[McKay (OP)]

So, collecting microwaves/ radiowaves for electric current is relatively easy using a length of wire and a diode aka an antenna, right? The current is small, unless high power microwave source is nearby or the antenna is very big, but thats irrelevant for my question. 
What I am wondering - is there some (very low) temperature where this process ceases to?
Why am I wondering this, you might ask? Well: what if we put such an antenna in a very cold environment - 12 kelvin, where the black body radiation is microwaves?

Edit: changed "background thermal radiation" to "black body radiation"

[hamdani yusuf]

So, collecting microwaves/ radiowaves for electric current is relatively easy using a length of wire and a diode aka an antenna, right? The current is small, unless high power microwave source is nearby or the antenna is very big, but thats irrelevant for my question. 
What I am wondering - is there some (very low) temperature where this process ceases to?
Why am I wondering this, you might ask? Well: what if we put such an antenna in a very cold environment - 12 kelvin, where the background thermal radiation is microwaves?

Interesting question.
If CMB can be harvested, then it means free energy for all. I haven't found anything that could prevent it, at least in principle.

[evan_au]

The CMBR currently has an effective temperature around 2.7Kelvin.

Your antenna at 12K would radiate more energy than it receives.

If you cool it to 1K, you would gain energy from the CMBR.

[hamdani yusuf]

The CMBR currently has an effective temperature around 2.7Kelvin.

Your antenna at 12K would radiate more energy than it receives.

If you cool it to 1K, you would gain energy from the CMBR.

I think some mechanism is missing in your argumentation. In a microwave oven, the temperature of the magnetron can be significantly cooler than the object being heated. The same is true for induction cooker.

[McKay (OP)]

I am not even talking about CMB, but any cold environment - a crater on some distant asteroid, a lab, whatever. I mean, the wavelength of black body radiation is dependent on temperature and if the temperature is low enough, it is microwaves.

[evan_au]

collecting microwaves/ radiowaves for electric current is relatively easy using a length of wire and a diode

With our current technology, the diode is a weak link.

Common Silicon diodes don't conduct any electricity until the voltage exceeds 0.7V, and that requires a high power, nearby source, as you suggest. You could go to Germanium diodes (0.3V) or even Schottky diodes (0.2V) - but this is still a high power source.

Another problem is capacitance; our present diodes don't work so well at high frequncies (lik GHz); their internal capacitance "shorts out" the signal.

The antenna is also a challenge - a piece of wire tends to collect energy best at one frequency - it's resonant frequency. But black-body radiation is spread out over an immense swathe of spectrum, so you need a broadband antenna.

The intensity of the radiation is also extremely low. Carl Sagan made this comment:

The total amount of energy from outside the solar system ever received by all the radio telescopes on the planet Earth is less than the energy of a single snowflake striking the ground.

That was a long time ago, and was probably an exaggeration at the time, but you get the general idea.

Maybe that's why the Chinese have now built a 500m-wide radiotelescope?
See: https://spectrum.ieee.org/tech-talk/aerospace/astrophysics/china-unveils-worlds-largest-singledish-radio-telescope

[Bored chemist]

https://en.wikipedia.org/wiki/Backward_diode
And we can pick up energy from the CMB, just like we can pick up star light at night.

I think the power density is about 3 millionths of a watt per square meter.

If CMB can be harvested, then it means free energy for all. I haven't found anything that could prevent it, at least in principle.

Not worth it.

[McKay (OP)]

Ok, so the limitations  of resonant frequency, diode perfotmance requiring large voltage and grneral low intensity of such cold Black Body radiation, but could some power br collected in principle?

[Bored chemist]

It would be interesting to get a big parabolic dish and set up a resonant antenna at the focus then point it at a "blank" bit of sky and  listen to the signal with what would amount to a "crystal set" based on  a back diode.