Noise Temperature Distribution
I have a antenna project for which I want to determine a noise temperature figure. In the reference material I have, a figure of 290 K to 300 K is given for ground noise temperature and 4 K for sky background temperature. I am interested in the gradient from a zenith angle of 75 degs to 90 degs. In other words what does the transition from ground temperature to sky temperature look like?
Has a rule of thumb been establish such a thing?
Best regards,
Fred B
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Analyzing antennas for noise temperature output is not simple. At Ku-band, for instance, a good parabolic dish 3 m diameter generates typically 30 K when pointed to zenith. The reason are the side lobes.
To get down to 4 K, Penzias and Wilson were luck to choose the horn-parabolic antenna with extremely low side lobes. With such good antenna only they could discover the 3.2 K relict noise temperature.
If you need to obtain the output noise temperature for zenith to 25 degrees down, it will be a good idea to add a metal screen on the ground around your antenna to reduce side-lobe contribution. Expect rather worse result.
In some antenna books you can see the function of noise temperature from pointing horizontal to zenith as a smooth curved line, from about 300 K to the lower limit I indicated above.
You will need a good calibrated radiometer to get reliable data. Let me know if you need a good design, and at what frequency band your antenna works.
I'm designing a quadrifilar helical antenna for amateur satellite communications on the 70cm band. This antenna is stationary with an omnidirectional circular polarized pattern. The satellites it communicates with are in low earth orbit around 1000 km high.
I want to use the antenna noise temperature figure alone as comparison measure for determining the best shape for the pattern.
From that graph it look like at at the 70cm band, 440 MHz, the the sky noise temperature is about 10 K minimum. There is still the question of the gradient from ground temperature to sky temperature at zenith.
I found a method in this paper that at first glance seems like what I want, although it's going to take some work to verify that.
https://www.researchgate.net/publica...UTUNSE_ANTENNA
I have an experience limited to microwaves, above 4 GHz. At 70 cm, with a helical antenna, I think the side lobes picking up the earth radiation would make the antenna noise higher than 50 K.
Please check publications by EME amateurs, maybe someone measured a similar antenna at 70 cm.
For a theory on the "gradient", the real function strongly depends on antenna side lobes, if the surrounding ground is flat, also trees would contribute. A real-antenna test will show the real results.
On page 98 of "Antenna Theory" 2nd Ed. Balanis says the sky temperature varies from 5 K looking up to 100-150 k at the horizon, although he omits mention of a frequency with that and inclusion of a graph that would show what kind of curve the gradient takes.
I think this paper will do for now:
http://publications.lib.chalmers.se/...text/67232.pdf
This is a nice study but no real data, same as Balanis. You will have to try and see . Please let me know what you find by measurement.