arXiv:2611.07954v1 Announce Type: new Abstract: The radiation of pinching antennas, dielectric elements placed on a dielectric waveguide, depends on the element shape. We explain it with a phase-matching, or prism-coupler, picture: the guided rod mode couples into the denser antenna plate at an angle set by the plate-mode effective index, about 31 degrees at 60 GHz, and radiates through the face it reaches first. This yields a simple sizing rule linking element height to contact length. Rectangular, isosceles and right-triangular elements of equal contact length and height are compared from 53 to 67 GHz in full-wave openEMS simulations including the waveguide-to-rod transitions. The rectangle peaks sharply at the rule, reaching 15.9 dBi. The right triangle keeps its gain within 0.7 dB and its side lobes at least 10 dB down over a two-to-one height range. At the rule, both outperform the isosceles element by 1.2 to 2.1 dB, and all remain matched.