The present disclosure relates to a planar antenna structure (100, 200, 800, 900) comprising at least one radiating aperture (101, 201 801, 901), adapted for a certain working frequency band, and an electrically conducting surface structure (102, 202, 802, 902) that is constituted by at least one surface part and is surrounding at least one radiating aperture (101, 201, 801, 901) and having a certain extension (T). The planar antenna structure (100, 200, 800, 900) comprises at least one continuous groove (103, 104; 203, 204; 803, 804; 903, 904) that forms a slot in the surface structure (102, 202, 802, 902), where each groove (103, 104) is defined by an at least virtual electric wall that is electrically connected to the surface structure (102, 202, 802, 902) and forms a continuous electromagnetic wall in the surface structure (102, 202, 802, 902) at the working frequency band. In this manner, that propagation of surface waves via the at least one groove (103, 104; 203, 204; 803, 804; 903, 904) is reduced.