Abstract: An electronic device may have an upper housing and a lower housing separated by a slow. A substrate may be mounted in the lower housing and may have an inner surface facing an interior of the lower housing and an outer surface facing the slot. Ventilation port openings may extend from the outer surface to the inner surface. Ribs in the substrate may separate the ventilation port openings. The ribs may include notches at the inner surface. An antenna may include conductive traces within multiple ventilation port openings and extending through the notches. The conductive traces may be offset from the outer surface. The outer surface of the substrate may be free from conductive material from the antenna. This may serve to maximize antenna efficiency while protecting the antenna from external forces and debris or other contaminants that pass through the slot during use of the electronic device over time.