Leaky wave antenna (LWA) is one of the travelling wave antennas and exhibits many attractive features. The most important features of these antennas are high directivity, wide bandwidth, non-resonant nature, capability of frequency scanning, easy feeding structure relative to array antennas, easy design, and easy fabrication. Among all the structures of leaky wave antennas, substrate integrated waveguide long straight slot leaky wave antenna (SIW-LSSLWA), has many applications in microwave and millimeter-wave frequency band because of easy design and low cost features. However, one drawback of SIW-LSSLWA is the high sidelobe level (SLL). So, one of the major concern in design of SIW-LSSLWA is its SLL reduction. In this thesis, firstly the theory of leaky wave antennas is expressed. Secondly, to control radiation pattern, the conventional methods of tapering is introduced. Thirdly, the method of effective radiation sections (ERS) for SLL reduction is explained. Then, the design procedure of a SIW-LSSLWA in X band is presented. The gain of the SIW-LSSLWA is more than 15 dB, SLL i lower than -17 dB and cross-polarization level is lower than -25 dB on 450 MHz bandwidth with center frequency of 10.15 GHz are achieved. Key Words: Leaky wave antenna, SIW, SLL
