Re: [amsat-bb] P3D Dreaming

[Jeff Davis <jeff@xxxxxxxxxxxxxx>]

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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