An optimized self-diplexed slot antenna is presented. The antenna is designed using the novel low loss inverted microstrip gap waveguide technology, formed between two parallel metal plates without the requirement of electrical contact between these plates. A texture of screws is used to provide the needed AMC condition in order to make the manufacturing low cost. The diplexer layout has been optimized in order to ensure compactness showing an excellent performance in terms of high isolation and low insertion loss.