[Moon-Net] IQ+ PREAMP GAIN NEEDED

serge ve1kg at eastlink.ca
Thu Nov 3 13:54:30 CET 2016


Thank you Leif & ED & all
I should have done that  long time ago. What I never understood is why WSJT 
decodes more often than MAP65 using the same audio chain. Some of the mail I 
got private mail that is shows that I am not the only one having this 
problem & most people use both MAP65 & WSJT for the same reasons & that is 
OK.
Thank you again & will keep you updated.
73, Serge VE1KG
----- Original Message ----- 
From: "Leif Asbrink" <leif at sm5bsz.com>
To: <moon-net at mailman.pe1itr.com>
Sent: Thursday, November 03, 2016 2:37 AM
Subject: Re: [Moon-Net] IQ+ PREAMP GAIN NEEDED


> 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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