Standing Wave Ratio Bridge
Understanding SWR Bridges
SWR bridges provide a simple, standalone method for measuring impedance match quality. While VNAs have largely replaced them for precision work, SWR bridges remain useful for production testing and field measurements.
SWR Bridge Operation
- Apply RF signal to the input.
- Signal splits between reference arm (precision 50-ohm load) and measurement arm (DUT).
- Detector measures the imbalance between the two arms.
- Perfect match (DUT = 50 ohms): detector reads zero (infinite return loss).
- Mismatch: detector reads proportional to reflection coefficient.
Applications
- Antenna tuning (adjust antenna for minimum SWR at desired frequency).
- Cable fault location (TDR mode).
- Field service testing of installed antenna systems.
Frequently Asked Questions
What is an SWR bridge?
An SWR bridge measures VSWR by comparing forward and reflected signals in a bridge circuit. Balanced (zero detector reading) means perfect match. Used for antenna tuning and field testing. Largely replaced by VNAs for lab work.
How accurate is an SWR bridge?
Standalone SWR bridges: +/- 5-10% accuracy for VSWR. VNAs with full calibration: +/- 0.1% accuracy. SWR bridges are adequate for field work and tuning but not for precision measurements.
When would I use an SWR bridge instead of a VNA?
Field work (lighter, battery-powered), quick antenna tuning (real-time display), production testing (simpler operation), and budget constraints. VNA is preferred whenever accuracy and phase information are needed.