Abstract: Sealing a TEM horn antenna in a nylon radome filled with transformer oil can enhance the power capacity, and improve the performance of the TEM horn antenna as long as a beam focusing lens is formed by the nylon shell filled with transformer oil. On the basis of the ellipses geometrical properties and the shrinkage of wavelength in a medium, an ellipsoidal transformer oil lens was designed with the help of geometrical optics theory. The theory says that the shrinkage of the wavelength in transformer oil enlarges the electrical size of the TEM horn will preliminarily improve the performance of the TEM horn antenna, and the beam focusing characteristics of the ellipsoidal transformer oil lens will further improve the performance of the TEM horn antenna. In order to prove the correctness of the theoretical analysis, a spherical transformer oil lens without beam focusing characteristics was designed as comparison, and the simulation results agreed well with the theoretical analysis. Compared with the TEM horn in atmosphere, the ellipsoidal transformer oil lens narrowed down the main beam and increased the rEpp by 56.97%, and the low boundary point of the impedance bandwidth reached 0.2 GHz from 0.28 GHz. The power capacity of the ellipsoidal transformer oil lens TEM horn antenna reached 18.43 GW.