Re: [amsat-bb] P3D Dreaming

[Bob Bruninga <bruninga@xxxxxxxxxxxxx>]

Good point Jeff.  I was using 300 deg noise temp for an omni.  In a high
noise RF environment with computers all around you, I guess it could go
to the 1200 deg figure he used..  None of my comments should be taken as
anti-Sband. I am ALL FOR IT.  I was just clarifying the receive apature
and link budget sentence in the original post.

Bob

 On Tue, 1 Aug 2000, Jeff Davis wrote:

> Quoting Bob Bruninga (bruninga@nadn.navy.mil) [000801 15:00]:
> > As he noted, this is all based on the premise that 2 meters is 10 times
> > more noisy.  Otherwise the required atnenna sizes are equal due to the
> > capture area he also mentioned.
> 
> it is!
> 
> Quoting from the G3RUH article that was referenced:
> 
> : To quantify noise we use a measure called noise temperature.  Higher is
> : worse.  It's the temperature of a 50 ohm resistor that, if connected to
> : the radio instead of the antenna, would make the same racket.  The noise
> : temperature on 145 MHz is of the order of 1000K-1500K.  In some
> : conditions it may be less, but usually it's a lot more.  Let's just use 
> : 1200K for now. (K = Kelvins)
> :
> : The important thing to grasp is that there is nothing you can do about
> : this noise.  You cannot reduce it.  Even with a hypothetical zero-noise
> : pre-amp, you'll still have 1200K of noise.
> :
> : Now lets look at noise on 2400 MHz.  Sky noise nil.  Environmental
> : noise nil.  Cable losses nil (no cable).  FT736R noise nil; there's a pre-
> : amp/converter at the antenna.  Antenna noise almost nil; just some
> : pick-up of the warm Earth from the sidelobes, 20K maybe.
> :
> : The only real source of noise is the 2400 MHz to 144 MHz down-converter.
> : And this noise is totally under the control of the equipment designer.
> :
> : The noise level of a typical low cost converter is about 100K.  Let's
> : assume this, although lower is achievable and add 20K for antenna
> : sidelobes.  That makes a total of 120K of noise for 2400 MHz.
> : Guaranteed, everytime, everywhere, worst case.
> 
> I'll take 2.4Ghz downlink, anytime! :-)
> 
> Vy 73,
> -- 
> jeff davis, n9avg
> http://n9avg.org/
> 
> 

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