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Re: My preamp coming out...



Hi Dominico, ( I hope I'm replying to a legitimate post by Dominico...given 
the advanced nature of Dom's postings, I find it hard to believe he would 
not understand the difference between Noise Figure and Gain.) I hope I'm not 
feeding a troll, who somehow hijacked I8CVS's email address.


Gain is input signal level vs. output signal level.

Noise figure is a measure of the internally generated noise of the preamp 
device.

Noise Figure is much more important than gain at vhf/uhf to improve receive 
sensitivity. All the "noisy" gain in the world will not help receive 
sensitivity at all.

What I'm saying is very simple: Low noise preamps only need moderate gain to 
set the system noise figure. A preamp without a high noise figure will not 
improve sensitivity in the slightest, no matter how much gain it has.

The proper place for a low noise preamp is before all other losses ahead of 
it (at the antenna, in other words). Of course if there are overload or 
intermodulation problems, then some filtering may be placed ahead of the 
preamp. Whatever you put between the antenna and the preamp degrades the 
noise figure of the preamp by adding directly to it. There is no way around 
it. Coax connectors, coax jumpers or runs of coax to the house (if the 
preamp is in the house) all compromise the NF that the preamp is trying to 
establish.

Typical preamps have a gain of 15 to 20 dB. These same preamps have noise 
figures associated with them. A 20 dB gain preamp with a noise figure of 5 
dB will add very little to sensitivity of a receiver unless it (the 
receiver) has a terrible noise figure. It has lots of gain, but it is so 
"noisy" that the net sensitivity is not improved (due to the internal noise 
of the preamp).

The function of a preamp at vhf/uhf is to "set" the system noise figure. 
This is accomplished by putting a very low noise figure (< 0.5 dB) preamp at 
the antenna, and having this preamp have enough gain to overcome the losses 
that follow it (coax). It should have no more gain than what is required to 
permit the low noise figure of the preamp "dominate" or "swamp" the poor 
noise figure of the following coax and receiver front end.

Playing with the aforementioned spreadsheet will demonstrate precisely what 
I'm saying far better than trying to describe it. Put a low noise preamp at 
the antenna (in the spreadsheet). Change it's noise figure and gain...watch 
what happens to your receive sensitivity. Put a 100' run of coax ahead of 
the preamp and recalculate. Put that same 100' run of coax AFTER the preamp 
and notice how much your receive sensitivity improves.

All of this, of course, assumes normal coax with a loss of perhaps 2.8 dB 
per 100'. If you can afford super low loss coax, then you can put the preamp 
in the shack. Just keep in mind, whatever coax loss you put in front of the 
preamp DIRECTLY adds to the noise figure of the preamp. So a fancy 0.5 dB NF 
preamp with 100 feet of LMR-400 in front of it now has a NF of 0.5 dB + 2.5 
dB or 3.0 dB...which is very very poor  for a good gasfet preamp.

That's about as well as I can explain it.

73,

...hasan, N0AN

----- Original Message ----- 
From: "i8cvs" <domenico.i8cvs@tin.it>
To: "hasan schiers" <schiers@netins.net>; "Luc Leblanc VE2DWE" 
<lucleblanc6@videotron.ca>; <amsat-bb@amsat.org>
Sent: Wednesday, March 22, 2006 4:41 PM
Subject: Re: [amsat-bb] My preamp coming out...


