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'"
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
10Almost 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 F3-1Ft = 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
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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 ======================================
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