[Moon] LNA FOR 144MHZ WITH 30DB GAIN

[Leif Asbrink]

Hello Gedas and Serge,

The IQ+ system may have a noise figure of 10 dB. It depends to some
extent on the soundcard used. That corresponds to a noise temperature
of 2900K. Assuming a 26 dB amplifier and 6 dB losses the gain from the
antenna input to the IQ+ input is 20 dB or 100 times in power as well
as temperature. That means that the IQ+ system would contribute
29K to the system noise temperature.

The total noise temp referenced to the LNA input would be:

LNA 0.5 dB NF       36K
IQ+ system          29K
Quiet sky          180K
Antenna loss         5K
Total              250K

With 4 dB more gain:
LNA 0.5 dB NF       36K
IQ+ system          12K
Quiet sky          180K
Antenna loss         5K
Total              233K

The improvement in S/N would be 250/233 = 1.07 times or 0.3 dB

Using a LNA with T=36K with 26 dB gain would give a RX system temp
of 36+29=65K or system NF=0.88 dB but with 4 dB more gain the
system temp would be 36+29/2.5=48K or NF=0.66 dB.

Quiet sky is well below room temp so the 0.2 dB NF improvement gives a
0.3 dB S/N improvement.

Near the passband edges as well as near the center soundcards are
usually a bit more noisy. At those frequencies the improvement would
be bigger.

I would suggest a mast mounted LNA with low NF and 23 dB gain
with one more amplifier in front of the splitter. The second 
amplifier should have limited gain and a modest NF. Maybe
NF 2 dB and gain 10 dB. More gain and lower NF would not
hurt as long as no strong signals cause overload. A band pass
filter in front of the second LNA would eliminate saturation
caused by out-of-band signals.

As I understand it, Serge does not have overload problems
at his QTH so there is no reason for him to make the usual
compromise between sensitivity and dynamic range.

0.3 dB is not much, but if you look at the detect probability
of jt65 at the threshold level would bring the decode probability
up from 50% to about 63%. That would be a 25% improvement. The
probabnility for a signal to be at the threshold is of course very
low....

73

Leif





> Hi Serge. Please do not take this the wrong way but I am having a hard 
> time understanding why you need so much gain.
> 
> I run 200' of heliax and have just under 1 dB of loss on 2m. So let's 
> say your coax loss is also 1 dB and with the splitter you now have 4 dB 
> loss. Now you have 5 dB loss.
> 
> Now you have several relays and jumpers etc so worst case you might add 
> 1 dB more loss. So you have a total of about 5-6 dB. Let's call it 6 dB 
> just to be pessimistic about your setup.
> 
> Why would a LNA with ~26 dB gain not work ok but one with 30 dB be ok?  
> Given your losses an LNA with ~26 dB gain would result in about 20 dB 
> net gain at the rig.......surely that is more then enough gain esp if 
> the LNA has a good NF of say 0.5 dB or less.
> 
> What am I missing here? I honestly think you will never notice or see 
> the difference between net ~20 dB gain and net ~24 dB except on the 
> s-meter. But I think your final S/N between the two may be too close to 
> even measure. Am I missing something? 73
> 
> Gedas, W8BYA EN70JT
> 
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them speak.
> 
> On 5/11/2020 10:52 AM, Serge Szpilfogel via Moon-net wrote:
> >
> > Hi folks I am looking for a LNA with good N/F but more important with 
> > a gain of 30db. Most of my LNA’s I have are between 23db/26db I have 
> > 120Feet of good coax then it goes to a splitter
> >
> > One side goes to my Icom & the other side goes to my IQ+. So right 
> > there I lose 3db then up the tower there is 2 relays connectors 
> >  adapters, God knows what the loses are??
> >
> > Is there anyone on this earth that can meet my needs??? The struggles 
> > goes on!!!!!!
> >
> > Serge VE1KG
> >
> >
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