David Anderson david at gm4jjj.co.uk
Thu Nov 3 22:27:02 CET 2016


I am trying to get my head around the test you propose with comparing the
noise floor of the receiver with a dummy load and that with the preamp and
antenna connected to the same receiver.

What if any assumption is made of the noise figure of the preamp or the
antenna noise level?

Many thanks,

David GM4JJJ

On Thu, 3 Nov 2016 at 02:37, Leif Asbrink <leif at sm5bsz.com> 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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