The Antenna Pattern

Joachim Köppen Strasbourg 2011

The Half-Power Beam Width (HPBW) is an important parameter of the antenna pattern. It specifies the angular width within which the antenna is most sensitive. Hence it tells us the minimum angular extent that a source must have, if we want to resolve its structure. Sources smaller than this width - such as the Sun - will fill only part of the antenna beam, and hence the received signal will be less than if the source was extended. Conversely, in order to get the true temperature of the Sun, we have to correct the measured antenna temperature by the beam filling factor, which is the ratio of the squares of HPBW and source diameter.

The HPBW can be measured in several ways:

This is what we did in our first solar observation on 18 june 2009: we let the Sun pass through the position where the fixed telescope is pointing. The data recorded shows therefore a peak flux when the Sun moved through the main lobe of the antenna:
We waited until the signal was much lower than the maximum value, then we moved the telescope to a position at the same elevation, but at about 90 to the East of the Sun to measure the background by observing an empty patch of the sky. Finally, we moved to stow position for flux calibration with the library wall.

By fitting a gaussian curve (the thin blue curve) to the solar data, we determine its width sigma = 11.04 minutes of time. As the Sun was at declination +23.40, it moved across the sky with angular speed of 0.25/min*cos(declination), so we can express sigma in degrees. The half-power beam width of the gaussian curve is given by HPBW = 2.3548*sigma. Taken all together, this gives a HPBW of our antenna of 5.95.

But we also notice that the measured curve deviates from the gaussian curve slightly, but significantly ...!

A more recent observation - done in the same manner - gave HPBW = 6.2

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last update: Apr. 2011 J.Köppen