[amsat-bb] Re: Pre amp question

kc6uqh kc6uqh at cox.net
Tue Dec 12 23:12:21 PST 2006


The preamp is working fine. The earth is at 290 degrees Kelvin. Unless the 
antenna sees only cold sky and no earth, trees & buildings you will have 3dB 
of noise going into your preamp. If your radio has a 1 dB noise figure and 
your preamp has a 1 dB noise figure no improvement of S/N is noticed only 
the level has changed by the gain of the preamp. Preamps only help to cover 
cable loss and poor receiver sensitivity. Low noise figures ( less than 3 
dB) are only achieved when the antenna is pointed into space and has a good 
front to back ratio.
Frequencies below 70 cm are subject to galaixy noise that becomes larger as 
frequency goes lower. expect a 3-4 dB increase at 2M, no noise reduction 
from cold sky @ 2 M and longer wave lengths.


----- Original Message ----- 
From: "Edward R. Cole" <al7eb at acsalaska.net>
To: "Steve Raas" <sraas at optonline.net>; <amsat-bb at amsat.org>
Sent: Tuesday, December 12, 2006 9:37 PM
Subject: [amsat-bb] 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 
>>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 
>>go out on a limb here and assume that its either:
>>a)       an incredibly broad band pre-amp with a completely fictitious ' 
>>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 
>>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
> =========================================
> BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
> Amsat #3212
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