[amsat-bb] Re: New Preamp
k0vty at juno.com
Wed Sep 19 09:53:34 PDT 2007
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
On Tue, 18 Sep 2007 22:18:32 -0800 Edward Cole <kl7uw at acsalaska.net>
> Thanks Domenico,
> I was about to insert my comments to Gary.
> I would challenge his belief that manufacturers achieve better NF,
> 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
> 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
> dB loss ahead of the preamp. A good ham radio VHF satellite
> will be about 100K or NF=1.2 dB with a sensitivity of -145 dBm at
> 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
> For the majority, a good low-noise preamp mounted at the antenna
> 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 at tks-net.com>
> >To: "'i8cvs'" <domenico.i8cvs at tin.it>; "'Amsat-BB'"
> <amsat-bb at 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
> >corresponding to T2 = 51 kelvin then using the same antenna the
> >of S/N using the above preamplifier is: 10 log (1005/51) = 13
> > 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
> >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
> >adding an attenuator and this is particularly easy to do if you use
> >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
> >gain as the S-parameters of a device for the lowest NF are
> > > 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
> >do not correspond to the maximum gain the gain G1 obtained from it
> >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
> > > 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
> > > 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.
> > > that, true magic is hard to come by. And I am not knocking the
> > > Again, I own and use them all the time.
> >Once I get the best receiver and the best antenna I must reduce the
> >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 at amsat.org. Opinions expressed are those of the
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> 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 at hotmail.com
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