This section describes the various Cable and Antenna Analyzer measurements. Note that not all measurements are supported in all instruments. Some of the measurements are option dependent. Please refer to your product’s technical data sheet for the list of supported options.

From the main menu select MEASURE > MEASUREMENT menu to select one of the desired measurements.

 

This is a measurement made when the antenna is connected at the end of the transmission line. It provides an analysis of how the various components of the system are interacting.

Return Loss is used to characterize RF components and systems. The Return Loss indicates how well the system is matched by taking the ratio of the reflected signal to the incident signal, and measuring the reflected power in dB.

This is a measurement made when the antenna is connected at the end of the transmission line. It provides an analysis of how the various components of the system are interacting.

VSWR is a ratio of voltage peaks to voltage valleys and allows you to view the impedance match. This measurement is similar to return loss, but is the ratio of voltage peaks to voltage valleys caused by reflections.

To make return loss or VSWR measurements:

The cable loss measurement verifies the signal attenuation level of a cable. Cable loss sweep is made when a short is connected at the end of the transmission line. This insertion loss test allows analysis of the signal loss through the transmission line and identifies problems in the system. High insertion loss in the feed line or jumpers can contribute to poor system performance and loss of coverage.

Different transmission lines have different losses, depending on frequency and distance. The higher the frequency or longer the distance, the greater the loss. This measurement returns the energy absorbed, or lost, by the transmission line in dB/meter or dB/ft. The average cable loss of the frequency range is displayed on the trace card. Figure: Cable Loss Measurement is a cable loss measurement example.

To make a cable loss measurement:

Distance-to-Fault (DTF) measurements are made with the antenna disconnected and replaced with a 50 Ω precision load at the end of the transmission line. This measurement allows analysis of the various components of the transmission feed line system in the DTF mode.

This measurement 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 the cable or moisture intrusion.

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 will reflect most of the RF energy from the RF test port and the true value of a connector might be misinterpreted or a good connector may 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 since the antenna is typically only designed to have 15 dB or better return loss in the passband of the antenna.

DTF measurement is a frequency domain measurement and the data is 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.

Because of the nature of the measurement, maximum distance range and distance resolution are dependent upon the frequency range and number of data points. DTF Aid (FREQ/DIST > DISTANCE > DTF AID) shown in Figure: DTF Aid explains how the parameters are related.

The maximum usable distance may be extended by either increasing the number of sweep data points or by reducing the current frequency span. Distance resolution is inversely proportional to the frequency span and may be increased by setting a wider frequency span. Windowing can also affect distance resolution. For the best distance resolution, select Rectangular Windowing.

Selecting the cable type is critical for accurate DTF measurements. Incorrect propagation velocity values (PV) affect the distance accuracy, and inaccurate cable attenuation values affect the accuracy of the amplitude values. The Cable and Antenna Analyzer is equipped with a cable list (FREQ/DIST > DTF SETUP > CABLE LIST) including most of the common cables currently used. Once the correct cable has been selected, the instrument will update the propagation velocity and the cable attenuation values to correspond with the cable. For setups with several different cable types, choose the main feeder cable.

For cables not on the list, select NONE and manually enter the propagation velocity (PROP VEL) and cable loss in DTF AID or the DTF SETUP menu.

The name, propagation velocity, and cable loss of the selected cable are displayed in the status panel’s trace card during distance measurements (MEASURE > MEASUREMENT > DTF Return Loss or DTF VSWR) as shown in Figure: Cable List Selection.

The cable list window has the following options:

 

To add a custom user cable to the user cable list follow the instructions below:

 

 

 

Figure: User Cable List - Custom Cable List is an example of the custom user cable.

 

Distance resolution is the Field Master Series’s ability to separate two closely spaced discontinuities. If the resolution is 5 meters and there are two faults 3 meters apart, the Field Master Series will not be able to show both faults until the resolution is improved by widening the frequency span.

with Rectangular Windowing applied.

Figure: DTF Measurements at 100 MHz vs. 500 MHz (Field Master Series) is an example of the same DTF measurement with a 100 MHz span vs. a 500 MHz span. The increased span provides additional detail that there may be several unique issues with the first 10 meters of the cable. This detail was not available in the narrower span.

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 end of the frequency sweep. The main lobe widens as the side lobes are reduced, thus 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 close to each other, then spectral resolution is important. In this case, use Rectangular Windowing for the sharpest main lobe (the best resolution).

To select the windowing type, follow this key sequence:

The maximum horizontal distance that can be analyzed is displayed in the DTF Aid Info dialog, of which the stop distance cannot exceed. If the cable is longer than this max distance, the setup needs to be improved by increasing the number of data points or lowering the frequency span. Note that the data points can be set to 130, 259, 517, 1033, or 2065 (SWEEP > DATA POINTS).

