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Insertion Loss Measurement Methods

 

Site Master

 

Introduction

Transmission feed line system performance plays an important role in wireless network coverage. Insertion loss measurement is one of the critical measurements used to analyze transmission feed line installation and performance quality. This application note explains how Site Master is used to measure cable insertion loss with different test methods and how to predict the maximum allowable cable insertion loss through manual ­calculations.

In wireless communication systems, the transmit and receive antennas are connected to the radio through coaxial cable and/or waveguide transmission lines (Figure 1).

Insertion loss measures the energy absorbed by the transmission line in the direction of the signal path in dB/meter or dB/feet. Transmission line losses are dependent on cable type, operating frequency and the length of the cable run. Insertion loss of a cable varies with frequency; the higher the frequency, the greater the loss.

Insertion loss measurements help troubleshoot the network by verifying the cable installation and cable performance. High insertion loss in the feedline or jumpers can contribute to poor system performance and loss of coverage. Measuring insertion loss using Site Master assures accurate and repeatable measurements.

1.1 Problematic Cables, Connectors and Antennas are Primary Sources of Cell Site Problems

More than sixty percent of a typical cell site’s problems are caused by problematic cables, connectors, and antennas. Faulty components are a primary source of overall antenna degradation and can result in dropped calls. This puts pressure on the field technician who must fix the problem while minimizing any delay in system operation. Quickly troubleshooting and identifying these problems offers Service Providers a way to improve quality, while reducing cost – an important differentiating factor for any company working in today’s highly competitive wireless market. However, locating and fixing these problems, especially without the right equipment, can be a tricky and time-consuming proposition. Traditionally, it has been easier to simply replace the suspected cable or antenna system component.

This task is further complicated by the need to continually perform instrumentation calibration, using such techniques as the Open-Short-Load (OSL) calibration method, every time a change occurs in either start frequency and/or stop frequency – an action which would invalidate the calibration. Despite the difficulty, calibration is necessary in order to maintain the integrity of any measurement.

2.0 Identifying Faulty Cables and Antenna System Components

The Anritsu Cell Master and Site Master FlexCal™ broadband calibration feature is an OSL-based calibration method. It offers field technicians a simple and convenient way to troubleshoot and identify faulty antenna system components because it eliminates the need for multiple instrument calibrations and calibration setups. Field technicians can now perform a broadband calibration and change the frequency range after calibration without having to recalibrate the instrument. A zoom in/zoom out capability is available in Return Loss, Cable Loss or VSWR mode. Because the resolution and maximum distance are dependent on the frequency range, field technicians - in DTF mode - can even change the frequency range to produce the desired fault resolution and horizontal range needed for the measurement without performing additional calibrations.

The Site Master has a measurement range of 25 to 4000 MHz. When FlexCal is selected, Site Master is calibrated from 25 to 4000 MHz. FlexCal has the same accuracy as the OSL calibration method for this frequency range. For frequency ranges other than 25 to 4000 MHz, measurement uncertainty may degrade slightly due to calibration data interpolation. Therefore, it is recommended that you perform another OSL calibration to obtain the maximum possible measurement accuracy when the data is to be used for archiving and reporting.

Figure 1

Measurement Methods

During network deployment, maintenance, and trouble shooting phases, insertion loss can be measured by disconnecting the antenna and connecting an enclosed short at the end of the ­transmission line. If a Tower Mounted Amplifier (TMA) is used in the transmission feed line system, it is best to remove the TMA and antenna from the system configuration to perform an insertion loss measurement. It is best to always disconnect the cable at the same location so the measured data can be compared to the historical data for accuracy and repeatability.

Using Site Master, cable insertion loss can be measured in CABLE LOSS or RETURN LOSS mode. In Cable Loss mode, Site Master automatically considers the signal traveling in both ­directions thus making the measurement easier for the user in the field.

The following section explains the procedure to measure insertion loss in cable loss mode and return loss mode. The measurement setup and equipment required is the same for both modes.

