[amsat-bb] Re: Preamp advice, pse

[i8cvs]

----- Original Message -----
From: "ANTHONY JAPHA" <tjjapha at earthlink.net>
To: <amsat-bb at 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