How does the VSWR of a load affect the power handling capability of a transmission line?
Standing Waves and Power Limits
When a transmission line is terminated in a mismatched load, the reflected wave interferes with the forward wave, creating periodic voltage and current maxima along the line. At voltage maxima, the voltage is VSWR times the nominal (matched) voltage. At current maxima, the current is VSWR times the nominal current. These enhanced voltages and currents can exceed the component ratings even though the forward power is within specification.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Where do the maxima occur?
Voltage maxima occur at multiples of λ/2 from the load, starting at the load for an open circuit or λ/4 from the load for a short circuit. The exact positions depend on the load impedance phase angle. Current maxima are offset by λ/4 from voltage maxima.
Does cable loss help?
Cable loss attenuates the reflected wave, reducing the VSWR seen at the source end of the cable compared to the load end. This reduces the standing wave amplitude and the voltage/current maxima. However, the cable near the load still sees the full VSWR and is at higher risk.
What about flexible cables?
Flexible cables are more susceptible to standing wave damage because the inner conductor is often thinner and has higher resistance than semi-rigid or rigid coax. The localized heating at current maxima can damage the dielectric or cause the center conductor to shift, permanently degrading performance.