Read in a study guide: 'Since the noise temperature of the antenna and the feeder could not be reduced below a limit, noise temperature of LNA is kept as low as possible.' I am trying to look for a definite answer why the antenna noise temperature and feeder noise temperature have limitations in their reductions? I have read that antenna elevation angle is a factor for antenna noise temperature with inverse proportionality. So can it be said as a reason, that as antenna elevation angle of earth stations are fixed w.r.t satellite's position in space its noise temperature can't be reduced a certain value? So my questions are:
1) Why can't the noise temperature of antenna and feeder cable be reduced a certain limit or in other words its limitations?
2) Does that mean LNA's noise temperature can be reduced by some technique. Its certain even then a limit is there. What are its limitations?