A broadband planar TEM horn antenna is proposed in this paper. Two triangular patch arrays are added in the front of the traditional planar TEM horn antenna for enhancing its radiation performance. Both simulated and measured results are provided to exhibit the characteristics of the proposed TEM horn antenna. For the very thin substrate with a thickness of about λ 0 /20 at 8 GHz, the proposed planar TEM horn antenna demonstrates good performance and has a broad bandwidth of about 800 MHz.