Measure it and calculate the frequency to which the signal generator must be set. When using the Slotted Line connect a 3dB attenuator between the signal generator and the slotted transmission line to reduce level changes in the RF source due to changes in load impedance. Measure the impedance of the antenna at resonance. How will you know that the antenna is at resonance? (At resonance the antenna will be purely resistive.) How is the impedance of the monopole measured?