[amsat-bb] Re: sun noise and other natural noise sources

i8cvs domenico.i8cvs at tin.it
Thu Sep 4 10:41:03 PDT 2008

----- Original Message -----
From: "Michael Heim" <kd0ar at sbcglobal.net>
To: <AMSAT-BB at amsat.org>
Sent: Wednesday, September 03, 2008 2:06 AM
Subject: [amsat-bb] sun noise and other natural noise sources

> I am working on an S-band receive setup.  I currently operate X band
> terrestrial, and used sun noise as a test of my receive equipment there.
> It is quite weak on 10 GHz.  Noise from the earth (aimed at the ground) is
> many times stronger.  Aiming at trees, buildings, etc produce substantial
> noise on 10 GHz.

Hi Michael, KD0AR

At 10 GHz you can use with easy your antenna to receive the Sun Noise
or the noise generated by the temperature of the ground or by the
temperature of the trees and buildings and why not by the human body.

You don't mention the size of your dish but supposing that the diameter
is 60 cm or 2 ft like my own dish then the gain at 10 GHz considering 50%
efficiency is 33 dBi and if the overall Noise Figure of your receiver is
1 dB equivalent to 75 kelvin it follow that the receiving System Temperature
Tsys is due by the sum of the following sources of noise

-Temperature of the area at wich the antenna is looking  that is 5 kelvin
  for the "cold sky" at 10 GHz

-Temperature resulting from the side lobes looking at the ground and ohmic
  losses estimating about 10 kelvin for that size of dish

-Temperature of the receiver = 75 kelvin for a receiver with an overall
  Noise Figure  = 1 dB

If you point the antenna at the cold sky the noise radiated by the sky at
5 kelvin fills completely the aperture area of your dish and the total
System Temperature becomes: Tsky = 5+10+75 = 90 kelvin

Be aware that 90 kelvin is the equivalent noise temperature of the
"radiation resistance" of your dish no matter the physical temperature of
the antenna.

If you now point your dish in direction of the ground or of a big wall at a
room temperature of say 290 kelvin corresponding to +17 degrees
centigrade then the noise generated at 10 GHz fills completely the aperture
area of your dish and the System Temperature becomes
Tsys= 290+10+75 = 375 kelvin

The ratio 375 / 90 = 4.16  or 6.2 dB represents the increase of signal
level due of noise when the dish is moved  from the cold sky to the

If you point your dish to a human body at +36 degrees centigrades or
309 kelvin and if  the body fills complely the aperture of the dish then
the System Temperature Tsys = 309+10+75 = 394 kelvin

The ratio cold sky / human body is now 394 / 90 = 4.37 or 6.4 dB
and if you transmit then the human body becomes a "dummy load"

We satellite users or EME'rs are mainly interested to calculate and to
measure the increase of noise level or the Sun Noise when the antenna
is pointed at the Sun because it figure out the real performance of
any receiving system

To do so we look at the NOAA pages:



and we select the Solar Flux Unit at 2800 MHz (10.7 cm)

For day 3 september 2008 the Solar Flux is 66 sfu at 2800 MHz (10.7 cm)
as measured by the NOAA stations of Learmonth or Penticton.

Then we go to the ARRL UHF / MICROWAVE Experimenter's Manual article
             "Noise Temperature, Antenna Temperature and Sun Noise"
written by Bob Atkins, KA1GT and with the aid of the diagram in Fig-2 we
convert the value of sfu at 2800 MHz (10.7 cm) into the corresponding value
at 10368 MHz resulting 240 sfu

The equation (5) at page-7-58  calculates the Antenna Temperature Ta of the
antenna when pointed at the Sun

             F G L
  Ta = ---------              [kelvin]


Ta = Antenna Temperature looking at the Sun in kelvin
F   = Solar Flux 2800 MHz (10.7 cm) converted into the amateur bands by
         diagram in Fig-2
G   = Antenna gain as a ratio (that is, 13 dB = 20 or 33 dB = 2000 )
L    = wavelenght in meters.

                                       240 x 2000 x  0.000838
In our situation:  Ta = ------------------------------- = 116  kelvin

and Tsys = 116 + 5 + 10 + 75 = 206 kelvin

The ratio 206 / 90 = 2.29 or + 3.6 dB represent the Sun Noise received
with the antenna pointed at the Sun with reference to the antenna pointed
at a quite direction of the sky called "Cold Sky"

The above Sun Noise = + 3.6 dB computed with the equation (5) match
well with the more precise value supplied by the software NOISE.EXE
because using the same parameters and option F1 we get a Sun Noise
of + 4.28 dB with a range of uncertainty between + 3.96 to + 4.58 dB

>  The question I have is... Can I use the same techniques on 2.4 GHz?   Of
>  course antenna gain is lower, but noise figure should be fairly close -
>  1dB or so....
Yes you can use the same technique for 2,4 GHz

> I have 2 units for 2.4 I'm testing.  one has an integral yagi antenna,
> supposedly about 16dB gain.  The other has an N connector, of which I have
> not designed an antenna for it yet.
16 dB gain is a too low gain to receive Sun Noise with the actual Solar Flux
of 66 sfu at 2800 MHz
Using 1.2 meter dish with a gain of  27 dBi at 50 % efficiency and a
receiver with an overall noise temperature of 72 kelvin (about 1 dB Noise
Figure ) and 66 sfu I actually receive only 3.0 to 3.5 dB of Sun Noise

> So, the question is... is sun noise stronger, weaker on 2.4 than on 10
> ghz?

The Solar Flux at 2.4 GHz is weaker than on 10 GHz

> If equipment is functioning properly should I be able to detect the
> sun?

Using an antenna with a gain of only 16 dBi and analog receivers it is
impossible to receive the Sun Noise

> I actually didnt think about trying to hear a tree on it, but they
> should also be noise sources on that band as well.
With the antenna looking at the ground you should receive about 3 dB of
noise against the noise received from the cold sky as the following
calculation shown.
It is difficult to say if you can receive the noise from the trees or not
because it depends on the density and umidity of the foliage.

>  Michael Heim

Using the above procedure or the software NOISE.EXE we can now compute
the Sun Noise received by the 2.4 GHz system for wich you are actually


- Solar Flux Units = 66 sfu at 2800 MHz converted to 65 sfu at 2400 MHz
- Antenna gain 16 dBi = 40 in power ratio
- Cold Sky Temperature at 2400 MHz  = 50 kelving
- Temperature due to side lobes and losses  = 60 kelvin
- Temperature of receiver with NF= 1 dB = 75 kelvin

With the antenna looking at the cold Sky: Tsys = 50+60+75 = 185 kelvin

With the antenna looking at the ground Tsys = 290+75 = 365 kelvin

The noise received by the ground against the noise received from the cold
sky is 365 / 185 = 1.87 or 2.9 dB

With the antenna looking at the sun the antenna temperature Ta becomes

           65 x 40 x 0.0156
 Ta = ----------------------- = 11.7  kelvin

With the antenna looking at the Sun Tsys = 11.7+50+60+75 = 196.7 kelvin

Sun Noise = 196.7 / 185 = 1.06 = + 0.26 dB

The software NOISE.EXE is showing 0.3 dB of Sun Noise using the same

Best 73" de

i8CVS Domenico

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