How to Troubleshoot Coaxial Cable Power

Larger coaxial cables have less power loss than smaller ones.

Coaxial cable carries high frequency electronic signals for telecommunications and computer applications. The power of a signal, the product of multiplying voltage times current (P=IE), is measured with a power meter and expressed in watts or decibels. To troubleshoot coaxial cable power, measure it at the input source and at the end of the cable, then compare the two measurements. The difference represents the power loss in the cable. Compare the results with the cable's specifications. Use this test only for industrial or professional cable applications, not home entertainment systems cables.

Things You'll Need

• Power meter
• Termination of the same impedance as the cable
• Specifications for the coaxial cable
• Calculator (optional)

Instructions

1. 

   Coaxial cables carry signals to antennas for hundreds of applications.

   Disconnect the coaxial cable from its power source and connect the output of the source to the power meter. Terminate the signal with a termination plug of the same impedance as your cable to ensure a proper reading. Record the signal level in dB, as read on the meter.

2. Disconnect the power meter and termination, then connect the coaxial cable to the power source.

3. Connect the power meter and termination to the end of the coaxial cable and measure the power. Record the reading. Subtract the second reading from the first to find the power loss of the cable.

4. Measure the cable and calculate its acceptable loss. Most coaxial cable specifications show acceptable loss in dB per 100 feet. If your cable is exactly 100 feet long, you won't need to make any calculations. For other lengths, divide the specification for the loss by 100, then multiply it by the length of your cable. For example, for a cable 47 feet long with a specification of 3 dB per 100 feet, divide 3 by 100 (0.03), then multiply by 47. Your cable loss should not exceed 1.41 dB. For a cable 187 feet long, multiply 0.03 by 187. Your power loss should not exceed 5.61 dB.

Tips & Warnings

• To test coaxial cables in home entertainment systems, simply replace them. Power reduction in these applications is not a common problem. More common problems are physical damage from corrosion, crushing, kinking or pets, or faulty or loose connectors. These problems cause complete signal loss or distortion, but not a reduction of power. Power troubleshooting tests are for longer cables such as LAN connections or transmitter to antenna connections.

• Do not use "bridged" measurements with the cable connected. Reflections in a defective cable could give erratic readings.

• For industrial cables, severe damage from crushing, corrosion or water will cause excessive distortion or reflections known as VSWR, but may not cause power loss. If you suspect bad coaxial cable, always troubleshoot for VSWR along with power to make a thorough test. Never assume that a coaxial cable is good only because it passes troubleshooting tests for power.

References

• Anritsu: Power Meter ML2437A
• RF Cafe: Coaxial Cable Specifications

Resources

• Jaybeam Wireless: Basestation Antenna - VSWR Test Procedure
• Barry McLarnon: VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters