planar loop antenna port

hye.im trying to make a waveguide port for microstrip loop antenna.but there are errors.
"waveguide for port number 1 is too short.the waveguide must be homogeneous for at least three mesh planes along propagation direction."

i dont understand this and dont know how to overcome it.

I can imagine a planar loop antenna, but no microstrip loop antenna, because the ground plane would short the loop.

im sorry cause make you confuse.i men it by planar loop antenna.it has no ground