[amsat-bb] Re: New Preamp
nate at natetech.com
Thu Sep 20 14:14:59 PDT 2007
Gary Memory wrote:
> 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.
I haven't jumped into this conversation, but really the issue is that a
lot of hams seem to want to "do it once, and never change it" or they
want someone ELSE to do the experimentation, real measurements, and WORK
for them. They want "Plug and Play"... instant gratification.
This leads to endless online debates by those who WON'T go DO, and keeps
the rest of us who HAVE played with pre-amps and the resulting
real-world complexity from even commenting.
You might see a "XYZ setup works great for me, but..." from a few
people, but not many -- when these threads come up.
The reality is, designing an RF *SYSTEM* for a particular location, site
noise floor, receiver, transmitter power level, etc... and making it
perform to the absolute best that it theoretically can -- is hard work,
that many simply aren't willing to do.
RF Engineers/Professional RF Technicians get paid big bucks to do this
level of work in non-Amateur systems, and many are also Amateurs, so
they benefit from their knowledge and experience, and know WHICH
trade-offs they'd like to make... since every particular setup of any
complete RF system, always has a few.
> 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.
Yep, you just HAVE to measure, make a theory, test that theory via
experimentation, and then wash-rinse-repeat until you're either happy
with the result, or you've run out of ideas.
> 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.
Depends on your knowledge level. Those who've "been there done that"
can always benefit from running the numbers ahead of time and planning,
while taking their experience into account.
Some *might* even share what "works for them" as some do, but your
advice is dead-on for newbies. Go try something, learn how to measure
it properly, learn the math, and get hard numbers to see how YOUR
station works out.
(And frankly, since a lot of us are "just Amateurs" one of the distinct
reasons that some of us don't pipe up and say anything is because we
don't feel comfortable with the math, or whatever reason... we're not
"classically trained" as they say, and people that are sometimes
accidentally come across as harsh when they correct mistakes in online
forums... I say accidental, because I've done it on other topics, and
didn't know I hurt someone's feelings.)
> 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!
This is what really made me want to comment. Ignore the cowards behind
their keyboards who can't be civil. So many people do this, that there
are psychological studies about how people's personalities change when
they feel "safe" from retribution behind a keyboard and a screen. In
any "normal" multiple-person conversation they'd either be shunned or
someone would hit them upside the head with a clue-bat. But anyway, it
really isn't worth worrying about them.
Let 'em be unhappy, that's their choice to be sour/mean/whatever, and
has no impact on you, if you don't allow it to. It's just an extension
of their own reality, and once you realize that, you realize that some
people live in very drab/angry/sad worlds of their own creation. Not
Back to the pre-amps and duplex operation, for those that have never
worked on them, remember that people building repeaters do exactly
that... all the time. And most add pre-amps in some fashion or another
to their repeater systems (as someone pointed out, many commercial FM
rigs only do anywhere from .2uV - .3uV sensitivity for 12dB SINAD (since
we are talking about FM at the moment, you gotta have that 12dB SINAD
reference point), and repeater owners/operators everywhere want better
receivers than THAT!
There's tons of information on the Net and in books about how to duplex
a repeater system, while maintaining high receiver sensitivity, and
pushing lots of power out the TX side... which are all similar goals to
what Satellite ops are trying to do, perhaps.
I only mention this as a reminder to those that hadn't thought about it,
that sites like http://www.repeater-builder.com and others have lots of
"basic starting point" information about that type of setup and how to
measure it... just change "repeater transmitter" to "uplink", and
"repeater receiver" to "downlink" in your mind and keep in mind that
you're often not on the same BAND in satellite operations for those two
frequencies, so duplexer/filter stuff won't be "quite right"...
But, it gives your brain a frame of reference to get your head around
the measurements the super-smart folks are doing on their RF systems.
And most of all, folks gotta remember to HAVE FUN with all this stuff...
it's not a job, it's a hobby... And it's not even more important than
the family dog, when the dog is sick and needs to go to the vet.
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