Radio Astronomy Cold Load Question

[racshot65]

Hi,

I'm reading about the discovery of the cosmic microwave background radiation and don't understand this part ...

... It seemed likely this discrepancy was due to a slight excess of electrical noise in the amplifier circuits. In order to eliminate such problems they made use of a device know as a cold load.

The power coming from the antenna was compared with the power produced by an artificial source cooled with liquid helium.

The electrical noise in the amplifier circuit would be the same in both cases and therefore cancel out in the comparison. Allowing a direct measurement of the power coming from the antenna.

What power does a source called with liquid helium produce and how is it related to the amplifier circuits ?

I don't get what they did and why ?



Thanks !

 

[davenn]

I understand what they are doing ... I am just trying to think of a ways to easily describe it to you.

Thermal noise is a major problem when trying to detect very low level radio signals from deep space, that may only be say 20 Kelvin. The thermal noise of Helium is known, so measuring that noise level with your receiver, you get a given signal noise from the receiver. You can then determine what the thermal noise created by the receiving electronics is. Then when you switch the receiver to the antenna and measure the thermal noise from the deep space object ... even the widespread cosmic background radiation, you have already worked out what the thermal noise of your receiver is and it can be taken away (subtracted) from the received noise level when looking at your object.
The noise remaining is therefore the noise level (signal strength) of your object.

cheers
Dave

 

[davenn]

as a further response, it is normal for receiver electronics in radio telescope use to be substantially cooled by liquid nitrogen during operation.

actually this is done even in commercial situations like ground based receiving stations for Earth orbiting satellites etc.


D