If measuring directly at the WE-2705P antenna analyzer test port then SWR accuracy is within +- 10% over the temperature range of 0 to +45 deg celsius, and over the frequency range of 1.5 to 2700 MHz. This level of accuracy is sufficient for most radio communication antenna testing applications, so calibration is not necessary. For accuracy within +- 5% (3>SWR>1.1) and 5.0 MHz < Frequency < 2700 MHz, then calibrate at the antenna analyzer test port using the optional terminations.
If measuring at the end of a short intermediate N male to N male cable, then the error introduced by the intermediate cable, mainly loss can be removed by calibrating at the end of the cable. To do this using the supplied terminations, connect a good quality N female to N female adapter to the end of the cable. Perform the calibration at the end of the adapter, and then remove the adapter. Errors will result due to non ideal impedance characteristics of the adapter, but these are usually small. The reference plane will also have shifted by the length of the adapter, but this is not an issue for Return Loss and SWR Measurements.
With the WE-2705P, the intermediate cable length should not exceed 1m.
See article, Antenna System Measurements With The WE-2705P, for more information on the effect of cable loss on antenna system measurements.