Can A HP8756A Network Analyzer Measure The Resonant Frequency Of A GPS Chip Antenna?
Yes, you will need an external RF source of some sort (ideally a sweeper) and a directional detector (e.g. http://www.ebay.com/itm/HP-Agilent-85021C-Directional-Bridge-10MHz-18GHz-/301374539011) or directional coupler and detector (e.g. http://www.ebay.com/itm/HP-11664A-18Ghz-Detector-for-HP-8756-HP-8757-/380853230821).  These units have no RF electronics in them at all; they are basically glorified power meters - you need to supply the source and the detectors.  I have not used one of these myself, but I think the way it's intended to be used is you put a splitter on the source so part of the signal is sent to a detector for the R (reference) port, then you can put a directional coupler and another detector between the other port on the splitter and your DUT port to measure the reflected power.  Other detectors can then be used for transmitted power out of other DUT ports.  

Some of the older VNAs need an external source as well, but it gets sent in through the back of the test set instead of the front.