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Re: [SPAM][Linrad] Re: system gain

Good morning John,

Thanks for your reply. My system looks like this:

The antenna, preamp, power amp and transverter are mounted remotely at 
the antenna. The system nf and gain to the IF is quite good, say .7 dB 
nf and 25 dB gain. From there, the 144mhz IF travels through about 7 dB 
of loss to a 3 dB splitter. One side of the splitter goes to the IF 
radio, the other side to the .6 dB nf, 18 dB amp, and then into the 
SDR-14 and on to the computer.

The IF radio hears signals better than the LINRAD system displays them. 
My whole point is to bring the LINRAD system up to or better than the 
sensitivity of the IF radio. Of course this is a better problem to have 
than the other way around--I think :-).

I hope this helps you visualize my system. I don't believe the problem 
is at the remote site since both the IF radio and the LINRAD system see 
the same signal. The question is how to tweak the LINRAD parameters to 
bring its performance up to that of the IF radio.

Thanks for your thoughts.


Dave, K4TO

radio1@xxxxxxxxxxxxx wrote:
> In the old days, when every thing was analog, we would say that the 
> system noise figure was set by the first amplifier's noise figure.  Assuming 
> you have enough gain.  From the first amplifier noise is just added.  I can't 
> visulize your system do you have any gain near the antenna?  The losses 
> introduced by coax also adds to the system noise if the amplifier is after 
> the line loss.
> Maybe this will help.
> John K1VBM
> ---- Original Message ----
> From: Dave Sublette <k4to@xxxxxxxx>
> To: linrad@xxxxxxxxxxxxxxxx
> Cc: 
> Subject: [Linrad] system gain
> Date: Wed, April 08, 2009, 10:11:00
> Good morning,
> I have been watching the email thread on setting optimum gain and other 
> settings for the LINRAD program. I have used the program, rather 
> casually, for two years or so to look at weak signals on the 144 mhz IF 
> outputs from my microwave transverters. I put a gain stage ahead of my 
> SDR-14's direct input and that stage measures at 0.9 dB nf and 18 dB 
> gain or so at 144 mhz.
> When I remove this amplifier from the system, the noise floor on the 
> main spectrum display on LINRAD drops 8 dB. I read in the emails that it 
> should drop 15 dB, if I understand things correctly. I am looking at 
> about a 160-170 Khz slice of the spectrum around 144.1 Mhz,
> I do note that the visual display on Linrad is not as sensitive as my 
> receiver on a splitter from the same IF.
> My question is,  "Where should I look next to either find the problem or 
> adjust the LINRAD parameters to bring the LINRAD system sensitivity up 
> to standard?"
> I continue to read the web page and previous emails from VK2DU on 
> setup. 
>   Nothing I have tried seems to be working. I would list all my settings 
> here, but perhaps that would be premature.
> Any advice to be had out there?
> Thanks,
> Dave, K4TO
> > 

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