[amsat-bb] Re: Preamp location tech question

[i8cvs]

Hi Elan,

You don't mention the model of preamplifiers from ARR that you are
using but I suppose that you got the better between the tree models
available i.e. the SP144VDG 144-148 MHz with NF = 0.55 dB and the
SP432VDG 420-450 MHz with  NF= 0.55 dB

Now if you add between the antenna and preamplifiers 50 feet of coax cable
LMR400 then the loss of 0.75 dB at 144 MHz and the loss of 1.35 dB at
435 MHz adds to the Noise Figure of your preamplifiers so that it is like
to have:

a 144 MHz preamplifier with an overall NF = 0.55 + 0.75 = 1.3 dB
a 435 MHz preamplifier with an overall NF = 0.55 + 1.35 = 1.9 dB

Supposing that you have the preamplifier antenna mounted working out
the numbars for 144 MHz we get:

Noise Figure NF = 0.55 dB
Noise Factor F = 10^(NF/10) = 10^0.055 = 1.13
The equivalent Noise Temperature T  = (1.13 -1 ) x 290 = 39 kelvin
If you live in a rural area at 144 MHz the equivalent Noise Antenna
Temperature T1=200 kelvin so that your Tsys1=39+200= 239 kelvin

Let see what happens if you mount the preamplifier in the shack:

Overall Noise Figure = 0.55 + 0.75 = 1.3 dB
Overall Noise Factor  F = 10^(10/1.3) = 10^0.13 = 1.35
The equivalent Noise Temperature T = (1.35-1) x 290 = 101 kelvin
Since the antenna is the same Tsys2 = 101+200 = 301 kelvin

The improvement putting the preamplifier antenna mounted
against shack mounted is

10 log     (Tsys2 / Tsys1 ) = 10 log   (301 / 239 ) = 1.0 dB
           10                                         10

The improvement at 144 MHz is very small because the Antenna
Temperature = 200 kelvin predominates.

Let see the situation for 435 MHz were the Antenna Temperature is
less and only 50 kelvin in a rural area

Preamplifier antenna mounted:

Noise Figure NF = 0.55 dB
Noise Factor F = 10^(NF/10) = 10^0.055 = 1.13
The equivalent Noise Temperature T  = (1.13 -1 ) x 290 = 39 kelvin
If you live in a rural area the equivalent Noise Antenna Temperature
T1 = 50 kelvin so that your Tsys1 = 39 + 50 = 89 kelvin


Preamplifier in the shak:

Overall Noise Figure = 0.55 + 1.35  = 1.9 dB
Overall Noise Factor  F = 10^(10/1.9) = 10^0.19 = 1.55
The equivalent Noise Temperature T = (1.55-1) x 290 = 159 kelvin
Since the antenna is the same Tsys2 = 159 +50 = 209 kelvin


The improvement with the preamplifier antenna mounted against
shack mounted is

10 log     (Tsys2 / Tsys1 ) = 10 log   (209 / 89 ) = 3.7 dB
           10                                         10

The improvement in 435 MHz is greater because the Antenna
Temperature = 50 kelvin do not predominates over the equivalent
Preamplifier Noise Temperature.

By the way 3.7 dB of more swing in the S meter over the noise is
more than half of a S point so that in your situation I would put
only the 435 MHz preamplifier at the antenna and would leave
the 144 MHz preamplifier in the shack.

CONCLUSION:

Since the loss of 50 feet of LMR400 is very small the advantage
in (S+N)/N ratio to put the low noise preamplifiers antenna mounted
is only 1 db at 144 MHz and 3.7 dB at 435 MHz in a rural area.
If the installation is made in a suburban area or in a urban area where
the equivalent Antenna Noise level is greater and predominates then
the advantages are accordingly less.

I hope this helps.

73" de

i8CVS Domenico

----- Original Message -----
From: "Elan Portnoy" <elanportnoy at yahoo.com>
To: "amsat bb" <amsat-bb at amsat.org>
Sent: Sunday, December 20, 2009 6:40 PM
Subject: [amsat-bb] Re: Preamp location tech question


> Well, it's clear what I should do. Will try remounting on masts as soon as
weather permits. Looking forward to improvements in performance.
>
> Thanks to all and happy holidays!