I'm working on designing a radio system for CubeSats and I want to make sure that the sensitivity of my receiver is as high as it can be. I'm trying to understand what parameters are going to set the lower bound for the performance I can squeeze out of my design.
One thing I am still confused about is antenna temperature and thermal noise. The closest question I could find is here: https://physics.stackexchange.com/questions/289893/is-antenna-noise-temperature-relevant-if-the-physical-system-temperature-is-high
I don't think the top answer on that post answers the question that the original poster was asking.
Basically, the noise power for a system is given by the following formula for Johnson-Nyquist noise: \$ N = k_bT\Delta B\$. As I understand it, this is present at all points of a system and exists regardless of resistance, material, input loss, or any other parameters. Basically, this sets a lower limit on the amount of noise power you can get your system down to.
I've started learning about antenna noise temperature though and I know that this is a function of what the antenna is actually "seeing" in its radiation pattern. When pointing at the sky, the antenna noise temperature can go as low as 3 K, which is the microwave cosmic background.
For receivers at 300 K, does a low antenna noise temperature matter at all? As I understand it, any antenna at a physical temperature of 300 K or so will have a much higher thermal noise power than anything received as antenna noise.
The advantages of putting the LNA close to the antenna have to deal with avoiding resistive losses over a transmission line and not with minimizing thermal noise because thermal noise is omnipresent in equal amounts in the circuit, correct?