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11 GHz Measurements

Measurements made at around 11 GHz with satellite TV dishes and Low Noise Blocks (LNBs) were some of the first experiments made whilst learning about the practicalities of Amateur Radio Astronomy.

In fact, this is where most people start to explore the subject as this type of equipment is cheap and easily obtainable.

However apart from detecting the Sun there are few astronomical sources that are strong emitters in this waveband and Ku band TV antennas / LNBs of < 1m diameter designed to receive satellite broadcasts are not particularly sensitive.
Much has been published about developing and using this type of equipment, with articles for example on the "Ity Bitty Telescope"   [http://www.aoc.nrao.edu/epo/teachers/ittybitty/procedure.html].


                                      















                The NRAO 'Ity Bitty Telescope'                                                                Satellite TV LNBs


Equipment like this is unfortunately rather unstable, with receiver gain changing quite markedly with temperature as such stability is not important in its TV application. It is however a problem if the amateur radio astronomer wants to make measurements at different times or days and then add them together to produce a sky map for example.

It is possible to overcome this problem by carefully modifying one channel of the dual polarisation LNB, to disconnect it from the antenna horn and place a 50 ohm resistor across the receiver channel input.
Any temperature variations will show up in both channels and the signal from the resistively loaded channel can then be subtracted from the' live' channel readings.


There is an excellent series of articles available on the internet produced by L'Observatoire Astronomique de Strasbourg  [http://astro.unistra.fr/]   written largely by Joachim Koppen.

These articles cover a very wide range of topics in some depth and present the science and maths behind using an 11GHz radiometer to measure the temperature of the Sun and Moon.
They are well worth consulting in some detail for anyone wishing to build an 11Ghz receiver system.


They show the construction of an LNB and explain how it works.    
   






This is the inside of an 11GHz LNB showing the microwave circuitry and components. 







Koppen points out that "at frequencies above about 10 GHz, the Earth's atmosphere becomes progressively less transparent, because the molecules of air are able to absorb this radiation by converting the energy into rotation of the molecules. Water vapour plays a rather important role, and thus the transmission of microwaves is sensitive to the weather, especially the humidity."  This makes these systems difficult to use for any astronomical work as signals will vary as clouds pass through the antenna beam, which is generally about 2 degrees wide and can easily resolve the size of clouds.



Koppen shows an illustration of a drift or transit scan of the antenna beam across the Sun, the cold sky and a calibrator which was the large wall of a building at ambient  temperature (~200C  or 2930K)



Joachim Koppen's 11GHz Radiometer measurement





The pictures below show the simple 11GHz set up which was first assembled at this observatory as part of the learning process to develop some skills in amateur radio astronomy.


                                                                                                    
                                                                                                                                                                                                             

                   
          



On the left is a picture of the 80cm diameter TV satellite dish and LNB used in some initial experiments to detect the Sun.                              

The Graph above show the signal detected from the Sun







A small receiver system operating at around 11 GHz - such as shown here - is a very good temperature detector. A radiometer !




All hot / warm bodies such as houses, trees and the ground emit wide band thermal radiation, some of which is
in the microwave band. These receivers can detect this signal and therefore can be used to measure the temperature
of such items.  A human being is quite hot compared to the ground (in the UK) and shows up well as a large signal
if the dish is pointed at a person.

Whilst these 11 GHz receivers are not particularly useful for astronomical measurements they demonstrate quite well the
properties of a microwave radiometer.


Their use as a radiometer was the basis for a School Science project for 6th form pupils set up as part of the STEM
(Science Technology Engineering & Maths) program which runs in the UK.  This is described below.








11 GHz Radiometer School Experiment



This practical experiment program was conducted at the Monmouth School for boys under their MSI (Monmouth Science Initiative) program.   [ http://www.monmouthschool.org/academic/monmouth-science-initiative/]



The aims of the program were :
  • Use a 1m diameter offset dish working at 11 GHz 
  • Build part of the receiver and make a map of the sky at 11 GHz 
  • Observe the radio emission from the SUN, moon, Direct Broadcast Satellites                                     
  • Also measure microwave signals from trees, ground, buildings, people 
                                                                                               
                             
 
     


The diagram below shows the main components of the system the students were to build and use.


                                                      

                             Concept diagram                                                                                                  Actual Equipment


The 'Detector' is a purchased item - a satellite TV antenna positioning meter which contains an RF amplifier and detector that enables the front panel meter to display signal strength.

The DC voltage across the meter is fed through a connector into the post detection amplifier which provides gain, offset and time constant (integration) functions. The signal strength can be read from the meter and the output voltage fed to an analogue to digital converter (A to D) connected to laptop or PC to log the signal levels.

The circuit of the Post Detector Amplifier is shown below.





                              


 
                                                                                           The Post Detector Amplifier



The equipment can be seen below being used outside to measure the position of Direct Broadcast TV satellites


       



                          








           









The 11GHz sky map shown below was produced by repeatedly scanning the antenna beam in Azimuth at a number of elevation angle - every 5 degrees. The data were combined in a software package called 'Stanford Graphics' to produce the false colour image. The positions of TV satellites can be clearly seen. Some fainter satellites can be seen in locations further to the West than the main group of European satellites  at about 200 East of South.     




                                                                
The thermal microwave emission from the ground and buildings is clear to see.


More details of the experiments can be found by opening up the Power Point presentation 
{School 2013 Dish Starter.ppt} at the base of this page.



Making  a Microwave image of surroundings

One of the experiments the students undertook was to use the equipment as a microwave radiometer to produce a temperature map of the surroundings.
This was done by repeatedly scanning in Azimuth at a series of increasing Elevations every 5 degrees. The signal strengths during the scans was logged in laptop and then combined to create a false colour map.

A panoramic picture of the surroundings was produce by combining wide angle shots taken with a normal digital camera. See below.




From this a line outline drawing was created:


Finally the microwave radiometer map was combined with the line drawing.




The students engaged in all aspects of this project and a final presentation was produced which can be seen by opening the Power Point file {Consolidated Presentation.ppt) at the base of this page.









Making a Radio Image of the Sun

The aim of this student experiment was to make a radiometric picture of the Sun. This was done by repeatedly scanning the beam across the Sun at slightly different elevations and combing the results. Two examples of Sun scans are shown below. They both have a high signal to background ratio which will lead to a clean high contrast image.






A comparison of signal levels from the Sun, a building wall at ambient temperature and the cold sky can be seen in the figure below. With this information and the analysis process as detailed by Koppen  [ http://astro.ustrasbg.fr/~koppen/10GHz/solar.html#OBS] it is possible to calculate the temperature of the solar disc.




The false colour picture of the Sun is built up from the azimuth raster scan data shown below. The highest readings are produced when the antenna beam scans across the middle of the disc, with the levels the decreasing as the beam scans higher or lower towards the Sun's limbs.




The half power beamwidth of the dish is approximately 3 degrees, which is of course much larger than the Sun's diameter of  0.5 degrees. Therefore the diameter of the image produced is dominated by the antenna beamwidth.  To properly resolve the disc a much larger diameter antenna with a correspondingly smaller beamwidth is needed.  

The experiments however demonstrates to the students the manner in which real astronomical measurements can be made.






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David Morgan,
1 Feb 2016, 07:15
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David Morgan,
1 Feb 2016, 06:13
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