Bandwidth Measurement

I would like to know how to measure an antenna bandwidth? If I have a 915MHz dipole antenna, how should I measure the bandwidth of this antenna by using a VNA?

There could be VSWR bandwidth, or gain bandwidth.

VSWR bandwidth could be measured using the VNA and generally indicates the frequency range between margins where VSWR of the antenna is better than 2:1 (or lower than -10dB Return Loss).

For gain bandwidth need to use a radiated setup for antenna gain using the VNA and a reference antenna (usually wideband). The gain bandwidth is given by the frequency range between margins of your target gain.

Alternatively, you can use two identical antennas in gain measurement setup and get the doubled gain variation.

By the way, a standard λ/2 dipole can be expected to have a well known gain vs frequency characteristic. It shouldn't be
necessa ry to perform a measurement, usually.