Chapter 4 Measurements
The performance of a transmission feed line system may be affected by excessive signal re- flection and cable loss. Signal reflection occurs when the RF signal reflects back due to an impedance mismatch or change in impedance caused by excessive kinking or bending of the transmission line. Cable loss is caused by attenuation of the signal as it passes through the transmission line and connectors.
To verify the performance of the transmission feed line system and analyze these problems, three types of line sweeps are required:
Return Loss MeasurementMeasures the reflected power of the system in decibels (dB). This measurement can also be taken in the Voltage Standing Wave Ratio (VSWR) mode, which is the ratio of the transmitted power to the reflected power. However, the return loss measurement is typically used for most field applications.
Insertion Loss MeasurementMeasures the energy absorbed, or lost, by the transmission line in dB/ft or dB/meter. Different transmission lines have different losses, and the loss is frequency and distance specific. The higher the frequency or longer the distance, the greater the loss.
The different measurements are defined as:
Return Loss - System SweepA measurement made when the antenna is connected at the end of the transmission line. This measurement provides an analysis of how the various components of the system are interacting and provides an aggregate return loss of the entire system.
Distance To Fault - Load SweepA measurement made with the antenna disconnected and replaced with a 50 precision load at the end of the transmission line. This measure- ment allows analysis of the various components of the transmission feed line system in the DTF mode.
Cable Loss - Insertion Loss SweepA measurement made when a short is connected at the end of the transmission line. This condition allows analysis of the signal loss through the transmission line and identifies the problems in the system. High insertion loss in the feed line or jumpers can contribute to poor system performance and loss of coverage.
This whole process of measurements and testing the transmission line system is called Line Sweeping.
Information Required for a Line Sweep
The following information must be determined before attempting a line sweep measure- ment:
System Frequency Range, to set the sweep frequency
Cable Type, to set the cable characteristics for DTF measurements
Distance of the Cable Run, to set the distance for DTF measurements
