You CAN use a scalar network analyzer to measure antennas.
But you have to understand the limitations:
1) Scalar network analyzers have a limited dynamic range....so if the antenna is far away the received signal might be too low to see it. You might have to add a calibrated gain amplifier between the receive antenna and the detector
2) Scalar analyzers receive ALL input signals and display them in unison. So if your antenna has a lot of 2nd harmonic gain, and your signal source is outputing a lot of 2nd harmonic, you will get some odd results. You COULD add an external lowpass filter to fix this