[Linrad] Re: system gain

[Dave Sublette <arrl.net; k4to@xxxxxxxxxxxxxxxx>]

Lief,

Many thanks for your reply. This has given me much to think about and I 
don't yet understand everything you have said. Let me add a few notes 
down through the text and see if it makes any difference in your reply--

Leif Asbrink wrote:
> Hi Dave,
> 
>> 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.
> OK. 
The measured nf and system gain quoted are at the IF output connector on 
the box at the top of the tower...


> 
>> From there, the 144mhz IF travels through about 7 dB 
>> of loss to a 3 dB splitter.
> Now we have 10 dB loss in total.

The 7 dB loss is the connecting feeedline from the top of the tower to 
the shack--it is quite long as the box is 185 feet above ground on a 200 
ft tower which is located 250 feet behind the shack.

Yes--there is a total of 10 dB loss presented to each input--the IF 
radio and the SDR-14 side.
> 
>> One side of the splitter goes to the IF 
>> radio,
> Let me guess that the noise figure of the IF radio is 5 dB.
> That would give a noise figure at the other end of the
> cable of 15 dB. The composite system noise figure would 
> be 1.04 dB
The input of the IF radio is 4 dB nf with unity gain. It is a DEMI 
144-28 transverter board mounted inside the K2 transceiver.

> 
>> the other side to the .6 dB nf, 18 dB amp, and then into the 
>> SDR-14 and on to the computer.
> At the antenna it looks like 10.6 dB NF and 8 dB gain. The composite
> noise figure becomes 0.82 dB with an associated gain of 33 dB.
> This is then connected to the SDR-14 direct input with a noise
> figure of 37 dB. The compsite noise figure for your Linrad
> part would be 5.7 dB. Totally unacceptable.....
This is where I am getting confused. The input to the SDR goes through a 
.6 dB nf, 18 db LNA (with T/R relay control to protect it and the 
SDR-14) Wow--I didn't know the SDR 14 direct input had that bad of a nf....

So--comparing the two systems...The IF radio system seems to have almost 
a 5 dB better NF than the SDR system?  Am I figuring this correctly? 
That is about the difference I think I am seeing  :-((  Perhaps if I put 
a higher gain stage in front of the SDR it would improve the system nf?


> 
> You should use the 0.1 to 30 MHz input which has a noise figure
> of about 17 dB. Then your system noise figure would be 0.91 dB.
> Probably just a little better than your IF radio.
> 
> If I were you, I would move the 0.6 dB NF amplifier to the other 
> side of the splitter and possibly insert an attenuator in front
> of the IF radio. The system noise figure for Linrad would then be 
> 0.84 dB for Linrad and the same for your IF radio if the attenuator
> is selected to provede a NF of 17 dB.
>

This would call for a significant redesign of my system. Moving that 
splitter is the problem. The splitter also passes the 17 dBm Tx signal 
back to the transverter. Unfortunately I do not run a two cable system 
up the tower for TX and RX. I will have to think about it.  Thanks for 
your help.

> 73
> 
> Leif / SM5BSZ
>  
>  

73,

Dave
> 
> 
>SNIP>

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