[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: New Preamp



Thanks to Dom and Ed.

>From you two gentlemen, a few words please on field proofing antenna
systems using sun noise and a EME signal should add nicely to this
discussion.  

Joe  K0VTY
Amsat #860
===========================
On Tue, 18 Sep 2007 22:18:32 -0800 Edward Cole <kl7uw@acsalaska.net>
writes:
> Thanks Domenico,
> 
> I was about to insert my comments to Gary.
> 
> I would challenge his belief that manufacturers achieve better NF, 
> as 
> the evidence shows the contrary.  If you can show me a commercial 
> radio with 0.5 dBNF on the 150-MHz band, then I would love to know 
> it.  Almost all commercial two-way radios have a sensitivity of 
> about 
> 0.15 to 0.25 uV at 15-KHz BW.  This will result in about MDS of > 
> -124 dBm and a NF well over 3-dB.  Commercial equipment is designed 
> 
> for immunity from high RF/noise urban environments and that trades 
> off noise figure in the process (commercial radios are designed for 
> 
> strong signals - hams* are the crazy weak-signal nuts!)  *and a few 
> 
> weird radio astronomers, NASA engineers, ....
> 
> My 2m eme station has a sensitivity of -147.5 dBm with 2.2 KHz SSB 
> BW; that is a receiver temp= 58K or NF=0.79 dB.  This is assuming 
> 0.5 
> dB loss ahead of the preamp.  A good ham radio VHF satellite 
> receiver 
> will be about 100K or NF=1.2 dB with a sensitivity of -145 dBm at 
> 2.2 
> KHz SSB BW.
> 
> Now if you add sky noise, industrial noise, and antenna noise, the 
> system sensitivity (Te) will suffer:  Te = Tr+Tsky+Tant+Tindustrial
> e.g. using Tsky = 210K, Tant = 45K (very good low sidelobe eme class 
> 
> antenna), and no man-made noise (my situation):
> Te = 313K, and Pn = -140 dBm  (note no antenna gain is included in 
> this number)
> so my environment cost me about 7.5 dB in sensitivity.  Obviously if 
> 
> you have several hundred degrees of industrial noise then things do 
> 
> get worse which will minimize the advantage of the low NF 
> preamp.  Here the use of well engineered filters may help.  Preamps 
> 
> with better strong signal characteristics (though a bit higher NF) 
> may also help.
> 
> Tradeoffs of NF vs gain are made in low noise amplifiers (preamps) 
> usually in favor of low NF.  As long as there is sufficient gain to 
> overcome the higher NF of the following receiver the overall system 
> 
> will benefit.  Usually this means a min of about 16 dB gain.  This 
> will lower the noise contribution of the following circuits by 
> 1/40.
> 
> For the majority, a good low-noise preamp mounted at the antenna 
> will 
> result in significant increase in hearing ability.
> caveat emptor
> 
> 73 Ed
> 
> At 12:33 PM 9/18/2007, i8cvs wrote:
> >----- Original Message -----
> >From: "Gary Memory" <gmemory@tks-net.com>
> >To: "'i8cvs'" <domenico.i8cvs@tin.it>; "'Amsat-BB'" 
> <amsat-bb@amsat.org>
> >Sent: Tuesday, September 18, 2007 8:26 PM
> >Subject: RE: [amsat-bb] Re: New Preamp
> >
> > > Domenico:
> > >
> > > Greetings once again.  You sent me a note some time ago that 
> helped to
> > > amplify my current belief concerning SNR.
> > >
> > > I am willing to bet two things.
> > >
> > > First, it is difficult (not impossible, just difficult) to 
> obtain a better
> > > front end noise figure than what the original receiver 
> manufacturer has
> > > created.
> >
> >Hi Gary, N7BRJ/DA1BRJ
> >
> >This is why we use a low noise preamplifiers antenna mounted.
> >Make an example:
> >The noise figure of a commercial receiver like a TS-736 is about 
> NF=6.5 dB
> >corresponding to an equivalent noise temperature T1= 1005 kelvin
> >If you add a low noise preamplifier antenna mounted with a NF= 0.5 
> dB and
> >if the overall noise figure of the system is degradated to NF= 0.7 
> dB
> >corresponding to T2 = 51 kelvin then using the same antenna the 
> improvement
> >of S/N using the above preamplifier is: 10 log     (1005/51) = 13 
