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Re: New Preamp



No doubt, making a front end better can be done.  I don't remember saying it
couldn't be done.  Maybe I am way off base here.  And, perhaps I should have
said, it is difficult to increase the overall performance of a given
receiver.  Not impossible....just difficult.

I don't disagree for a minute your and Domenico's comments.  On paper, it
works great.  In practice is often something totally different.  Your final
comment, "caveat emptor" is really what it is all about.  If it works, do
it.  The end result, no matter what, is to somehow dig out enough signal
above the noise floor so that your demodulator can have something to chew
on.  If overdriving or underdriving or whatever works to that end, nothing
else matters.

Adding a preamp in my case is a total disaster.  I could probably put a
diode in my coax line and then charge batteries with the resulting DC.

The only thing I would add to your comments is that to amplify signals
before an existing front, with a given antenna and feed line, can't help but
have a negative impact somewhere else.  Maybe that negative is no big deal.
The less stages of anything before demodulation, the better chances you have
at a super receive chain.  And with another stage of amplification or a
preamp, certainly you just have to hope that no strong signal will come up
anywhere nearby in frequency or distance to what your receiver is set to.
What about dynamic range....wouldn't a preamp toss that into the toilet?
Again, maybe it is no big deal to the end you are after.

My advice about advice is this:  Listen to others, study and learn about it
yourself, and then go do whatever you want.  In the world of radio, no
amount of paperwork, planning and talking beats trying it to see if it
works.  I have several preamps of all kinds and I use them whenever
possible, but I realize their limitations.

Finally, to those who sent some perty nasty comments via private mail re
this thread...Gee...I am sorry.  Really I am.  I had no idea my comments
were so flammable.  I'm betting that stealing your wife or girlfriend
(boyfriend?) would not be as traumatic?  What can I say?  I didn't mean to
cause trouble.  To me, this is a hobby.  A fun pastime.  I promise, I won't
make comments again.  I rarely do make comments on this board...because
every time I do, there are those who take it so seriously.  Geezzooo!

73!

Gary, N7BRJ/DA1BRJ



  

-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Edward Cole
Sent: Wednesday, September 19, 2007 8:19 AM
To: i8cvs; Gary Memory; 'Amsat-BB'
Subject: [amsat-bb] Re: New Preamp

 
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

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