The key is the SWR bridge or directional coupler.
If your SA has a tracking gen then this is much easier - Mine doesn't so what I did before I go the network analyzer was:
1. Attach signal generator to the RF Input port
2. Attach antenna to the DUT port
3. Attach SA to the SWR Output Port
4. Set SA to trace hold max trace A
5. Sweep generator across 0 - 1GHz range
The resultant plot should show where the antenna reflects the least energy and these will be the points where it has the best load matching - It should look like this:
Pleased note that this was setup quick and dirty (you'd expand range, align amplitude etc) and I took a photo because I haven't yet reinstalled the 7440 utilities due to some forced NVIDIA fun time...
TonyG