Two-Port vs. One-Port Handheld Insertion Loss Measurements
2019-04-02 | 7 min read
Verifying and troubleshooting electrical performance of RF and microwave transmission systems and antennas starts with accurate cable and antenna measurements. Installation, operation, and maintenance of communications systems cause wear and tear on connecting cables, adapters, and antennas over time. This degradation can result in poor cellular signal, reduced coverage area, and unnecessary cellular handovers. Before testing system components, it is important to verify the performance of the cables and antennas so you can better troubleshoot system level issues.
In this blog, I cover different techniques and configurations to help you make cable and antenna test (CAT) quick and easy.
The insertion loss of a transmission line or coaxial cable is the amount of energy dissipated in the cable. Insertion loss is also defined as the energy lost due to mismatch reflection between the source and the load. Ideally the source (transmitter), transmission line (coaxial cable), and the load (antenna) are all designed for the same characteristic impedance, usually 50 or 75 Ohms. You can also represent insertion loss by the S-parameters S21, or S12 in a 2-port network. The ratio between the input and output signals, from one end of the cable to another, or from one port to the other, represents the total insertion loss of the cable. Ideally, lossless cable will have an insertion loss of 0. If you’d like to learn more about insertion loss and s-parameters, check out this previous blog.
Cable Test Basics
When a cable is installed in a system, it is often difficult to physically connect test equipment to both ends of a very long cable. Adding very long cables to the test set up is impractical. Under these conditions, you may need to perform a one-port insertion loss measurement. FIeldFox analyzers can operate in Cable and Antenna Test (CAT mode), where you can perform traditional two port insertion loss measurements, or in a one-port measurement mode, where insertion loss is measured from only one end of the cable.
A traditional two-port set-up requires you to connect the cable under test to two separate ports on the FieldFox, as seen in the image below:
The FieldFox injects a signal into the cable from the RF OUT port, conveniently positioned along the top of the analyzer. As the signal passes through the cable, the cable absorbs a small portion of the energy, typically due to resistive and dielectric losses in the cable. Discontinuities from cable connectors, including bends or other damage, will also reflect some energy back into the source. This results in an increase in the measured insertion loss. The remain signal coming out of the cable is then measured by FieldFox at the RF IN port.
When testing a very long cable, it can be difficult to access both ends of that cable under test. In situations like this, you can execute the test using the one port measurement technique in CAT mode on the FieldFox. This allows you to measure cable insertion loss from only one end of the cable under test, eliminating the need to carry extra-long, high-quality field test cables. In this configuration, the FieldFox injects a signal into the cable from the RF OUT port, just like the 2-port measurement technique. The test signal passes through the cable, reflects at the open end, and passes bacj through the cable a second time, where the Fieldfox measures the result on the RF OUT port. Once the reflected measurement is complete, the FieldFox uses a built-in model for coaxial cable dispersion to compute the cable insertion loss as a function of frequency.
Another configuration for measuring cable insertion loss that does not require a return path cable involves setting up the test with a power meter connected to a power sensor. One end of the cable under test is directly connected to the RF OUT port of FieldFox. The other end of the cable is connected to the USB power sensor (Keysight U2000 Series USB power sensor). The FieldFox generates a CW signal at the RF OUT port, which is transmitted along the cable, and measured by the USB power sensor. The USB sensor can be connected back to FieldFox through a USB cable extender. With this configuration you do not need to run an expensive coaxial cable from both ends of the instrument. As with any test set-up, there are pros and cons; the main constraint with this configuration is that swept frequency measurements are not available, so changing test frequencies must be done manually.
With these techniques, you can easily perform insertion loss measurements on any cable, anywhere – rain or shine. For more information on making handheld insertion loss measurements, check out the app note, “Techniques for Precise Cable and Antenna Measurements in the Field.”