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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