> dB
> >                                                                    
>       10
> >
> >   Almost certainly the addition of an outboard amp will add more
> > > noise.  And this is not always a bad thing.
> >
> >In the above example the more noise that you see on your S-meter is 
> only
> >apparent because the gain of the system probably is too great but 
> you can
> >reduce the noise reading of the S-meter without reducing the S/N 
> ratio
> >adding an attenuator and this is particularly easy to do if you use 
> a
> >preamplifier + downconverter + attenuator + IF receiver
> >
> > >  A preamp can be a great
> > > addition despite the addition of more noise.
> >
> >If you get more noise degrading the S/N it meens only that the NF 
> of your
> >preamplifier is greater then the NF of your receiver or its gain is 
> too low.
> >
> > > But then you get into a curve
> > > of cost vs true value of gain.  Is it worth it?  Maybe so.
> >
> >Adding a low noise preamplifier with the capability of improving 
> the S/N
> >ratio is always wortwhile.
> >
> > > Second, it is very unlikely that maximum signal strength will be 
> at the
> > > same
> > > point as maximum signal to noise ratio (SNR).
> >
> >The best noise figure NF of a preamplifier cannot correspond to the 
> maximum
> >gain as the S-parameters of a device for the lowest NF are 
> showing.
> >
> > >  Again, maybe this is not a
> > > bad thing in any one individual case.  Almost certainly the best 
> SNR will
> > > be
> > > at a point of gain somewhat less than maximum.
> >
> >The important of the point is that even if the lowest noise factor 
> F1
> >do not correspond to the maximum gain the gain G1 obtained from it 
> be
> >sufficient to get a satisfactory overall noise factor Ft of the 
> system as
> >the formula for more stages in series is showing:
> >
> >                   F2-1       F3-1
> >Ft = F1 + --------+ --------- + ..................
> >                    G1       G1xG2
> >
> > > I work with receivers and preamps that cost dozens of thousands 
> of
> > > dollars.
> > > Cost and receive noise figure is almost a logarithmic curve.  
> And it seems
> > > that only with the very best front ends are SNR and gain at the 
> same
> > > point.
> >
> >This is not strictly mandatory for as Radio Amateurs.
> >
> > >
> > > Once you get the best receiver, coax and antenna you can afford, 
> that is
> > > all
> > > you can do....which seems like a dumb and obvious thing to say.  
> Beyond
> > > that, true magic is hard to come by.  And I am not knocking the 
> preamp.
> > > Again, I own and use them all the time.
> >
> >Once I get the best receiver and the best antenna I must reduce the 
> thermal
> >noise of the coax cable wich noise factor is F1 in the above 
> formula so that
> >to improve the S/N ratio I have to add a low noise preamplifier in 
> F1 with
> >the maximum possible gain in G1
> > >
> > > 73!
> > >
> > > Gary, N7BRJ/DA1BRJ
> > >
> >Best 73" de
> >
> >i8CVS Domenico
> >
> >
> >
> >_______________________________________________
> >Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the 
> author.
> >Not an AMSAT-NA member? Join now to support the amateur satellite 
> program!
> >Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
> 
> 73,
> Ed - KL7UW
> ======================================
>   BP40IQ   50-MHz - 10-GHz   www.kl7uw.com
> 144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
> DUBUS Magazine USA Rep dubususa@hotmail.com
> ====================================== 
> 
> _______________________________________________
> Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the 
> author.
> Not an AMSAT-NA member? Join now to support the amateur satellite 
> program!
> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
> 
> 
_______________________________________________
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb



AMSAT Top AMSAT Home