[amsat-bb] Re: New Preamp
kl7uw at acsalaska.net
Tue Sep 18 23:18:32 PDT 2007
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.
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 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
> 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
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Ed - KL7UW
BP40IQ 50-MHz - 10-GHz www.kl7uw.com
144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
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