Figure 2

Measuring Insertion Loss Using CABLE 

Required Equipment

  • Site Master Model S11xx, S33xx, or S251x
  • Precision Open/Short, Anritsu 22N50 or Precision Open/Short/Load, Anritsu OSLN50LF
  • Precision Load, Anritsu SM/PL
  • Test Port Extension Cable, Anritsu 15NNF50-1.5C
  • Optional 510-90 Adapter, DC to 7.5 GHz, 50 Ω, 7/16(f)-N(m)


Insertion Loss Measurement Setup

The insertion loss measurement set up for a typical transmission feed line ­system is shown in Figure 2. Remove the antenna and connect an enclosed precision "short" at the end of the ­transmission line.

If a Tower Mounted Amplifier (TMA) is in the transmission feed line system, remove the TMA and antenna and ­connect an enclosed short at the end of the transmission line. Insertion loss measurement for a transmission feed line system with a tower mounted ­amplifier is shown in Figure 3.

Figure 2
Figure 3

Procedure

  • Step 1.Power On the Site Master and press the MODE key.
  • Step 2.Select FREQ-CABLE LOSS using the Up/Down arrow key and press ENTER.
  • Step 3.Set the start and stop frequencies F1 and F2. As an example, F1=750 MHz and F2=850 MHz for a typical cellular frequency band.
  • Step 4.Connect the phase stable test port extension cable to the RF port.
  • Step 5.Calibrate the Site Master at the end of the phase stable test port extension cable. (See the ­section entitled "Calibrating Site Master with the Phase Stable Test Port Extension Cable" for details.)
  • Step 6.Disconnect the antenna and connect an enclosed precision "short" at the end of the ­transmission line.
  • Step 7.Connect the other end of the transmission line to the phase stable cable of the Site Master. A trace will be displayed on the screen when the Site Master is in continuous sweep mode.
  • Step 8.Press the AMPLITUDE key and set the TOP and BOTTOM ­values of the display. In Figure 4, the TOP is set to 2 dB, and the BOTTOM is set to 5 dB.
  • Step 9.Press the MARKER key.
  • Step 10.Set M1 to MARKER TO PEAK.
  • Step 11.Set M2 to MARKER TO VALLEY.
  • Step 12.Calculate the insertion loss by averaging M1 (MARKER TO PEAK) and M2 (MARKER TO VALLEY) values as follows:

  • Step 13.Press SAVE DISPLAY and name the trace using the soft keys and press ENTER. Saving the display is recommended for historical documentation. Saved traces can be used in the future to compare, check or verify changes in the transmission feedline performance.
Figure 2
Figure 3

Measuring Insertion Loss using RETURN LOSS MODE.

  • Step 1.Power on the Site Master and press the MODE key.
  • Step 2.Select the FREQ-RETURN LOSS using the Up/Down arrow key and press ENTER.
  • Step 3.Set the start and stop frequencies F1 and F2. As an example, F1=750 MHz and F2=850 MHz for a typical cellular frequency band.
  • Step 4.Connect the phase stable test port extension cable to the RF port.
  • Step 5.Calibrate the Site Master at the end of the phase stable test port extension cable. (See the section entitled "Calibrating Site Master with the Phase Stable Test Port Extension Cable" for details.)
  • Step 6.Disconnect the antenna and connect an enclosed precision "short" at the end of the transmission line.
  • Step 7.Connect the other end of the transmission line to the phase stable cable of the Site Master. A trace will be displayed on the screen when the Site Master is in continuous sweep mode.
  • Step 8.Press the AMPLITUDE key and set the TOP and BOTTOM values of the display. In Figure 5, the TOP is set to 4 dB, and the BOTTOM is set to 10 dB.
  • Step 9.Press the MARKER key.
  • Step 10.Set M1 to MARKER TO PEAK.
  • Step 11.Set M2 to MARKER TO VALLEY.
  • Step 12.Calculate the insertion loss by averaging M1 (MARKER TO PEAK) and M2 (MARKER TO VALLEY) values and dividing by two as follows:

  • Step 13.Press SAVE DISPLAY and name the trace using the soft keys and press ENTER.

