[amsat-bb] Re: Pre amp question

i8cvs domenico.i8cvs at tin.it
Wed Dec 13 11:55:50 PST 2006

Hi Steve, N2JDQ

If your receiver has the capability to switch OFF the AGC than the following
simple test will tell you if your preamplifier connected to the actual
antenna is working or not for you:

1) Switch OFF the AGC, power OFF the preamplifier and tune the receiver
     in a free frequency to receive only the white noise

2) Connect an AC voltmeter (5 volt f.s) to the high impedance output of
     your RX audio hadphone jack

3) Switch the mode to CW or SSB wich uses the product detector wich is

4) Reduce the RF gain to minimum

5) Increase the Audio gain to maximum

6) Aim your antenna at 90° elevation to the cold sky and adjust the RF gain
     until you read  1 volt due to the white noise and call this voltage V1

7) Without changing anything aim your antenna to the horizon at 0° elevation
     where half of the antenna lobe should now pick up more ground noise and
     read the new voltage and call this voltage V2

8) Make the ratio V2/V1 wich is your (S+N)/N with preamplifier OFF

Repeat the above steps with the preamplifier ON

If the ratio V2/V1 with preamplifirer ON is a little bit greater than the
ratio with the preamplifier OFF than that preamplifier is usefull othervise
if the ratio is unchanged it is not usefull because or the noise figure of
the preamplifier is too high or your antenna temperature is too high
because the antenna pick up too much thermal noise due to side lobes or
pick up man made noise particularly in urban or suburban areas particularly
if the frequency is 144 MHz or belove.

If you live in an area with high density of FM and TV transmitters located
in top of a hill (my situation) you can get the phase noise of the above
transmitters as a reference signal because the phase noise of their
oscillators extends well in to the amateur band in form of a withe noise
( moving the S meter to me in 70 cm at 10 km distance from the top hill )

If you measure V2 with the antenna aimed toward the source of phase noise
(the hill) and than V1 with the antenna aimed to the cold sky 90° elevation
than you can better evaluate the ratio V2/V1 with preamplifier ON and
preamplifier OFF

I don't mention the possibility to use the sun for the above evaluation
because using small gain antennas for satellite communications in to 2
meters and 70 cm the sun noise is not well detectable over the noise of a
receiving system.

Greatest is the ratio V2/V1 with preamplifier ON and lower is the overall
noise figure of your receiving system or lower is your antenna temperature
or both togheter.

For the evaluation of a receiver when is antenna connected is the ratio
V2/V1 i.e. the swing between V2 and V1 that count.

73" de

i8CVS Domenico

----- Original Message -----
From: "Steve Raas" <sraas at optonline.net>
To: <amsat-bb at AMSAT.Org>
Sent: Wednesday, December 13, 2006 3:30 AM
Subject: [amsat-bb] Pre amp question

> 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
> 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.
> I really want to do this the correct way.. and if this thing is un-fixable
> junk.. its going in the junk drawer fast. I don't want to configure rx
> system around sompthing that wont produce results.
> I'd like to thank everyone for their input so far.. Ive learned a lot w/o
> even Transmitting and R.F. yet!
> -Steve Raas
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