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Re: Fw: SUISAT forensics...The reverse procedure



Hi Ed, KL7UW

Your Tsys = Tsky + Tgnd + Trx

You know Trx because you can measure the overall Noise Figure of your
system.

You don't know exactly Tgnd because it depends from the magnitude of the
side lobes of your antenna looking at the groung depending on Elevation.

Your Tsky depends on where the antenna is looking at  the sky.

I use a very simple metod to measure if my Tsky + Tgnd  is above or is
belove 290 kelvin

1) Disconnect the antenna from the preamplifier input
2) Connect a 50 ohm termination to preamplifier input

The 50 ohm termination is at room temperature nominally 290 kelvin and
replaces Tsky + Tgnd

3) Disconnect the AGC of your RX and use CW or SSB mode because
     the product detector is linear.
4) Reduce to minimum the RF gain
5) Put to maximum  the Audio Gain

Connect a 5 volts f.s. voltmeter to the audio output of your RX preferably
to a high impedance point.

With the 50 ohm termination connected to preamplifier input increase the RF
gain of your RX until you read a reference voltage of say 5 volt and call
this voltage V1

Now without changing the RF gain of your RX disconnect the 50 ohm
termination and reconnect the antenna.

Move the antenna in the sky  and since the voltage is changed read the new
voltage and call this voltage V2

If V2 il less than V1 than your Tsky + Tgnd is less than 290 kelvin and the
reverse is true.

If you want to know how many dB your Tsky + Tgnd  is greater or less than
290 kelvin compute

20 log    (V1/V2)
          10

Since connect and disconnect the 50 ohm termination from the preamplifier
is tedious I have a low loss coax relay antenna mounted

Any calibrated S-meter is unusefull for accurate measurements and even a
very good voltmeter with a large scale is not accurate enough.

For the above reason I use a well know linear dB meter built over an
Analog Device chip AD 636 or alternatively a G3WDG 144 MHz
Wideband Noise Amplifier with an HP 435A power meter for maximum
accuracy.

In a separate email I have sent the electrical diagrams to you.

Obviously Tsky + Tgnd  mostly depends from the site in wich you are
located and in my case the bay of  Naples is very hot so that when the
preamplifier input is relay switched from the 50 ohm termination to the
2 meters antenna looking at the sky there are not sky places in wich
Tsky + Tgnd are less than 290 kelvin but mostly they are in the range
of 1500 kelvin and you probably in Alaska are living in a peaceful
paradise.


Best 73" de

i8CVS Domenico

----- Original Message -----
From: "Edward R. Cole" <al7eb@acsalaska.net>
To: "John B. Stephensen" <kd6ozh@comcast.net>; "AMSAT-BB"
<amsat-bb@amsat.org>
Sent: Thursday, February 09, 2006 4:43 PM
Subject: Re: Fw: [amsat-bb] SUISAT forensics...The reverse procedure


> John,
>
> Unfortunately, the bulletin board cannot view the the sky noise chart that
> you sent me.  Also, unfortunate that it is largely useless for frequencies
> below 1GHz (obviously designed to show the celestial noise "window"
between
> 1 and 10 GHz (roughly).  I would point you to fig. 8-4 and 8-6. Radio
> Astronomy-2nd Ed., Kraus -1966.  In the first figure it shows many areas
of
> the sky at 200-MHz (close enough for use at 144-MHz) where sky temp<200K.
> Even along the Galactic Plane.  The second figure is less detailed but
> shows the spectrum of sky temp is approx = ambient temp of 290K away from
> the Galactic center.
>
> I could cite several Moon tracking programs that predict sky temperature
> background.  They can show sky temps as low as 215K at 144-MHz.  I use
this
> antennas system to track the Moon for eme and am well award of sky
> temperature.  I disagree that I have to assume that I was looking at the
> Galactic center where Tsky=1200K or even 600K.  If Tsky were that high for
> 144-MHz, in general, then use of low noise preamps below 3 dB (290K) would
> be unnecessary.  Think about it!
>
> This does imply that the main-beam is clear of the horizon or other
> terrestrial thermal radiators.  I allowed that my beam was close to the
> horizon by using 275K and if you insist I will use 290K which will not
> alter the results substantially.
>
> respectfully,
> Ed - KL7UW
>
> At 09:42 AM 2/9/2006 +0000, John B. Stephensen wrote:
> >Tsky is probably twice earth thermal noise levels at 144 MHz due to
galactic
> >noise.
> >
> >73,
> >
> >John
> >KD6OZH
> >
>
> 73's,
> Ed - KL7UW
> =========================================
> http://www.qsl.net/al7eb - BP40iq
> 144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
> 432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
> =========================================
> ----
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----
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