Elevation Scan of the Sky Measures Antenna Spill-Over
Joachim Köppen Kiel Feb 2021

Antenna
focus/diameter ratio
angle to the rim

Measured skyprofile data
Elevation

ground noise

System temp. [K]
Zenith temp. [K]

Spill-over pattern
Angle     Weight
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50
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60
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80
85
90
Mouse position

Some brief explanations

• The feed of a parabolic dish antenna may pick up ground noise from just outside the rim of the dish. This adds to the overall system noise, but its amount varies with the elevation angle ε.
• The measured radio noise from the empty sky at a number of elevations is composed of the thermal emission of the Earth atmosphere, the cosmic microwave background, and the spill-over noise. This tool interprets this 'sky profile' and derives the system temperature of the radio telescope and the zenith temperature for the atmospheric emission. The enhanced signal levels usually found at elevations near the zenith then can be matched by adding necessary amounts of spill-over contributions.
 The lines of sight from the feed to the rim of the dish touch the ground at these places, making ellipses of hyperbolae, depending on the antenna's elevation angle ε. This is from where the antenna will pick up ground noise When the antenna points close to the zenith, the noise is high, as all lines of sight around the dish's rim will reach the ground, making an ellipse. But when it points lower, only the lower lines of sight hit the ground in an hyperbola, while the higher ones look at the cold sky.
• How to use it
• Enter your data. The main plot shows a descending blue curve which models the emission from the sky.
• Use the radio buttons at those high elevation angles where data and the red curve rise again to switch off these data for the analysis of the system temperature. These data are plotted with smaller blue circles.
• Enter suitable weights at suitable angles in the spill-over pattern to make a good match between the red curve and your measured data. The small plot shows the spill-over contribution for any elevation angle

• Antenna: enter the ratio of focal length and diameter to get the maximum angle with which the antenna rim is seen by the feed. This angle is marked by a vertical blue line in the main plot. Any radiation from the feed that goes beyond this angle does not reach the antenna and is lost as spill-over. Thus, at these angles a receiving antenna picks up radiation from behind the reflector, e.g. from the ground.
The default data are from the 9m antenna at DLØSHF on 1.3 GHz.
• Measured skyprofile data: enter your data by specifying the elevation angle - this can be done in any order - along with the received power - the values can be in dB or linear units. A field left empty or filled with non-numerical content lets this data line ignored.
• ground noise: in order to determine the system and zenith temperatures, a measurement of the ground noise is required, e.g. from the ground, a large building, or a dense patch of tree, large enough to fill completely the antenna beam. If the system temperature is known or can be estimated from other means, one may enter some value which results in the desired value of the system temperature.
• Data with radiobutton=ON: the radio button to the right of each data line selects whether this datum is used for the analysis of the system temperature. One should use only elevations below the angle to the rim, as they are not affected by the spill-over contribution.
• System temperature: is the temperature corresponding to the overall noise of the receiving system, i.e. including cable losses etc.
• Zenith temperature: the thermal emission of the Earth atmosphere is modelled by the antenna temperature as a function of elevation:
TA(ε) = Tzen/sin(ε)
Please note that this does not include any absorption by the atmosphere. Thus this tool can only apply to data below about 15 GHz.
• The main plot: shows the measurements as blue circles (smaller circles indicate data not selected for the fit), the fit of the data with the displayed system and zenith temperatures as a blue curve, and as a red curve the interpretation with the spill-over contribution.
• Spill-over pattern: enter values for the weights at various angles so that the red curve in the main plot gives a best match to the measured data points. This ist best done by trial-and-error. One might find that the solution is not unique, or that it is not as perfect as one would have wished. However, a look at the smaller plot often reveals that the overall spill-over noise has not changed much!
• Mouse position: displays the x and y values of the position of the mouse in the two plots.
• Spill-over noise plot shows the contribution to the system temperature by the ground noise picked up at various elevations. Since the value is nearly constant between elevations 0 to 60°, one may use the value at 0 to characterize the spill-over of an antenna. Note that this analysis assumes that the antenna stands on an empty horizontal plane. The presence of any building or structure of appreciable angular height adds to the spill-over ...