Calculate Antenna Q from VSWR bandwidth measurement

This calculator allows calculation of the antenna Q from VSWR bandwidth for matched antennas that are well represented as a simple series resonant circuit with constant R near resonance.

The calculator is usable even if R varies a little, the error will be quite small if dR/df<<dX/df.

The calculator will not give accurate results for antennas with multiple resonances spread over nearby frequencies, eg some types of Yagi antennas.

If Min VSWR is greater than 1, it matters whether R is less than or greater than R0, select as appropriate.

The calculator does not do a lot of error checking, if you enter nonsense, it will produce nonsense.

The technique for calculating bandwidth from VSWR other than 2.618 is an approximation that is valid for loads where X changes much faster than R with frequency about minimum VSWR.

VSWR should be significantly larger than Min VSWR. For best accuracy, min VSWR should be near to 1, and VSWR at which the bandwidth is measured should be near to 2.6 (which is the half power bandwidth for a matched antenna).

References / Links

Antennas and Q

Changes

Version	Date	Description
1.01	27/05/2014	Initial.
1.02	21/04/2016	Calculation from min VSWR added.
1.03
1.04
1.05

Inputs:

Frequency for min VSWR (<1.5) (MHz)
Min VSWR
VSWR
Bandwidth @ VSWR (kHz)

Results:
Half power bandwidth (kHz)
Q