Penzias and Wilson discovered the 2.7K Cosmic Microwave Background (CMB) Radiation using a 6 meter horn antenna, along with a cryogenic low-noise detector measuring at 4 GHz:
My question: Did they need such a large antenna to do this?
I think that according to the Antenna Theorem, an isotropic and uniform distribution of thermal background radiation will deliver the same power to a detector regardless of the antenna's size (i.e. it's gain), as long as it is directive enough to look up in the sky and not at the hot earth background. This measured power should be kTB, where B is the bandwidth and T the 2.7K background temperature. So could the CMB have been discovered with a much smaller horn antenna, for example one maybe just ~10 inches in diameter?