> Hi Hasan, N0AN
>
> If I know what is the difference between Noise Figure and Gain I can
> understand your explanation otherwise I cannot.
>
> Try to explain it in simple words please.
>
> Tnx
>
> 73" de
>
> i8CVS Domenico
>
> ----- Original Message -----
> From: "hasan schiers" <schiers@netins.net>
> To: "Luc Leblanc VE2DWE" <lucleblanc6@videotron.ca>; <amsat-bb@amsat.org>
> Sent: Wednesday, March 22, 2006 10:56 PM
> Subject: Re: [amsat-bb] My preamp coming out...
>
>
>> Luc, (if I understand your post correctly)
>>
>> 1. An S-meter change is not the appropriate measure of the effectiveness
> of
>> a preamp.
>>
>> 2. The fact that your S-meter went full scale both at the antenna and at
> the
>> radio means nothing. Absolutely nothing (other than it has gain).
>>
>> 3. The preamp was designed to have a low noise figure (let's say < 1 dB).
>> When you mount it at the antenna you get the benefit of that low noise
>> figure. When you mount it at the radio, you add the loss of the coax to
> the
>> noise figure of the preamp. Typically, you might see 2.5 dB loss in your
>> feedline at 70 cm. for normal lengths of coax (80'). Now this nice fancy
> low
>> noise preamp has a noise figure of 3.5 dB (which stinks), due to the
>> additional loss of 2.5 dB from the coax in front of it.
>>
>> Unless you have VERY low loss in your feedline run, the preamp belongs at
>> the antenna.
>>
>> For a proper analysis of this issue, it would benefit everyone to look at
>> Gene's satellite link budget analysis spreadsheet, which evaluates the
>> effects of antenna gain, sky noise, preamp noise figure and gain, 
>> feedline
>> losses, receive bandwidth and front end noise figure of the radio. Just a
>> few minutes manipulating the values in the spreadsheet and seeing their
>> effect on receive sensitivity will demonstrate convincingly WHY a preamp
>> belongs at the feedpoint, and WHY preamp gain is of only minimal
> importance
>> compared to noise figure.
>>
>> Noise figure (and overall system noise temperature) is the ONE variable
> that
>> absolutely must be paid attention to, when it comes to weak signal work.
>> Assessments based on S-meter readings are worthless and worse, often
>> misleading the casual satellite operator.
>>
>> If a person has any interest at all in learning about how your satellite
>> receive system works, and how to optimize it, Gene's spreadsheet (Excel),
> is
>> an indispensible and informative tool.
>>
>> 73,
>>
>> ....hasan, N0AN
>>
>>
>> ----- Original Message -----
>> From: "Luc Leblanc VE2DWE" <lucleblanc6@videotron.ca>
>> To: <amsat-bb@amsat.org>
>> Sent: Wednesday, March 22, 2006 1:02 PM
>> Subject: [amsat-bb] My preamp coming out...
>>
>>
>> > Yes i make my coming out...
>> >
>> > I was not a preamp fan. I was not beleiving too much in preamps virtue
> but
>> > i should now getting out
>> > the closet in admitting that i try a hamtronic LNG 432 on a SO-50 near
>> > overhead pass.
>> >
>> > Results:
>> >
>> > From a RX signal who was never indicated on the FT-847 meter i was able
> to
>> > get a S9 plus 60db on
>> > the same meter!! (still tryng to figure out the problem..) I quickly
>> > installed the preamp at the
>> > transceiver not even at the antenna and i got this result.?
>> >
>> > Many folks always recommend to install the preamp at the antenna feed
>> > point to acheive the best NF
>> > figures but when you got a full led scale i am just asking why should i
>> > put the preamp there?  to
>> > be battered by the WX ?
>> >
>> > With the official spring start yesterday it will be now possible to 
>> > make
>> > further test but it is
>> > still hard to believe how much amplification this little box can do. I
>> > have also an other one in
>> > standby on VHF just awaiting to be tested too.
>> >
>> > For those who are thinking that my FT-847 is probably deaf or dead i
> just
>> > checked it with a cushman
>> > signal generator and he is better than the Yaesu specs.
>> >
>> > Too bad for suit sat it will be an excellent source of weak signals.
>> >
>> > A former preamps unbeleiver...
>> >
>> >
>> > "-"
>> > "It is not the class of license the amateur holds, but the class of the
>> > amateur that holds the
>> > license."
>> >
>> > Luc Leblanc VE2DWE
>> > AMSAT 33583
>> > Skype VE2DWE
>> > www.qsl.net/ve2dwe
>> > ----
>> > Sent via amsat-bb@amsat.org. Opinions expressed are those of the 
>> > author.
>> > Not an AMSAT member? Join now to support the amateur satellite program!
>> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>> ----
>> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>> Not an AMSAT member? Join now to support the amateur satellite program!
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> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org 
----
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