[Moon-Net] IQ+ PREAMP GAIN NEEDED

Edward R Cole kl7uw at acsalaska.net
Thu Nov 3 10:06:24 CET 2016


Leif,

I made the 50-ohm/preamp comparison by placing the 50-ohm term at the 
input of my transverter.
I measured noise floor using MAP65 audio level indicator:
channel:    50-ohm   Preamp   delta
Vpol         8.2 dB     23.5 dB   15.3 dB
Hpol         6.8 dB     25.0 dB   18.3 dB

antenna on horizon

Recently, I lowered the IF gain in my transverter for both 
channels.  Appears that they both are a little too low (at least on 
the Vpol channel).
Audio is being converted by a Delta44 and no AGC is present to front 
end of the IF dual-Rx (Elecraft K3).
Vpol preamp gain = 25.3 dB
Hpol preamp gain = 26.7 dB
I probably need to do a delta-phi calib (MAP65) to equalize gain 
between channels.

Max transverter conversion gain is 17-dB and I probably lowered it to 
under 10-dB.  IF s-meter reads S3 vs S5 before adjusting the IF gain.
2nd IF and s-meter ckt is not in use for MAP65 as I divert the 1st IF 
of the Rx to dual-channel SDR that drives the delta44.

But seems enough gain ceiling for good operation of MAP65.

73, Ed - KL7UW

At 06:37 PM 11/2/2016, Leif Asbrink wrote:
>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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73, Ed - KL7UW
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