Calibrating Site Master with the Phase Stable Test Port Extension Cable

The phase stable test port extension cable is used as an extension cable to the test port of the Site Master, and ensures accurate and repeatable ­measurements. The phase stable cable can be moved and bent while making a measurement without causing errors in the measurement. When poor quality cables are used as an extension test port cable, large error will be introduced in the measurements when the cable is moved.

For accurate results, Site Master should be ­calibrated at the ambient temperature before making any ­measurements. The Site Master must be re-calibrated whenever the setup ­frequency changes, the temperature exceeds the calibration temperature window or when the test port extension cable is removed or replaced.

Connect the phase stable test port ­extension cable to the Site Master RF Port. The Site Master with phase stable cable combination can be calibrated manually using Open, Short and Load (OSL) precision components (Figure 6), or by using the InstaCal® module. Manual calibration is explained here. For the InstaCal procedure, refer to the Site Master user guide.

Note: The InstaCal module is not compatible with the Site Master S251C Model.

Figure

Manual Calibration Procedure

  • Step 1.Power on the Site Master.
  • Step 2.Select the appropriate ­frequency range.
  • Step 3.Connect the phase stable test port extension cable to the RF port.
  • Step 4.Press the START CAL key. The message "Connect Open to RF OUT port or connect InstaCal module and press ENTER" will appear in the display.
  • Step 5.Connect the OPEN precision calibration component to the end of the test port extension cable. Press the ENTER Key.
  • Step 6.The message “Measuring OPEN” will appear, and after the measurement "Connect SHORT to RF OUT" will appear.
  • Step 7.Remove the "open" and connect the "short" precision calibration component to the test port extension cable. Press the ENTER key.
  • Step 8.The message "Measuring SHORT" will appear, and after the measurement "Connect TERMINATION to RF OUT" will appear.
  • Step 9.Remove the "short" and connect the "precision termination" at the end of the test port extension cable. Press the ENTER key.
  • Step 10.The message “Measuring ­TERMINATION” will appear. After the measurement, the "CAL OFF" message will change to "CAL ON" on the upper left-hand corner of the display.
  • Step 11.Remove the precision ­termination from the text port extension cable.

Calculating Transmission Line Insertion Loss

Cables have different insertion losses at different frequencies. For example LDF4-40A attenuation at 1 GHz is 0.022 dB/ft (0.073 dB/m) and at 2 GHz it is 0.0325 dB/ft (0.107 dB/m). As the frequency increases or the length of the cable run increases, the amount of cable insertion loss increases.

To verify cable insertion loss measurements are reasonable, the expected insertion loss can be ­calculated manually using the ­following procedure:

  • Calculate the estimated worst loss of each component in the ­transmission line system.
  • Add all the component’s estimated worst losses together to ­calculate total insertion loss in the transmission line system.


For example:

  Cable Type Cable
Attenuation
(dB/ft)
  Cable
Length
  Insertion Loss
(dB)
Bottom Jumper LDF4-50A 0.0204 x 20 = 0.408
Main Cable LDF5-50A 0.0115 x 200 = 2.30
Top Jumper LDF4-50A 0.0204 x 10 = 0.204

Number of Connector Pairs   Loss per Pair in dB   Total Connctor Loss (dB)
4 x 0.28 = 1.12


Compare the measured insertion loss to the calculated insertion loss to verify transmission line performance. The measured cable insertion loss should be lower than the calculated cable insertion loss.

Summary

The preferred method to measure Cable Insertion Loss using Site Master is Cable Loss mode. Cable loss mode automatically considers the signal traveling in both directions and thus makes it easier to measure the cable insertion loss in the field. Measured Insertion Loss should always be compared to the calculated loss to verify accuracy thus assuring transmission line performance. The calculated insertion loss is usually a "worst case scenario".

Cable insertion loss may be difficult to measure on excessively long or highly lossy cables. When the cable insertion loss is greater than 20 dB, it will be hard to measure.

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