In this paper, a planar broadband leaky‐wave antenna (LWA) has been designed to achieve a full‐space scanning property with a stable high gain. The proposed LWA consists of 9 permeability‐negative transmission line unit cells. With the use of the unit cell, the space harmonic with n = −2 mode in a wide frequency band is generated, and thus a scanning range from −90° to 0° is realized when one port of the LWA is excited and the other is connected to a matched load. Considering the symmetry of the proposed antenna structure, the scanning range from 0° to 90° can be achieved by exchanging the excitation port. A prototype was fabricated and measured. The experimental results show that the proposed LWA has an impedance bandwidth for SWR ≤ 2 from 11 to 17.5 GHz, a scanning range from −85° to +85°, and a stable gain of 10.7 ± 2 dB.