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Re: Even more out the closet



----- Original Message -----
From: "Luc Leblanc VE2DWE" <lucleblanc6@videotron.ca>
To: <amsat-bb@amsat.org>
Sent: Friday, March 24, 2006 5:58 AM
Subject: [amsat-bb] Even more out the closet

> For those unbeleivers..
>
> I perform some test and here is some answers
>
> Undisclose commentators wrote:
>
> > Just keep in mind, whatever coax loss you put in front of the
> > preamp DIRECTLY adds to the noise figure of the preamp. So a fancy 0.5
dB NF
> > preamp with 100 feet of LMR-400 in front of it now has a NF of 0.5 dB +
2.5
> > dB or 3.0 dB...which is very very poor  for a good gasfet preamp.

Hi Luc, VE2DWE

This is correct.

> I'm still dont rely too much on the FT-847 meter to convert S units in
real DB but it looks like i
> have around 18 db on noise and if i substract the .5db NF of the LNG-432 i
end up with still a good
> 17.5 noise?

You cannoct subtract the 0.5 dB NF of your preamplifier from your 18 dB
S meter reading on noise and your assumption to get 17.5 dB noise is
conceptually wrong and means nothing.

> Just let see now if theses 3 S units of noise will be also there when the
preamp will be at the
> antenna? (Just to be curious) Could be i will be able to hear the favorite
Martian...

No matter if these 3 S units of noise will be there or not but when the
preamp will be at the antenna you will get more S meter swing on the same
signal.
More the S meter swing you get on the same signal and less is the overall
Noise Figure of your receiving system.
It's the range of the swing on the S meter that count for the evaluation of
the S/N ratio.
A greater level on the S meter (greater gain) at wich a less swing (higher
NF) happens produces a less S/N ratio.

The above represents the concept difference between Noise Figure and Gain.

If  one cannot understand the difference between Noise Figure (NF) and Gain
of an amplifier to make calculations he can use the following rule of
thumb.

Everytime you put a preamplifier in front of a receiver the Noise Figure of
this preamplifier will be not significantly degraded by the Noise Figure of
your receiver if powering ON the preamplifier the noise increase is at least
10 dB

Any loss between the antenna and preamplifier cannot be recovered and it
represents a signal loss producing a less S/N ratio.

>
> Luc Leblanc VE2DWE
> AMSAT 33583
> Skype VE2DWE
> www.qsl.net/ve2dwe

I hope this helps

73" de

i8CVS Domenico
----
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