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Re: Pre amp question



At 09:30 PM 12/12/2006 -0500, Steve Raas wrote:
>
>As I prepare for getting on the birds.. I decided to test out this AG-25
>preamp that came with my new radio. Now my experience with preamps is
>limited, but this one is rather baffling me. 
>
>I connected everything in line.. and powered up the preamp and at 1st glance
>it appeared that it worked fine, however after about 10 seconds I started
>questioning a few things.  
>
>#1 My noise floor went up 10db ( I've never seen this before I guess my
>experience with preamps is just the good ones ) 
>
>#2 My Received signal quality was unchanged all it did was make the signal
>'appear' stronger on the meter.
>
>Now I know if the preamp was dead.. I would hear nothing.. so I am going to
>go out on a limb here and assume that its either: 
>
>a)       an incredibly broad band pre-amp with a completely fictitious ' Low
>noise floor ' claim or
>
>b)       Detuned
>
> I guess what Im going after is a few things with this email .. has any one
>ever taken one of these AG-25's and given it a proper tuning? Or is this
>what I should expect with this particular item? 
>
>In my previous experience (terrestrial weak sig stuff) a decent preamp may
>up the noise floor from a s-meter reading of mabey 0 or 1 to a s5, but with
>a significant increase in received signal gain. and intelligibility.

Steve,

My guess is that the stock ICOM preamp is not especially low-noise.
Typically, they are more like 1.5 to 3 dB NF and that would give the
results you see...more gain but little quieting.

Years ago I ran a nuvistor preamp (3-dB NF with 15-dB gain) and all it
mainly did was add gain (made signals louder including the noise).

Now if you have the preamp installed in the shack next to the radio with
lots of coax up to the antenna, you will get less low-noise performance
(even if it is a good low noise amplifier).

Finally, have you checked receiving weak signals?  Testing with strong
signals will not show much difference other than higher s-meter reading. 
73's,
Ed - KL7UW 
=========================================
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Amsat #3212
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