[Moon-Net] IQ+ PREAMP GAIN NEEDED

Leif Asbrink leif at sm5bsz.com
Thu Nov 3 03:37:07 CET 2016


Hi Serge,

> Hello Folks over the years folks have been telling me I need 
> at least 26db at the input of the IQ+ i only have at the 
> most 20db & the IQ+ rarely decodes below -28db. 
It is a property of the jt65 decoder to rarely decode below 
-28 dB S/N. You might put a 10 dB attenuator in front of your
LNA, right at the antenna, and you would find that your
system rarely decodes below -28db. You would however find 
that stations that used to be easy decodes at -18 dB would 
be marginal decodes at -28 dB. (This is 144 where the sky temp
is similar to room temp.)

What I want to say is that the indicated decode level gives
NO INFORMATION AT ALL about your system performance.
 
What you need to do is quite simple:

1) Measure the level of your noise floor.
2) Disconnect the antenna cables from the IQ+ and replace
   them with 50 ohm resistors.
3) Measure the noise floor again and compare with the initial 
   noise floor.

If the difference is less than 10 dB you have a serious problem.

If you want "full EME performance" the difference should be
something like 17 dB.

> I could put an extra preamp in line before the splitter but 
> then i would need probably a 15 to 20db attenuator not to 
> overload the IQ+ & WSJT it just does sound practical.
Why? The IQ+ has a good dynamic range. Only if you have problems
because of strong local stations you would need to insert
attenuators (or switch out the extra LNA.)
 
> The eternal question has been for me WHY is WSJT7 decodes 
> lower & more often than the IQ+?????????????
Hmmm, decode is by software. IQ+ is hardware.Perhaps
you have not given your IQ+ an adequate signal level
while the signal level is adequate for your other receiver.
It is not clear to me how your setup is arranged, but "I 
could put an extra preamp in line before the splitter" indicates
you split at RF.

> BUT not always. Sometime the IQ+ will out decodes WSJT 
> so i guess you have to have both

Make sure to add LNAs to make the noise floor change by
17 dB (or more) when the cable from the antenna is replaced
by dummy loads. For both of your systems. Use the soundcard 
volume control to not make the stronger signal from the IQ+ 
saturate your jt65 decoder.

73

Leif





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