Even when using a purely resistive dummy load with no reflected voltage, you may notice that the VSWR meter in a FlexRadio SDR never reads a perfect 1.0:1. This is because the radio is not measuring reflected RF voltage the way many external SWR meters do.
FlexRadio transceivers measure RF return loss—the amount of transmitted RF power that is reflected toward the source instead of being absorbed or radiated by the load. Because the radio measures returned power rather than returned voltage, SmartSDR must compute VSWR from that power-based return-loss measurement.
Any VSWR value derived from return-loss calculations will inherently be less precise than one obtained by directly measuring the forward and reflected voltages. As a result, SmartSDR will never display a perfect 1.0:1 VSWR because that would correspond to infinite return loss, which is not mathematically achievable.
In practical terms, this means the VSWR shown in SmartSDR may not perfectly match readings from other meters in your antenna system, especially those that rely on directional couplers to measure RF voltage directly.
Does This VSWR Difference Affect Radio Operation?
No, it does not.
The small discrepancy does not affect performance. FlexRadio transmitters look primarily at return loss to determine whether the load is acceptable. Only when the return loss indicates a load so mismatched that the equivalent VSWR exceeds the radio’s protection threshold (typically around 3.0:1) will the radio begin reducing output power to protect the PA.
When Should You Be Concerned?
You only need to investigate further if SmartSDR’s VSWR reading differs significantly from the first SWR meter located immediately downstream in the RF path. A large mismatch between those two readings indicates a real problem in the cabling, connectors, or internal directional coupler—not a normal measurement-method difference.