What is VSWR, and why is it important for my AIS transponder?

What is VSWR (Voltage Standing Wave Ratio)?

AIS transponders are highly reliable devices that can operate efficiently for many years, continuously transmitting position data and receiving AIS signals from vessels within VHF range.

For an AIS transponder to function correctly, three key components must be in optimal condition. A simple way to remember them is the VGA rule:

• V = Voltage – The transponder requires a stable 12V DC power supply.
• G = GPS – The transponder must receive a good GPS position fix.
• A = Antenna – The AIS/VHF antenna must be in good condition and properly matched to the transponder.

Both voltage and GPS reception can be easily checked using a multimeter or the proAIS2 software, which provides real-time diagnostic readings. However, checking the AIS/VHF antenna is more complex and often leads to the most technical support queries.

Why Is VSWR Important?

A well-functioning AIS transponder depends on an efficient antenna system to ensure clear signal transmission and reception. However, over time, antennas can degrade due to harsh marine conditions, including:

• Saltwater and UV exposure
• Temperature fluctuations
• Vibration and mechanical stress
• Water ingress into the cable or connectors

In many cases, yacht owners install an AIS transponder with an antenna splitter, only to discover that their old mast-top VHF antenna is not performing as expected. While a VHF radio may still receive signals with a weak or degraded antenna, AIS transmission is far more sensitive to antenna quality.

One of the key measurements that determine antenna performance is the Voltage Standing Wave Ratio (VSWR).

What Is VSWR?

VSWR is a measure of how efficiently power is transmitted from the transponder (source) through the antenna cable (feeder) to the antenna (load).

• Ideally, all transmitted power should be radiated by the antenna into the air.
• In reality, some power is reflected back down the cable due to mismatched impedance.

Impedance is the AC resistance in an electrical circuit, and for VHF systems, it should be 50 ohms throughout the setup. When the impedance of the transponder, cable, and antenna do not match, the reflected signal increases, reducing transmission efficiency.

Understanding VSWR Ratios

A perfect system has a VSWR of 1:1, meaning all power is radiated by the antenna, and no energy is reflected back. However, in real-world conditions:

• A VSWR between 1.1:1 and 2.5:1 is considered normal for most pleasure boat installations.
• A VSWR above 5:1 indicates a serious problem with the antenna system, causing poor transmission performance.
• If the VSWR is too high, the transponder’s red status LED will illuminate, and a VSWR alarm will be displayed in proAIS2 software.

How to Check and Improve VSWR

1. Monitor VSWR Using proAIS2

• The diagnostic page in proAIS2 software displays real-time VSWR readings.
• The Transmit Counter allows you to track when transmissions occur and observe changes in VSWR values.
• Slight fluctuations (within decimal places) are normal, but large changes suggest poor connections or water ingress.

2. Inspect the Antenna and Cabling

• Check for corrosion, water damage, or loose connections.
• Ensure the antenna is properly mounted and not obstructed.
• If using a splitter, confirm it is AIS-compatible and functioning correctly.

3. Replace or Adjust the Antenna System

• Consider a lower-loss coaxial cable to reduce signal degradation.
• If possible, use a dedicated AIS antenna instead of a shared VHF antenna.
• Ensure the antenna has an impedance of 50 ohms for proper matching.

Conclusion

Maintaining a low VSWR is essential for optimal AIS transmission range. While AIS receivers may still function with a poor antenna, AIS transponders rely on a strong outgoing signal to ensure they are detected by other vessels. By regularly checking VSWR levels, inspecting the antenna, and ensuring proper installation, users can achieve reliable AIS performance and enhanced safety at sea.