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?
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.
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:
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
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:
- The Sun
- Electrical Devices
- The antenna itself
More information can be found by consulting the actual Site