Estimating System Noise Power
The fundamental formula for estimating system noise power (so that signal-to-noise ratio or SNR can be determined) is N = (k) (Tsys) (BW), where N is the noise power, k is Boltzmann's constant, Tsys is the effective receiver system temperature, and BW is the bandwidth. This formula assumes white noise with equal power per unit bandwidth. Be careful of units.
The bandwidth BW is set by the beacon receiver design. In general, since noise power is linearly proportional to bandwidth, a good system design should have as small a bandwidth as is practical to contain the signal frequencies which one wants to measure. This can be done either in the analog or digital domain; the latter is implemented using signal processing, and is much more flexible than physical analog filters.
In general, we can express Tsys as a sum of factors:
Tsys = Tsky + Trx
Tsky is the effective temperature provided by cosmic background radiation, which for our purposes is a noise source at a given radio frequency. In the neighborhood of any particular center frequency, this noise looks flat or white. However, the Tsky will vary significantly for center frequencies which are widely separated. Radio astronomers have been studying this variation for decades. A useful reference to get a feeling for background sky temperature variations is the book by Kraus, Radio Astronomy. This can be found in the Haystack library, and has some references to earlier papers which quote firm numbers. (Asking Alan Rogers is also a good idea; you have one of the world experts on radio astronomy in this building!)
Trx is the effective temperature added by the antenna, cabling, and receiver hardware, lumped together in this discussion. In principle, for a well designed system, this is set by the first preamplifier or "front end" noise performance, a system parameter which often has little relation to the physical temperature of the amplifier! A reference ![[WWW]](http://www.openradar.org//openradar_theme/img/moin-www.png){kind=small}
here gives some details about practical calculations. The design of the beacon receiver will set the appropriate noise figure.
References for cosmic background radiation calculation (Tsky): LINK1, LINK2
Preamplifier Noise Figures are approximately as follows:
150 MHz amplifier: 2.3 dB NF or 202K
243 MHz amplifier: 1.0 dB NF or 75 K
400 MHz amplifier: 0.9 dB NF or 67K
1066 MHz amplifier: 0.7 dB NF 51K
RG-58 cable which has the following losses: (current)
100 MHz: 4.5 dB/100 feet
400 MHz: 10.0 dB/100 feet
1000 MHz: 18.1 dB/100 feet
Andrews LDF1-50 coax cable which has the following losses: (once turnstile comes in)
150 MHz: 1.52 dB/100 feet
450 MHz: 2.71 dB/100 feet
960 MHz: 4.07 dB/100 feet