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Re: My preamp coming out...



Luc,

Here some numbers:
15-foot of RG-213, Loss (435-MHz) = 0.7 dB
60-foot of LMR-400, Loss (435-MHz) = 1.6 dB

Assumptions:
Preamp NF = 1.0 dB
Gain = 20 dB
FT-847 NF = 12 dB
FM bandwidth = 15 KHz
Tsky = 75K
Tant = 50K

Results:
1) Preamp at antenna followed by RG-213 and LMR-400 to FT-847:  systemNF =
1.8 dB, MDS = -132.4 dBm
2)  Antenna followed by RG-213, preamp, LMR-400 to FT-847:  systemNF = 2.4
dB, MDS = - 131.5 dBm
3)  Antenna floowed by RG-213, LMR-400, preamp to FT-847:  systemNF = 3.78
dB, MDS = -129.6 dBm
*MDS is the minimum detectable signal strength.

So placing the preamp in the shack will result in 2.8 dB less sensitivity.

Here are two things to check:
What does your s-meter read when you turn on the preamp with no signal (how
much noise does it show).
What does your s-meter show when you are hearing the satellite at the
horizon and then overhead?

>From your description the preamp is working pretty good for you.  You now
hear signals that you did not hear before and their s-meter readings are
much higher with the preamp.

What you may see if you put the preamp at the antenna is less s-meter
reading with only noise (no signal).  That would be good since you do not
want to amplify the noise (only the signal).

I should caution that there are many situations other than just line loss,
that can affect your performance.  Some have already been mentioned in
reply to your original post.

But I summarize that a preamp in the shack is better than no preamp at all
(in general).
Being a ham one can experiment to find the best set up...have fun!

73's Ed - KL7UW

At 05:46 PM 3/22/2006 -0500, Luc Leblanc VE2DWE wrote:
>On 22 Mar 2006 at 15:56, 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
>
>Hi Hasan
>
>Here is what i'm just telling in another e-mail. If it is valid for my
antenna it should probably 
>be the case for other antennas.
>
>That's another thing who puzzled me. When there is no signal the S meter
does not
>show anything 
>exactly the opposite of my GRE wide band preamp who indicate a near S9
signal of
>plain noise upon 
>switching.With the Hamtronic at AOS the S meter rise from no indication to
2 or 3 S
>units to the 
>+60 and he decrease at the same rate until LOS. I am using about 60 feet
of LMR-400
>and a short run 
>of rg-213 15 feet to the dish. I will try to install the preamp directly
at the feed
>point and at 
>the junction point of the 2 coax in a way to keep the preamp away of the
vhf beam
>radiation. At the 
>feed point the preamp will be at 5 inches from the VHF last reflector.
>
>I just added this at 450 there is a TV station control studio link at
70KM.Without the preamp their 
>signal was just barely audible when i switched the preamp the signal rise
near S9. I understand 
>there is a difference between theory and real case but what i failed to
understand is what benefit 
>i will gain in my case?
>
>Probably i will need an even weaker signal to demonstrate what you are
telling and to be able to 
>test it i will probably need an EME like signal to be able to effectively
tested it!
>
>But as a "casual" satellite user the preamp in the shack do the job as on
many low pass it was 
>impossible to work SO-50.
>
>Summer is coming and more time will be available here to make more test.
>
>Thank's for your inputs
>
>Luc Leblanc VE2DWE
>AMSAT 33583
>Skype VE2DWE
>www.qsl.net/ve2dwe 
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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> 
73's,
Ed - KL7UW  
=========================================
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
=========================================
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