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Re: [Discuss-gnuradio] a question about the antenna VERT2450

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] a question about the antenna VERT2450
Date: Wed, 22 Apr 2009 21:47:51 -0400
User-agent: Thunderbird (X11/20090105)

Matt Ettus wrote:
> Bill,
> The short answer is that there is no answer to your question.
> The long answer is that noise temperature is a function of where you
> point the antenna, and the ambient noise environment.  It is also not
> normally specified for low gain antennas, only for dishes and the like.
> I would suggest that you read the Wikipedia article on noise temperature.
> Matt
Even for dishes, you have to factor in the feed assembly as well (a dish
with a 15dB edge taper feed will have a lower spillover noise than
  one with the standard 10dB edge taper).

Ground noise should only be a problem due to spillover, and a mesh
surface that isn't well-matched to the wavelength of interest.

The ground is a blackbody radiator with an equivalent noise temperature
of about 300K, and how much of this noise leaks
  into your feed determines what the effective noise temperature is of a
parabolic reflector antenna.  A feed with an aggressive
  edge taper, and a good solid-surface dish should push the ground noise
down to under 5K.

For a low-gain antenna, it just isn't relevant.  The antenna "sees" all
noise sources within its 3dBi "torus".  It's rather similar for
  amplifiers in the HF bands.  The ambient noise temperature is so high
(100,000K) in HF that specifying the noise temperature of the
  amplifier just isn't meaningful.


Marcus Leech
Principal Investigator, Shirleys Bay Radio Astronomy Consortium

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