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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:

http://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation

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?

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    $\begingroup$ That instrument was not built to look for the CMB. It's design should be evaluated in terms of what they thought they were going to do with it (though I admit I can't recall what that was). $\endgroup$ – dmckee Jan 27 '14 at 2:54
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    $\begingroup$ This is not a critique of the instrument design, but rather a question of simply whether such a big 'hammer' is really needed for the CMB problem itself. If the huge Keck telescope just happened to be the first one to spot, say, a large new comet, wouldn't it be reasonable to wonder if an amateur's backyard telescope could see it too? That would not be an invalid question. $\endgroup$ – user38274 Jan 27 '14 at 5:06
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    $\begingroup$ Larger the antenna directivity (effective aperture) the easier is to maximize the ratio of the energies received in the mainlobe relative to the sidelobes by properly tapering the radiation pattern. $\endgroup$ – hyportnex Jan 27 '14 at 14:16

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