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I'm using a 14 element cross Yagi antenna in my system. For my link budget calculations, what antenna noise temperature should I consider? How does this vary with elevation?

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  • $\begingroup$ Very interesting question! I never found any graphs about noise temperature versus elevation. I'd guess that the higher you go, the lower is noise temperature, because you're farther away from noise sources. $\endgroup$ – AndrejaKo Oct 20 '16 at 16:01
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It's expected that the total noise temperature to vary with elevation.

Three major effects to consider:

  • Sidelobe pickup of noise from the ground, this is the dominant effect
  • Variation of atmospheric loss at different elevations, not really relevant for UHF frequencies
  • The sky temperature itself, which varies mainly depending on whether you're on or off the galaxy. This sets a minimum for your system noise.

Here's a graph from Phil Perillat of Arecibo, about a 12 m dish.
12 m dish tipping curve
The sidelobes of a yagi will look quite different so the curve will be different, but it'll probably have a minimum when you're pointing straight up, and be nearly 300K when you're pointing at the horizon. When taking these curves, you need to be careful you don't cross the plane of the galaxy.

Here's a [map of the sky temperature at UHF from Lambda/gsfc:

408 MHz sky map

They say "Log scale from 10 to 250 K" so it's safe to assume you could have from as little as no significant contribution from the sky, to a completely dominant contribution, depending on where you point your antenna.

QST has a whole long article about sky noise at 433 MHz. Read it here

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Wikipedia has a nice article on antenna noise temperature. Quoting here:

In telecommunication, antenna noise temperature is the temperature of a hypothetical resistor at the input of an ideal noise-free receiver that would generate the same output noise power per unit bandwidth as that at the antenna output at a specified frequency. In other words, antenna noise temperature is a parameter that describes how much noise an antenna produces in a given environment. This temperature is not the physical temperature of the antenna. Moreover, an antenna does not have an intrinsic "antenna temperature" associated with it; rather the temperature depends on its gain pattern and the thermal environment that it is placed in.

Antenna noise temperature has contributions from several sources:

  • Galactic Radiation

  • Earth heating

  • The Sun
  • Electrical Devices
  • The antenna itself

More information can be found by consulting the actual Site

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    $\begingroup$ This doesn't answer the question which was "How does the antenna noise temperature vary with antenna elevation?", not "What is noise temperature?" $\endgroup$ – Phil Frost - W8II Oct 23 '16 at 23:35
  • $\begingroup$ I guess it is an answer by its absence, the noise temperature is not merely dependent on elevation. It is depending on a variety of sources that each have their own affect where distance to the antenna is a factor but not necessarily elevation of the antenna. $\endgroup$ – K7PEH Oct 23 '16 at 23:41
  • $\begingroup$ @K7PEH But why not state that explicitly in your answer? By the way I did find a source from NASA's JPL about noise temperature and elevation, but it's mostly related to microwaves and high elevations. $\endgroup$ – AndrejaKo Oct 24 '16 at 11:40
  • $\begingroup$ @AndrejaKo -- sorry, been away for a few days. When I think if a concept such as dependence on elevation for an antenna I am thinking of the antenna itself in terms of improving or modifying its performance. However, other sources of effect or cause (e.g. microwaves) is not a direct performance parameter for an antenna, it is something specific to a situation and dealt with quite differently than other performance related parameters for the antenna. To me, it is a non-sequitur to associate elevation with something like noise temperature. $\endgroup$ – K7PEH Oct 26 '16 at 16:41

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