A flat lens will be proposed for measuring the antennas. In antenna measurements, the incoming waves do not always come from a normal angle but sometimes from any angle. So, the waves can't be absorbed perfectly. The function of a flat lens is to focus or deflect the incident wave at any angle to become an incident wave with a standard curve. The flat lens will consist of unit cells which arranged periodically with an area of 9x9 cm of FR-4. The unit cell is designed using the ring slot method. It will operate at a frequency of 2-4 GHz. The design of the unit cell will be simulated using CST. In this paper, simulation is carried out using vivaldi antenna as Tx and Rx with dimension area of 9x5 cm each. The simulation includes measurement using flat lens and without flat lens. Based on the simulations that have been carried out, it has been proven that using a flat lens produces better performance than before adding a flat lens. Judging from the radiation pattern, the main lobes are increase by adding a flat lens. From the comparison, the lowest S11 has achieved when using a flat lens than without using a flat lens. The value of return loss decreased 0.51 dB by using a flat lens. Using a flat lens has increased bandwidth than before adding the flat lens. From both simulations, we get a better VSWR when using a flat lens than without using a flat lens.