[Moon-Net] IQ+ MAP 65 TEST & CONCLUSION

Leif Asbrink leif at sm5bsz.com
Sun Nov 6 23:30:53 CET 2016


Hello Serge,


> 1)  50 ohms dummy load on the IQ+ INPUT 
>     FLOOR NOISE 5DB ON  MAP65
>  
> 2)  ANTENNA CONNECTED   (IQ+ TO SPLITTER LNA ON) 22DB WITH ELEVATION 
>     OF 15 DEGREES IN THE CLEAR OVER THE OCEAN
> 
> 3)  ANTENNA CONNECTED   (IQ+ to SPLITTER LNA OFF)  
>     FLOOR NOISE  5DB  ON MAP65
> 
> CONCLUSIONS: THE 16DB required are barley enough 22db -5db floor 
> noise =16db RF GAIN & very often depending on the AZ/EL it can go 
> down to -19db -5db = 14db RF GAIN well below of what is required.

What you measure is the right thing, but it is not RF gain.
It is how much the noise floor changes when you switch on/off the
LNA. As expected your #1 and #3 gives the same noise floor.

22-5=17 and that is OK, but you note that this difference can go
down to 14 dB and that is a bit low. If you add a LNA in front of
the IQ+ you can expect to gain 0.10 dB. That will be hard to notice
but your detect rate in map65 should improve by about 10% at
the detect limit with the FT decoder which shows the steepest
dependence on S/N. A station that detects with a rate of 50% would
improve to about 56% if you add a LNA - but that would happen only
when your antenna points in a direction where your noise floor 
is at 19 dB. The DS decoder is less steep and if you use that you
can only expect an improvement from 50 to 53%.

Maybe, if you look at a time when Leo (quiet sky) is in the clear 
over the ocean you might see an even lower noise floor.....

Anyway, your system is very good (provided your LNA NF is low)
I suggest you consider how often you work stations at the detect 
limit in a quiet direction. If you make 10 such contacts per year,
improving detect by 10% from 50% to 55% would make QSOs faster
but not often.

On the other hand, if you have a sked and detect only 10% of
the transmissions with DS, a 0.1 dB improvement on S/N should 
give a detect rate of 11.5%. Instead of detecting one in 10
transmissions you would detect one in 8.5 transmissions.

The advantage of adding more gain is small. There is a risk that
there are strong out of band transmitters that saturate your
system if you add too much gain.

There is no such thing as "the required gain" Gain is always 
a compromise. In a very quiet location (like mine was when
I was QRV) it is satisfying to use very high gain to know that
one has squeezed the last fraction of a dB from the system.

By the way, even if you can not hear it you probably have some
impulse noise that lifts your noise floor. There is a blanker
inside map65, but I do not know how good it is. You might use
a sound recorder to make a .wav file from your Delta44 while
rotating the antenna 360 degrees in azimuth. Noise pulses would
be visible with an audio editor such as audacity. If you make
such a recording I would be happy to look at it - and if you do,
switch off the LNA for 10 seconds at the end.

73

Leif



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