Antenna Efficiency: Why/how do we check for antenna efficiency?
We previously described how the shape of
the antenna changes as it points to different parts of the sky, and this
alters how effectively it collects radio waves. We describe this with a
multiplicative factor we call the antenna efficiency. The only way to
determine this reliably is to look at flux calibrators on the sky.
Fortunately for us, the antenna efficiency does not change much over
time - at a particular position on the sky today, the antena efficiency
should be pretty much the same next month. Therefore, periodically we
track a bunch of trusted flux calibrators all over the sky. This allows
us to make a map of the antenna efficiency as a function of antenna
position.
Antenna Efficiency Details
From observations of our Primary Flux Calibrators, we have determined
that the Antenna Efficiency at S-band, as a function of elevation, is
given by this equation:
AntennaEfficiencySband = As + (Bs * Elev) + (Cs * Elev*Elev) where
Elev is the elevation in degrees,
As = 0.589412
Bs = -0.001069
Cs = 1.240e-05
Since we know the declination of our antenna when we observed the
flux calibrator, we can use the above equation to calculate the antenna
efficiency for the location of the calibrator. We'll refer to that
quantity as AntEffSband_Cal.
Similarly, we know the declination when we observed our target
source, and can calculate the antenna efficiency at the location of our
source, and we call it AntEffSband_Source.
We can then use the ratio AntEffSband_Cal / AntEffSband_Source to
correct for changes due to the changing position of the antenna:
TimeAndPositionCorrectedGain = TimeCorrectedGain * AntEffSband_Cal / AntEffSband_Source
Another way of saying this is that the position correction to the
EndToEndGain is a multiplicative factor given by PositionCorrection =
AntEffSband_Cal / AntEffSband_Source.
The antenna efficiency at X-band is given by an equation with different coefficients:
AntennaEfficiencyXband = Ax + (Bx * Elev) + (Cx * Elev*Elev) where
Ax = 0.158,
Bx = 0.0034,
Cx = -6.1e-6.
and this can be used to determine the TimeAndPositionCorrectedGain (or
the PositionCorrection) for X-band, just as in the above S-band example.