The present invention relates to a compact planar antenna containing, on a substrate featuring at least one ground plane, a radiating slot forming at least one folded strand with parallel strand parts. The antenna contains at least one means of phase inversion between two successive strand parts, the means of phase inversion being positioned on the strand in such a manner that the field components of the parallel strand parts are added together. The use of phase inversion means makes it possible to reduce the dimensions of the antenna, facilitating its integration on a card.