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Plotting antenna pattern from Sun noise

Rex Moncur (VK7MO) used a series of measurements of Sun noise using NFM and an SSB receiver to plot the shape of the major lobe of his 2.3m dish on 2300MHz.

The technique involved making a measurement of cold sky using NFM's Instrument Calibration - Noise LO button, then a series of measurements of the Sun from on-bore sight to about 4° off-bore sight using NFM's Instrument Calibration - Noise HI button and recording the result of each measurement in the TSV file. To help in collection of the data, the Sun bearing can be stored in the Title field of NFM's main form, and it will then be recorded in the TSV file in a form ready for use in spreadsheet formulae.

Just a reminder that in this situation, the Noise LO measurement or 'cold' measurement is not due solely to cold sky, it also includes all other ambient noise (eg antenna spillover) and receiver noise. Similarly, the Noise HI measurement is all the contributions to Noise LO plus the Sun's contribution.

Fig 1:

Fig 1 shows the data captured in NFM (columns A - Q). The important fields are highlighted in pink, Title contains the Sun angle and Y2 contains the ratio of (SunNoise+OtherNoise)/OtherNoise.

To calculate the gain at each bearing, the ratio of SunNoise/OtherNoise must be calculated as S/O=(S+O)/O-1, or in decibels, S/O dB=10*log(10^(Y2/10)-1) dB. From that, the Sun noise relative to the bore sight value can be calculated.

In the case where the antenna beamwidth is large compared to the angle subtended by the radio Sun (as it is in this case) the calculated Sun noise can be taken as coming from a point source. The technique is most suited to antennas with half power beamwidth greater than about 4°.

Columns R and S calculate the relative Sun bearing and relative Sun noise.

Fig 2:

Fig 2 shows the calculated relative Sun noise power plotted against the Sun bearing relative to bore sight. A third order polynomial curves is fitted to the measurement data to give a smoothed curve of the response.

The indicated 3dB beamwidth is 2*1.88° or 3.76° which is very close to the rule of thumb estimate of 70/D*λ° (3.97°) for a 2.3m dish on 2300MHz.

This test is most valid if the antenna is not moved during measurement, ie that the antenna is set up so that the Sun transits the bore sight of the antenna. If the test is done at local noon, the Sun's angular position can be calculated from the recorded time (the Sun appears to move in the sky at a rate of 360°/day). Local noon means when the Sun is observed at its highest point in the sky.

To calculate angular offsets from time in Excel, follow these steps:

  1. Open NFMData.tab in Excel
  2. Open NfmFunctions.xls in Excel
  3. Select the Obstime column
  4. From the menu Tools/Macro/Macros run the CvtIsoDate macro, the Obstime column should now contain Excel datetime values
  5. If row 2 is on bore sight at local noon, calculate the offset in degrees of the other rows as (Obstime for that row less Obstime for the first row)*360 (where the bore sight observation is row 1, the formula for angular offset for a cell in row 2 would be =(C3-$C$2)*360).

If the observations are not made at local noon, the Sun's position can be worked out from an Almanac knowing the latitude and longitude of the antenna and the time recorded by NFM.

Moving the antenna during the measurements brings the risk of minor lobes picking up varying amounts of ambient noise which varies the OtherNoise component with pointing.

Links

NFM

Basic Excel Spreadsheet

An extended Excel spreadsheet with functions for calculating the Sun's position from time and station position.

NfmFunctions.xls contains some useful macros for reformatting the data and notes columns of NFMData.tab when opened in Excel.

NOAA Sunrise/Sunset Calculator

NOAA Solar Position Calculator

Changes

Version Date Description
1.01 16/10/2007 Initial.
1.02    
1.03    

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