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RE: Noise with MKU24



Cliff -- almost, but not quite right:

> You might try putting a fifty ohm resistor at the input and call what you
> observe "normal".  Anything else would then not be normal.  Cliff K7RR
>
> On Tue, 11 Dec 2001, Pieter Ibelings wrote:
>
> > Hi,
> >
> > I built a homebrew converter and it does exactly what you
> mentioned. I have
> > verified that it is NOT oscillationg. I think it is caused by
> not having the
> > correct load at the input. When there is nothing connected at
> the input, the
> > noise is low. As soon as I connect a connector or a cable the
> noise goes up
> > to normal levels. It almost seems like leaving the input
> totally open does
> > not leave any noise resistance to generate noise at the input.
> This should
> > be normal behavior. I wonder is someone has a better explanation.
> >
> > Pieter
> > N4IP

Let me explain why I pick nits here --

Let's assume that the preamp is working OK and you connect a 50 ohm load on
its input. The noise you then hear is due to the combination of the LNA's
noise (expressed in temperature unites, Tlna) plus the thermal noise from
the resistor (about 290 Kelvin), so what you are hearing is a noise equal to
(Tlna + 290). If you cooled the resistor with liquid nitrogen at 77K, then
the noise would go down to (Tlna + 77).

Now let's connect a real antenna to the input instead of the load. If the
antenna has zero side/back lobes then you see a temperature of (Tlna + Tsky
+ Tfeed). The Tfeed term comes from ohmic losses in the cables connecting
the amplifier to the antenna, ohmic losses in the feed itself;  I assumed
(not true in the real world) no spillover/sidlobes/backlobes, so there is no
contribution from the earth.

[ ASIDE -- At this point we have set the stage to answer one of the
questions in a different AMSAT-BB thread having to do with "sugar scoop"
"cornucopia" antennas. It was the US end of the link to Goonhill Downs in
Maine where Arno Penzias and Richard Wilson (Bell Labs) were trying to
account for the observed (Tlna + Tsky + Tfeed) and they kept coming up with
3K more noise than should be there. The discrepancy turned out to fit George
Gamow's prediction from the late '30s of the remnant radiation from the Big
Bang. For making this landmark observation, Penzias and Wilson received the
Nobel Prize in Physics.]

The contributions to Tsky include the 3K microwave background, noise from
the galaxy, the aggregate noise from all radio stars, and radiation from the
atmosphere. At 13 cm at a dry, cloudless like Goldstone, Tsky can be as low
as 10K. At 10M, galactic noise can raise this as high as 100,000 K. The
total noise tends to minimize in the 1-10 GHz range. Since I ignored
imperfections in the antenna (sidelobes/backlobes etc), let me make a WAG
that your Tsky+Tfeed+Timperfections is about 100K. Therefore when you hook
up a good antenna looking at the sky, the noise should be LESS than you see
with a load connected. So I disagree with your "everything else" statement.

If you want to get a crude feel for the "Timperfection" term, put your body
between the antenna and the sky. Since your body is also at a temperature of
~300K, the noise should go UP. [f you transmit while standing in front of
the feed, you bring new meaning to the term "dummy load"!]

73 de Tom, W3IWI


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