Coax/WG transition - probe radiation resistance

Radiation resistance of a quarter-wavelength monopole antenna is known to be 36.5 ohm. What is the radiation resistance of a quarter-wavelength probe in a waveguide?

Raidation resistance stands for a radiator's ability to radiate input power into free space. When working a waveguide, due to the variation of boundary condition, the probe's radiation capability should be worse than that in free space. As for my thoughts, the radiation resistance of quarter wavelength probe in wave guide should be smaller than 36.5 ohm. But the exacy value should be depend on the dimension of the waveguide, and the feeding location of the probe in the waveguide.