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Re: My preamp coming out...



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 
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