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With modern sensitive receivers, the system noise under good weather
conditions may be significantly lower than 300 K. Accordingly, measuring
the total noise when looking at a warm load at 300 K requires a linear
response of the whole detection system over a dynamic range greater than
2-3. This can be problematic.
There are several ways to bypass this limitation, for example
- Using lower temperature (controlled or monitored) loads. This is
probably the best, but it requires cooling devices somewhere.
- If the receiver is linear, but not the backend, it is possible to
place a switchable attenuator in the IF chain, preferably immediately
after the conversion to IF frequency.
- Covering partially the beam by the absorber, the other part still
looking at the sky.