[amsat-bb] Re: Pre amp question

Bruce Robertson broberts at mta.ca
Fri Dec 15 06:47:32 PST 2006

Quoting i8cvs <domenico.i8cvs at tin.it>:

> 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
>     linear.
> 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

Thanks, Dom. This is excellent helpful advice for those of us with
elevation rotors. The V2/V1 ratio with preamp on would provide a relative
benchmark to turn to when something seems 'not quite right'. For instance,
I know that ice buildup on my yagis due to freezing rain will degrade their
performance. I could get some idea of how badly the performance degrades by
comparing this 
ratio in icy conditions to the benchmark taken in ideal conditions. (This
would assume that the S component of the S+N was constant over time.
Assuming I'm using ground noise, does anyone know if this value changes
significantly with ground temperature? Transmitter phase noise as discussed
above would, of course, be beyond one's control.) 

I'm wondering if anyone knows of a reliable piece of software that might do
the voltage measurements through a computer and sound card. I've noticed
recent projects in QST that appear to entail this sort of measurement, but
I don't think any of them had software that performed this basic function.

73, Bruce VE9QRP
Bruce Robertson, 
Dept. of Classics, Mount Allison University

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