Difference between revisions of "Q Factor"

Revision as of 13:49, 26 August 2015

The quality factor, also known as the Q factor, Q value, or just Q, is a dimensionless parameter that characterizes the energy loss of a resonating cavity with respect to the energy stored in the cavity. See the Wikipedia entry for a more general and detailed description.

There are multiple, incompatible ways to measure this parameter, and there has been considerable debate on the NSF forums as to which is most appropriate for the EM Drive. Experimenters have so far reported the unloaded Q factor. However, the loaded Q factor is generally considered the standard technique for measuring the Q factor of closed resonant cavities outside of the EM Drive research community.Template:Citation needed This difference may explain why the published Q factors of EM Drives are much higher than what is considered possible outside of theory without using advanced techniques such as superconducting cavities.

There is no standard notation to distinguish between the method used. We use Q_l to explicitly denote the loaded Q factor and Q_u to denote the unloaded Q factor.

Loaded Q factor (Q_l)

This is typically used for resonant cavities.

Method

Measure the S12 or S21 2-port return loss. See http://docs.lumerical.com/en/diffractive_optics_cavity_q_calculation.html and http://www-elsa.physik.uni-bonn.de/Lehrveranstaltungen/FP-E106/E106-Erlaeuterungen.pdf (German version).

Unloaded Q factor (Q_u)

This is typically used for antennas.

Method

Measure the S11 return loss at the 3dB down bandwidth.