspond with the waveguide. For setups with several different types, choose the main feeder waveguide.
For waveguides not on the list, select NONE and manually enter the Waveguide Loss and Cutoff Freq in DTF Aid or the DTF Setup submenu.
Note
Custom waveguide settings that are entered manually are not saved when the instrument is preset, reset, or turned off.
Custom waveguides can be created and uploaded to the instrument using Line Sweep Tools (LST). Instructions for using the LST Waveguide Editor are available in the software’s Help menu. The latest version of LST is available from the Anritsu website: http://www.anritsu.com
The name, cutoff frequency, and waveguide loss of the selected cable is displayed below the graph during distance measurements (Measurement > DTF Return Loss or DTF VSWR).
Cable List Selection Displayed Under the Graph
Distance Resolution
Distance resolution is the ability of the Site Master to separate two closely spaced discontinuities. If the resolution is 5 meters and two faults are 3 meters apart, then the Site Master will not be able to show both faults until the resolution is improved by widening the frequency span.
Distance Resolution (m) = 1.5 x 108 x PV / ΔF (in Hz)
with Rectangular Windowing applied.
Figure: DTF Measurements at 100 MHz vs. 500 MHz is an example of the same DTF measurement with a 100 MHz span versus a 500 MHz span. The increased span provides additional detail that several unique issues may affect the first 10 meters of the cable. This detail was not available in the narrower span.
DTF Measurements at 100 MHz vs. 500 MHz
Windowing
The theoretical requirement for inverse FFT is for the data to extend from zero frequency to infinity. Side lobes appear around a discontinuity because the spectrum is cut off at a finite frequency. Windowing reduces the side lobes by smoothing out the sharp transitions at the beginning and the end of the frequency sweep. As the side lobes are reduced, the main lobe widens, thereby reducing the resolution.
In situations where a small discontinuity may be close to a large one, side lobe reduction windowing helps to reveal the discrete discontinuities. If distance resolution is critical, then reduce the windowing for greater signal resolution.
If two or more signals are very near to each other, then spectral resolution is important. In this case, use Rectangular Windowing for the sharpest main lobe (the best resolution).
In summary:
• Rectangular Windowing provides best spatial distance resolution for revealing closely spaced events, but the side lobes close to any major event (large reflection) may mask smaller events which are close to the major event. Excellent choice if you suspect multiple faults of similar amplitudes close together.
• Nominal Side Lobe Windowing provides very good suppression of close‑in side lobes, but compromises spatial distance resolution compared to Rectangular windowing. Closely spaced events may appear as a single event, often non‑symmetrical in shape. Excellent overall choice for most typical antenna system sweeps.
• Low Side Lobe Windowing provides excellent suppression of close‑in side lobes, but spatial distance resolution is worse than Nominal Side Lobe. The additional suppression of side lobes may be useful in locating very small reflection events further away from large events. This is not often used for field measurements.
• Minimum Side Lobe Windowing provides the highest suppression of side lobes but the worst spatial distance resolution. It can be useful for finding extremely small events spaced further apart than the distance resolution. This is not typically used for field measurements.
Effects of Windowing on a Sample Trace
DMax (Maximum Usable Distance)
DMax is the maximum horizontal distance that can be analyzed. The Stop Distance cannot exceed DMax. If the cable is longer than DMax, then DMax needs to be improved by increasing the number of data points or by lowering the frequency span (ΔF). Note that the data points can be set to 130, 259, 517, 1033, or 2065 (Sweep> Data Points).
DMax = (Data points – 1) x Distance Resolution
DTF Measurement Examples
1. Press the Measurement main menu key and select DTF Return Loss or DTF VSWR.
2. Press the Freq/Dist main menu key.
3. Press the Distance submenu key and then select DTF Aid. Use the touchscreen, rotary knob, or Up/DownArrow keys to navigate through all the DTF parameters.
l. Highlight a parameter in the DTF Aid table to edit and then press Edit or Enter to display a parameter for editing.
m. Edit all required parameters and then highlight Keep current values – CONTINUE and press Enter.
Note
If Stop Distance is greater than DMax, then increase the number of data points or reduce the frequency span accordingly.
4. Connect a phase‑stable Test Port cable to the RF Out/Reflect In connector on the Site Master. Press the Calibration main menu key to start calibration of the instrument. Refer to Calibration, CAA for details.
5. Connect the Site Master to the Device Under Test using the calibrated phase‑stable test port cable.
Example 1 – DTF with a Short to Measure Cable Length
To measure the length of a cable, DTF measurements 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. In Figure: DTF Return Loss with Short at End of Cable (15 m) the cable end is at 15 meters.
The cable end was found by selecting Marker 3 (Marker > Select M(1‑8) > M3) then using searching for the trace peak (Marker > Marker Search > Marker to Peak).
The Distance‑To‑Fault transmission line test verifies the performance of the transmission line assembly and its components and identifies the fault locations in the transmission line system. This test determines the return loss value of each connector pair, cable component and cable to identify the problem location. This test can be performed in the DTF‑Return Loss or DTF‑VSWR mode. Typically, for field applications, the DTF‑Return Loss mode is used. Figure: DTF Return Loss Measurement (Antenna at 15 m) shows the failure with the antenna still attached.
Failing DTF Return Loss Measurement (Load at 15 m)
The jumper connector at 1.5 m was found to be loose and dirty. After cleaning and tightening to specification, another DTF measurement showed that the connector now passed the carrier 20 dB specification, indicated by the limit line.
