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Re: My preamp coming out...



Hasan and Luc (and anyone else interested):

Of course what you are saying is correct, Hasan, but before you beat up Luc
too much let me make a statement:
"If its a choice of no preamp or a preamp in the shack, then by all means
put it in the shack".  

I had one of my 2m eme preamps go bad back a year or so in the winter...not
a good time in Alaska for tower climbing.  I had a spare 1.0 dB NF preamp
so I rigged up a T/R relay on shack end of my transmission line and
connected my preamp between my FT-847 and the Rx port of the relay.  It
worked not that badly and I made several eme contacts even though my
overall NF ~ 2.8 dB (My normal eme preamp is a 0.15 dB NF and that results
in a overall NF = 0.74 dB).  The difference in minimum detectable signal
was something like 1.5 dB so many extremely weak signal stations would not
be heard, but many not so weak signals were copyable.

Many of you probably are not aware how bad the NF is of your commercial
rigs.  The FT-847 has a noise figure of 14-dB on 2m (12-dB NF with internal
preamp turned on).  So even putting a mediocre preamp in the shack will
improve your overall NF significantly.

Gene's spreadsheet is great.  Let my offer another one:
http://www.qsl.net/al7eb/emelink.xls

Part-One calculates your receiving system noise figure, noise temp, and
minimum detectable signal (sometimes called the noise floor of your
receiving system).  The rest of the spreadsheet deals with calculating
signal levels for moon bounce (eme).

I do agree that playing with one of these programs will make you aware of
just how big a roll NF has in receiving performance.

Finally, to Luc...welcome aboard the preamp "club".  Once you have tasted
the difference there is no going back!

As a wise old ham once said: "you can't work them if you can't hear them"!

73's Ed - KL7UW



At 03:56 PM 3/22/2006 -0600, hasan schiers wrote:
>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!
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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