The nanovna is a great tool to measure this; really what it can measure is
the loss in your feed cable and the fraction of reflected power from the
antenna feedpoint.
An easy way to measure the cable loss is to disconnect the cable at the
antenna feedpoint, and from the shack end connect the nano and use the
Measure -> Cable function. You read the cable loss directly on the screen
(no need to do an S12 measurement). The frequency range needs to be set so
the smith-chart trace goes at least one-half circle; if it spirals in many
circles, it is best to reduce the upper frequency so there are only a
couple of circles to improve accuracy.
And as Steve noted, do an SWR measurement at the desired frequency (or to
directly read dB, use the Log Mag return-loss display).
With those measurements you can easily calculate the loss in power getting
to your feedpoint, and the amount being reflected back, to give an estimate
of the effective power at the antenna feedpoint.