To make DTF Return Loss or DTF VSWR measurements:

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. Provided you have selected the appropriate cable type from the Cable List, the peak indicating the end of the cable should be 0 dB or a value close to zero. In Figure: DTF Return Loss with a Short at the End of the Cable, the cable end is at 20.5 meters.

The cable end was found by selecting Marker 1, then searching for the trace peak.
(MARKER > MARKER SEARCH > 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: Failing DTF Return Loss Measurement (Antenna) shows the failure with the antenna still attached.

To perform this test, disconnect the antenna and connect the load at the end of the transmission line (Figure: Failing DTF Return Loss Measurement (Load)).

The jumper connector at the antenna end of the cable 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 (Figure: Passing DTF Return Loss Measurement (Load)).

Figure: DTF Return Loss Measurement (Antenna) shows the same system with the antenna re-attached. The reflection of the jumper connector is now reduced to 22 dB.

The Smith Chart is a graphical tool for plotting impedance data versus frequency. It converts the measured reflection coefficient data into complex impedance data and displays it in a manner that makes the Smith Chart a useful tool for determining and tuning input match. Markers can be used to read the real and imaginary parts of the complex impedance. See the example in Figure: Smith Chart Measurement. This impedance plot reveals which matching elements (capacitance, inductance) are necessary to match a device under test to the reference impedance (50 ohms).

To make a smith chart measurement:

 

The Cable and Antenna Analyzer can display the phase of the reflection measurements at the RF port. The Phase display range is from –450 degrees to +450 degrees. The 1-port phase measurement is most useful when making relative measurements (comparing the phase of one device to the phase of another) by utilizing the Trace Math function (Trace – Memory).

 

The Transmission measurement is used to measure the loss (or gain) in dB of a device. This measurement requires that one port of the device be connected directly (or through a test port cable) to the RF port of the S331P. The second port of the device connects to an external USB sensor. To make more accurate cable loss measurements, especially for cables with more than 10 dB of loss, you can use the transmission measurement with an external sensor. For this measurement, connect the cable under test to the RF port of the S331P and connect a USB power sensor to the other end of the cable. USB extenders can be used for long cable runs.

This measurement gives accurate results of cable loss up to 30 dB. This is a scalar measurement, providing only magnitude data (no phase) and, therefore, does not use vector error correction for its calibration steps. Instead, it uses a sensor reference calibration. Figure: External Sensor Transmission Measurement is a cable loss transmission measurement example with an external power sensor, which can be compared to the example shown in Figure: Cable Loss Measurement.

For best results when performing both transmission and return loss measurements on the same cable, the return loss should be measured with a good-quality termination at the end of the cable.

Table: Cable and Antenna Measurement Overview presents a summary of the typical measurements and required cable end tool. The typical values are for general information purposes. The carriers will provide final values in the acceptance testing specification.

To set up a, external USB sensor transmission measurement:

The 2-port transmission measurement is used to verify the performance of tower-mounted amplifiers, and duplexers, and to verify antenna isolation between two sectors. The excellent dynamic range makes it suitable for repeaters as well. The second port is a selective receiver which provides up to 100 dB dynamic range which makes it possible to test the band pass filters common on many networks.

Both high and low power settings are available. The high-power setting delivers approximately 0 dBm power at the RF out port, ideal for antenna isolation measurements and duplexers. When making measurements of a Tower Mounted Amplifier, Anritsu recommends to use the low-power setting (approximately –40 dBm). This will ensure that the RF In port is not over-powered and that the measurements are made in the linear region of the amplifier. This measurement is only available on Site Master instruments.

To set up a, 2-port transmission measurement:

Time Domain Reflectometry (TDR) is a technique that measures and displays the impedance of a network (cable, filter etc) over time.

A TDR measurement shows impedance against distance, with a normal 50 ohm line running across the center of the display. Different causes of reflections such as open circuits, short circuits, kinks to the outer cable conductor and water ingress will cause characteristic changes to the transmission line impedance. This in turn helps identify the cause of the fault and accelerates the repair process.

TDR Ohm measurement involves using TDR to analyze the impedance or characteristic impedance of a transmission line or cable. TDR instruments provide valuable information about impedance mismatches, cable faults, and variations in characteristic impedance.

The TDR Linear measurement is a linear representation of the return loss (or gain) in dB of a device in time domain. The time domain transform of the underlying s-parameter is integrated into a step response (and presumably plotted on a linear scale).

To set up TDR measurements: