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Observations at 12GHz


This 12GHz system was built as an educational program to detect microwave thermal radiation from the Sun, the cold sky, the ground, the human body,and so on.
Examples on what can be done with a similar system is described by William Lonc in Classroom Radio Telescope.
It is very easy to implement such a system: The receiving equipment is a 83 cm offset dish with LNB connected via a short coax cable to a satellite finder that replaces the receiver.
The LNB has a Noise Figure of 0.7dB, it receives from 12.50 GHz to 12.75 GHz, the output is from 950 MHz to 1750 MHz and feeds the satellite finder (Satfinder).
The Satfinder is normally used to set the dish towards a satellite when installing an equipment for the reception of satellite television. It is a very small box with an analogic meter that shows the intensity of the signal, the gain is varied via a potentiometer.
The built-in analog meter has ten graduation marks, when the dish is pointed to the Sun the meter shows a sharp peak.
The satellite finder was modified as follows: A shielded two-leads cable was soldered to the meter terminals and the other end of the cable is connected a A-to-D converter.
The output of the satfinder goes normally, via a coax, to the receiver and the receiver powers the satfinder and the LNB with 13/18 Volt through the coax.
In our case a 15 volt regulated power supply is connected directly to the output.
The data from the A-to-D converter are then processed by the Radio-Sky Pipe software.

Dish installation and orientation

Pointing the offset dish towards the Sun is the most difficult operation,the beam size is estimated to be 2 degrees, this means that it is quite challenging to point the dish towards the source (the Sun). The mean apparent diameter of the visible Sun is ~32 minutes of arc.
I found that the dish points towards the Sun when the shadow of the LNB is on the lower rim of the dish (see the picture).
The support must be very stable. The dish must be oriented for meridian transit and movable in Elevation.
In the configuration described here it can be moved in both Elevation and Azimuth. The Elevation indicator is quite rudimentary and will be replaced by a ten turn potentiometer controlling a DC amplifier driving a voltmeter as described by Chuck Foster in Antenna Position Indicator.

dish

This is the Satfinder , protected by a polystirene container, on the left is the coax cable to the LNB, on the right is the output F-plug connected to the power supply,and barely visible, the cable to the A-to-D converter.

satfinder
This is the Sky-Pipe plot of a meridian transit of the Sun. We can now calculate the Half Power Beam Width (HPBW)of the antenna.
The plot is a voltage plot, the -3dB level (HPBW) occurs at 0.707 times the voltage peak value, this gives 2.665 (Y-axis units). The width of the plot at that level (DeltaT) is ~8 minutes.
The Declination (D) of the Sun at that time was 18.28 degrees. The time taken by the Sun to transit through the -3 dB level and the HPBW are given by:
T=4/cos(D) expressed in minutes per degree, and HPBW=DeltaT / T in degrees.
The final result is therefore: T = 4.21 min and HPBW = ~2 degrees.

transit


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