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Re: Preamp advice, pse



Nice explanation Dominico!

Roger
WA1KAT

----- Original Message -----
From: "i8cvs" <domenico.i8cvs@tin.it>
To: <tjjapha@earthlink.net>; "AMSAT-BB" <amsat-bb@amsat.org>
Sent: Tuesday, November 04, 2008 1:31 AM
Subject: [amsat-bb] Re: Preamp advice, pse


> ----- Original Message -----
> From: "ANTHONY JAPHA" <tjjapha@earthlink.net>
> To: <amsat-bb@amsat.org>
> Sent: Tuesday, November 04, 2008 12:45 AM
> Subject: [amsat-bb] Preamp advice, pse
>
> > Bird Lovers,
> > Pse let me have advice on which of the following ARR 2 mtr preamps I
> should get: P144VDA: 1.0db nf, 15 db gain; or P144VDG: 0.5db nf, 24db
gain.
> >
> > The preamp will be used primarily for AO-7.  I operate only portable in
a
> > high noise environment here in Manhattan.  I doubt the nf difference is
at
> > all meaningful in my use.  The large difference in gain between the 2
> > preamps may not be impt. either, that is the real question.  My XCVR is
> > the FT-817.  Another 817 is used to transmit, running abt 15-20 watts
into
> > a small amp. with an Arrow split between 2 booms on 2 tripods only abt.
1
> > ft. from each other.
> >
> > So what's the verdict?
> >
> > Tnx and 73,
> > Tony, N2UN
> >
> Hi Tony, N2UN
>
> Do you remember our QSO via FO-29 on 21 april 2008 ? You writes to to me
>
> > Domenico
> > I thought you might like to see a photo of the station used to make our
> > QSO.  It is attached.  The Triborough Bridge and Hell Gate Bridge are in
> > the background.  The station is on a promenade next to the East River, 2
> > miles north of the United Nations.
> > 73,
> > Tony
>
> I am looking at the photo of your location and I see that you are living
in
> an urban area in which the man made noise temperature at 145 MHz is at
> best a minimum of 10.000 kelvin
>
> The sensitivity of a FT-817 at 144 MHz is specified to be 0.25 uV but the
> manufacturer do not refere for what ratio in dB
>
> Supposing that the sensitivity is 0.25 uV for a S/N ratio of 10 dB then
the
> corresponding computed Noise Figure of a FT-817 is 11 dB at 144 MHz
> corresponding to a noise temperature of 3360 kelvin
>
> Assuming from the photo that you are using 10 feet only of RG213 to
connect
> the output of your preamplifier to the input of your FT-817 then the loss
> of 0.25 dB at 145 MHz introduced  by the above coax cable can be omitted
> in our calculation.
>
> If you use a preamplifier with NF= 1 dB and Gain 15 dB in front of your
> FT-817 then the overall Noise Figure of your receiving system becomes
> 2 dB corresponding to a noise temperature of 170 kelvin
>
> If you use a preamplifier with NF= 0.5 dB and Gain = 24 dB in front of
> your FT-817 then the overall Noise Figure of your receiving system becomes
> 0.7 dB corresponding to a noise temperature of 51 kelvin
>
> The improvement in S/N using a FT-817 with in front of it a preamplifier
of
> NF= 0.5 dB versus a preamplifier with NF = 1 dB  in the New York City area
> with a man made noise of 10.000 kelvin will be only
> (10.000 + 170) / ( 10.000 + 51) = 0.05 dB i.e. a insignificant
improvement.
>
> You cannot see the improvement between preamplifiers because the man made
> noise of 10.000 kelvin on your antenna predominates but if you go in a
> country location where the man made noise is only 200 kelvin at 145 MHz
then
> the improvement in S/N using your FT-817 with a 0.5 dB NF preamplifier
> versus a 1 dB NF preamplifier will rise to  (200 + 170) / (200/ 51) = 1.7
dB
>
> In conclusion using preamplifiers with 0.5 dB  or 1 dB Noise Figure in
front
> of a FT-817 in the New Yok City area the advantage is basically ZERO but
it
> rises to 1.7 dB if you go in the country area very far from the city.
>
> On the other side:
>
> If you use the FT-817 alone in New York City area versus the FT-817  + the
> preamplifier with NF= 1 dB then the improvement in S/N will be
> (10.000 + 3360 ) / ( 10.000 + 170 ) = 1.18 dB
>
> If you use the FT-817 alone in New York City versus the FT-817 + the
> preamplifier with NF= 0.5 dB then the improvement will be
> (10.000 +3360) / ( 10.000 + 51) = 1.23 dB
>
> You cannot see too much difference with or not preamplifier because again
> here the antenna temperature of 10.000 kelvin in New York City
predominates
> and again the improvement with one or the other preamplifier is  only
> 1.23-1.18 = 0.05 dB i.e. inexistent
>
> But if you go in a country area where the man made noise is only 200
kelvin
> then the improvement in S/N using a FT-817 alone versus a FT-817 + in
front
> of it a 1 dB NF preamplifier will rise to : (200 + 3360) / ( 200 + 170) =
10
> dB wich is a great improvement ! !
>
> Identically if in a country area you use a FT-817 alone versus a FT-817 +
in
> front of it a 0.5 dB NF preamplifier then the improvement in S/N will rise
> to (200 + 3360 ) / ( 200 + 51) = 11. 5 dB
>
> In this situation where the antenna temperature do not predominates then
> the difference in the improvements 11.5-10 = 1.5 dB is greater than the
> difference between the Noise Figure of  the preamplifiers 1-0.5 = 0.5 dB
>
> In conclusion since using a FT-817 with a 0.5 dB NF preamplifier In the
New
> York City area the improvement in S/N versus the FT-817 alone is only
> 1.23 dB I suggest you do not use a preamplifier but to use an antenna with
a
> greater gain of at least 3 dB
>
> I hope to contact you via AO-7 wich is easier than our previous QSO via
> FO-29
>
> 73" de
>
> i8CVS Domenico
>
>
>
>
>
>
> _______________________________________________
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