[opendtv] Re: Noise Figure

Ron Economos wrote:

> It's not the temperature of the amplifying device that matters
> (except for the ultra low-noise space systems that Dale is
> talking about), it's the temperature of the background that
> the antenna is pointed at.
>
> Here's the equations that can be used for either terrestrial
> or space (satellite) communications:
>
> 1) Converts noise figure to noise temperature
>
> Tr = 290((10^(Nf/10)) - 1)
>
> Tr = preamp or receiver noise temperature in degrees Kelvin
> Nf = preamp or receiver noise figure in dB
>
> 2) System noise temperature (including antenna, feed-line and
> preamp/receiver noise)
>
> Ts = Ta + 290(Lr - 1) + LrTr
>
> Ts = system temperature in degrees Kelvin
> Ta = antenna temperature in degrees Kelvin
> Lr = feedline loss expressed as a ratio (3 dB = 2, 6 dB = 4,
> etc.)
> Tr = preamp/receiver noise temperature
>
> Note: Ta is consider to be 290K for terrestrial antennas
> pointed at the horizon (although it can be much higher due to
> man-made noise and cosmic noise at VHF). Ta can be much lower
> for antennas in space communications when the antenna is
> pointed at cold sky.
>
> 3) Noise floor
>
> dBm = -198.6 + 10 log B + 10 log Ts
> B = bandwidth in Hertz
> Ts = system noise temperature
>
>If Ta (antenna temperature) is 290K (the antenna is pointed on
> the horizon), then the equations above can be reduced to:
>
> Receiver noise floor in dBm = -174 + 10 log B + line loss (dB)
> + receiver noise figure (dB)
>
> So a -83 dBm sensitivity is approx. 7.7 dB noise figure (with
> no line loss, 15.5 dB C/N and 6 Mhz bandwidth)

Thanks, Ron. Which means that if A/74 specifies sensitivity, bandwidth,
and C/N margin requirements at TOV, in effect it is specifying a noise
figure. Alternatively, A/74 could have specified a noise figure,
bandwidth, and C/N margin instead, and left sensitivity unspecified.

It all comes out in the wash.

> Note that the typical run of 100 feet of good RG-6 has around
> 4 to 5 dB of attenuation. Even a 0 dB noise figure receiver
> ends up with a 4 to 5 dB system noise figure with that coax
> run. A low-noise preamp at the antenna is the only way to get
> low system noise figures, making low-noise receivers not so
> useful.

Good point. So overall, a noise figure of 10 dB, say 6 dB for the
receiver and 4 dB for the downlead, does not seem unreasonable. Unless
the FCC planning factors assume a mast-mounted antenna amp. (I certainly
hope not.)

Bert
 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: