R: [amsat-bb] Sun noise

["i8cvs" <domenico.i8cvs@xxxxxx>]

Ed  and All,

The S-meter is not a good instrument for accurate sun-noise measurements.

Tom calculation are based on the fact that he suppose to use a "true power
detector" at the output of the receiver or a square low detector in wich the
voltage reading is proportional to the power reading.

Also at the output of the receiver he is measuring a (S+N)/N in power ratio.

This means that when the antenna is directed to the cold sky he adjust
the instrument for a reading of  1 volt as a base line and when the antenna
is directed on the sun he read  3.66 volt for the worked example but since
he is measuring powers than  the ratio

3.66/1= 3.66 is the (S+N)/N ratio or Y factor= (1+2.66)= 3.66:1 as per
Toms calculations.

Now S/N = ((S+N)/N)-1 and so  3.66-1= 2.66 or 200/75 as per Tom
calculations for the equivalent temperature ratio.

The ratio (S+N)/N that we really see in our  S meter in reality is

10 log    3.66 =5.63 dB
          10

But  S/N= 10 log     2.66 = 4.25 dB
                            10

And 4.25 dB are those dB  having effect in hearing signal in our hears not
5.63 dB that we are happy to see in the instrument.

Following the above concept it is very easy to calculate that when the
power detector is showing an increment of 3 dB for  (S+N)/N in reality
we are receiving a S/N= 0 dB or a signal equal to noise even if the
pointer of the instrument is moving up by 3 dB

I don't know the metod of measurement used by the most part of us to
establish how many dB  of  (S+N)/N we are mesuring for sun-noise.

For us radio hams without a "true power detectors" i believe that the
simplest metod to measure the (S+N)/N at the audio output of our receiver
with acceptable accuracy for sun-noise is the following:

1) Switch OFF the AGC to get the audio follow  linearly the increment of
    noise

2) Switch in SSB to operate the product detector wich is a linear detector.

3) Reduce to minimum the RF gain control all CCW

4) Increase to maximum the RX audio control all CW and do not move it.

5) Connect an AC voltmeter 5 V fs or less to the audio output of receiver

6) Put the antenna to the cold sky and increase only the RF control to
    read 1 volt for the noise and call this voltage V2

7) Put the antenna on the sun  and read the new voltage and call this
     voltage V1

8) compute the (S+N)/N ratio in dB  using the following formula:

     (S+N)/N) =  20 log    (V1/V2)
                                    10

In the example worked out by Tom to get a (S+N)/N = 3.66 in power ratio
the voltage readind V2 for cold sky at the audio of receiver  or "0 dB base
line"must be ajusted to 1 volt and the voltage reading V1 with the antenna
on  the sun would be consequently  SQR 3.66 = 1.91volt or only 0.91 volt
of increment.

Than  (S+N)/N) = 20 log ( 1.91/1)=  5.62 dB

and consequently S/N= 4.25 dB

Unfortunately modern VHF receivers do not allow to switch the AGC ON/OFF but
the hold HF receivers do that.

To get  sun-noise measurements  i convert my 144-146 MHz  IF from my 2400
converter to a 28-30 MHz IF using a second converter and than i operate AO40
using an old Drake R4-C receiver with switching AGC capability and i am
happy to operate with only 11 knobs  in total while performing sun-noise
measurements giving a "new life to an old receiver"

73" de i8CVS Domenico

----- Original Message -----
From: Edward R. Cole <al7eb@ptialaska.net>
To: <w3iwi@toad.net>; 'Eric Van Offelen' <eric@vanoffelen.com>; 'Amsat-bb'
<amsat-bb@AMSAT.Org>
Sent: Sunday, December 22, 2002 3:32 AM
Subject: RE: [amsat-bb] Sun noise


> A quick comment:
>
> My 85cm offset feed dish with 0.6 dBNF/40dBG preamp directly connected to
> the feed produces about 1/2 to 3/4 s-unit rise...about the same rise I see
> from looking at the trees to cold-sky!  My s-meter is very approx. 4-5
> dB/s-unit.  I think that is close, considering how difficult making
s-meter
> reading on noise.  In other words enough rise that is definitely there,
not
> my imagination!
>
> Ed - AL7EB
>
> At 02:15 PM 12/21/2002 -0500, Tom Clark, W3IWI wrote:
> >
> >
> >> How much sun noise should i expect on 2.4 GHz with the
> >> following set-up:
> >> - 90 cm offset dish
> >> - +/- 0,2 dB estimated loss in adapters
> >> - 0,7 dB NF preamp
> >> Merry Christmas and 73's
> >> Eric
> >> EA5GIY
> >
> >Eric -- let's calculate it from first principles:
> >
> >Your dish is 90 cm and the wavelength is 13 cm. Therefore your dish has a
> >beamwidth of a bit more than 13/90 radians = 8.3 degrees. For a circular
> >aperture, Airy's criterion would predict the diffraction limited beam to
be
> >1.22 times this large, or about 10.1 degrees in diameter. To be simple,
I'll
> >call this 10 degrees.
> >
> >The radio sun at 13 cm, depending on sunspot count, is about 40,000
Kelvin
> >and is about 1 degree in diameter. Therefore the sun fills about (1/10)^2
or
> >~1% of the beam. Therefore, the sun should contribute about (1/100)*40000
=
> >400 degrees Kelvin if you dish has perfect (100%) aperture efficiency.
> >
> >Since I know nothing about the quality of the feed and dish, I will guess
> >that you actually have about 50% aperture efficiency (meaning that half
of
> >the power is in the main beam and half is scattered into sidelobes).
> >Therefore the sun probably contributes ~400/2 or about 200 Kelvin.
> >
> >You say the preamp has 0.7 db NF and you have 0.2 dB connector loss. You
do
> >not say how long the short piece of coax between the feed and the preamp
is,
> >so I will guess that it adds another 0.1 dB of loss. Therefore your real
> >Noise Figure is 0.7+0.2+0.1 = 1 dB.
> >
> >Many graphical and slide rule devices make the next calculation be
easier,
> >but we go back to first principles to find that the correspondence
between
> >Noise Figure and Temperature is
> >      T = 290 * [10^(Noise Figure/10)-1] Kelvin
> >Which, for NF=1, is 75 Kelvin as an estimate of your system temperature.
> >
> >So this says that the sun should have S/N = 200/75 = 2.66 times the noise
in
> >the receiver. Therefore if you have a true power detector on the output
of
> >the receiver and set it to 1 volt = receiver noise off the sun, it should
go
> >up from 1 volt to (1+2.66) =3.66 volts when pointing at the sun, or a
> >Yfactor of 3.66:1.
> >
> >You can check this by making the dish look at large trees with many
leaves.
> >These trees will contribute about 200-250 K of noise, but since they fill
> >the entire ~10 degree beam, they will also contribute about the same
noise
> >as we predict for the sun.
> >
> >Another idea is to have a 90 cm wide person stand in front of the dish
> >completely covering the dish. The person will contribute about 300-320
> >Kelvin of noise. If you happen to transmit, this makes the person bring a
> >new meaning to the words "dummy load" ;<}
> >
> >Hope this helped -- seasons greetings de Tom W3IWI
> >
> >----
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> >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> >
>
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