[amsat-bb] Re: Preamp location tech question

[Elan Portnoy]

Yes, thank you for that detailed explanation and analysis.

Best 73,
Elan

--- On Sun, 12/20/09, i8cvs <domenico.i8cvs at tin.it> wrote:

> From: i8cvs <domenico.i8cvs at tin.it>
> Subject: Re: [amsat-bb] Re: Preamp location tech question
> To: "Elan Portnoy" <elanportnoy at yahoo.com>, "amsat bb" <amsat-bb at amsat.org>
> Date: Sunday, December 20, 2009, 4:23 PM
> 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!
> 
> 
> 
>