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.
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.
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:
- measuring the solar flux at various positions offset from the
centre of the solar disk: Since the Haystack's positioning
system is accurate better than 0.5°, we observe the Sun, and
then move the telescope to several positions, offset in azimuth
and/or elevation. The ratio of the power measured at each position
relative to the maximum gives the antenna's sensitivity curve.
The offset angle where this curve dropped to half of the peak
value is half of the HPBW.
- the Haystack also has the option to make a map of 5 by 5
points in a rectangular pattern about the centre. After performing
this mapping, the HPBWs in elevation and azimuth are displayed on
the lower right hand side, along with the estimated offset angles
for the true maximum flux. We can also use the data recorded at
these 25 positions and interpret them ourselves ...
- finally, we can make a drift scan of the Sun. Here we point the
telescope to a position which the Sun will pass some time later,
and we record the spectra, as the Sun enters the antenna beam,
and then comes out of it. Together with a measurement of the
empty sky - preferably at the same elevation, but at a more easterly
azimuth - we can reduce the data, and obtain the antenna's
angular sensitivity curve ...
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
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