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Re: HRD and two radios: was dual IC-706 station



----- Original Message -----
From: "Wayne Estes" <w9ae@charter.net>
To: <dmg@bossig.com>
Cc: <amsat-bb@amsat.org>
Sent: Thursday, December 01, 2005 11:33 PM
Subject: Re: [amsat-bb] HRD and two radios: was dual IC-706 station

> Dave G. wrote:
> > Never, ever, generalize... not no-how, not never !!!
> >
> > Pre-amps are primarily needed to overcome losses in
> > transmission lines with he inherent increase in noise
> > factors!!
> >
> > A good short read is in the  discourse in the HAMSAT journal
> > on the wideband homebrew 432MHz pre-amp
>
> Wayne replies:
>
> I'll generalize a little more specifically this time.  Regardless of
> cable loss, a <1 dB NF preamp allows one to hear weak signals better
> than a transceiver whose internal preamp has a noise figure 4 or 5 dB.
>
> Wayne Estes W9AE
> Oakland, Oregon, USA
> ----

Hi Wayne, W9AE

I agree with you.

Regardless of cable loss if you connect to the antenna a receiver
with a noise figure of 5 dB and  adding a preamplifier the overall noise
figure of the system decreases to 1dB than your S/N ratio in dB
improves and the improvement can be much more than the difference
between the above noise figures particularly if  the noise contribution
of the antenna is low.

As an example:

In 70 cm a good antenna system has an equivalent noise temperature
of about 50 kelvin when the antenna is looking at the sky in a quite 
rural area far away from the city.

A noise figure NF= 5 dB is equivalent to a noise factor F = 10 E 0.5 =3.16
and the equivalent noise temperature T1= (3.16-1) x 290 = 627 kelvin

A noise figure NF= 1 dB is equivalent to a noise factor F= 10 E 0.1= 1.26
and the equivalent noise temperature T2 = (1.26-1) x 290 = 75 kelvin

If we add to T1 and T2 the equivalent antenna temperature of 50 kelvin we
get the  equivalent noise temperature Tsys of the 70 cm system connected to
the antenna without and with the preamplifier.

Tsys1 = 627 + 50 = 677 kelvin  without preamplifier

Tsys2 =   75 + 50 = 125 kelvin   with preamplifier

The improvement of the S/N ratio is thus 677 / 125 = 5.42 in factor

or 10 log      5.42 = 7.34 dB   
               10

The above improvement is very high because if  we like to get it without 
using the preamplifier we must increase the antenna gain by 7.34 dB

This improvement in dB is greater than the difference 5-1 in dB between
the noise figures because the noise contribution of the antenna in 70 cm is
very low and is only about 50 kelvin

Of coarse in 2 meters the equivalent noise temperature of the antenna
is at best a litle bit less than 290 kelvin in a very good antenna system
having small side lobes and located in a rural area.

If we repeat the calculation for 2 meters using 290 kelvin instead of
50 kelvin for the noise contribution of the antenna than the improvement
using the preamplifier will be only 4 dB

Considering to use a coax cable between the above preamplifier antenna 
mounted and the above receiver in the shack than the coax cable must be 
considered as a cascade stage with a noise figure equal to the loss in dB 
and with a negative gain equal to 1 - F where F is the noise factor of the 
cable.

Best 73" de

i8CVS Domenico
----
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