I am designing a phased array antenna in S-Band frequency running on RHCP for a project. One of my requirements is to have a receive noise figure better than 2 dB. I cannot find any resources on the receive noise figure, and was wondering if anyone is aware of any resources.
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2 Answers
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How to calculate the receive noise figure of an antenna?
Antennas and passive components don't have a noise figure (see last sentence of wiki quote below)...
Noise figure is: -
the difference in decibels (dB) between the noise output of the actual receiver to the noise output of an “ideal” receiver with the same overall gain and bandwidth when the receivers are connected to matched sources at the standard noise temperature T0 (usually 290 K). The noise power from a simple load is equal to kTB, where k is Boltzmann's constant, T is the absolute temperature of the load (for example a resistor), and B is the measurement bandwidth.
So, if your antenna (a passive device) has an output resistance (ignoring the complex part of the impedance) of 50 Ω, then the noise it produces is equivalent to a 50 Ω resistor.
See also Antenna Temperature for more information.
You then need to ensure that your receiver doesn't produce a NF of greater than 2 dB. This means that your receiver doesn't degrade the signal to noise ratio from input to output by more than 2 dB.
answered Jun 12, 2022 at 19:12
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\$\begingroup\$ Hey Andy, thank you very much for your answer. That has cleared a lot for me. I think I have a similar misunderstanding of how the maximum RF output is calculated as well. As far as I know, the max RF output of an antenna will mostly be dependent on what the input ports and the transmission lines can handle, as well as any thermal requirements. Is that correct? \$\endgroup\$ Commented Jun 12, 2022 at 21:13
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\$\begingroup\$ It's usually mush smaller than that; the level of a received signal is at best millivolts and more typically microvolts. Miles away from any electrical limit imposed by the receiver's port. \$\endgroup\$– Andy akaCommented Jun 12, 2022 at 21:21
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Antenna noise figure is a way of expressing the noise power produced by the antenna (presumably when it's pointing at the cold sky). Antennas produce a noise power independent of any active devices which might be connected to them.
Search for Antenna Temperature instead. There's plenty of literature, more under radio astronomy and satcoms than raw antennas & propagation.
For ground communications the antenna temperature will be about 300 K, or 3 dB, but antennas pointed at the sky can be 20 K or less, with a corresponding reduction in noise power.
You can further break it down into several components.
Sky temperature is the noise from the sky, unavoidable, but varies depending on where you point it, and there are good maps, also atmospheric losses matter slightly. Sometimes this is excluded from the antenna specification, and if the beam is narrow, the celestial poles provide a quiet place for testing it.
Losses in the antenna, which are like room temperature attenuators (unless your antenna is cooled), they will have a noise figure equal to their attenuation value etc; and
Ground pickup, the integrated gain of the antenna below the horizon where it will receive noise from the warm ground.
