The electrical length changes with frequency for microstrip. Higher frequencies are electrically longer or they travel slower. Normal measurement of length is from a vector network analyzer where you calibrate S21, then hook up your cable and measure in time domain mode how long the effective pulse takes to get to the other port. Using the time domain feature of the analyzer can get you the amount of electrical delay in the microstrip. But if your antenna (or material being tested) is wide bandwidth, then the time it takes the energy to travel changes with frequency. Alternately, after you calibrate for S21 and attach your item there is a button on the display to add "electrical delay" length to the test result. Measuring phase you can set this electrical length value to make the phase read zero degrees and then just reading the value you entered into electrical delay. Look up "electrical delay" in the Agilent/HP manuals.