John mathews test Feed
Sep 2004
A 430 Mhz test feed was installed on carriage
1 for john mathews to look into the 430 interferometer he wants to install.
It was placed about 5 feet below the paraxial surface (using the bottom
of the carriage house as the reference). To get an idea of the feed response,
4 cross scans were done on 3C286 (24.3 Jy at 430Mhz).
The system setup was:
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The signal from the receiver was passed through a 1 Mhz butterworth IF
filter and then detected with a .2 second time constant.
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The data was sampled at 10 Hz.
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The cross consisted of a 1 deg strip in azimuth and a 1 deg strip in za
(centered on the source). Each strip took 1 minute. The data was sampled
synchronously with the motion at 10 Hz.
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Great circle az,za offsets were installed to point this feed using the
ch model. The offsets were .5 degrees in azimuth and -1.6 degrees in za
(these value were added to the computed carriage house position).
4 cross were taken and then 2-d gauss fits were done to find the beam offsets,
amplitude, fwhm. No cal was available so the results were scaled to Tsys
of this receiver (probably warm). The plots
show the data and the fits (.ps) (.pdf).
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Fig 1. The top plot has the 4 azimuth strips and the fit results. The bottom
plot has the 4 za strips and the plot results. The y axis is in units of
Tsys. The dashed red line is 0 pointing error.
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Fig 2. This shows the azimuth pointing error (black) and the za pointing
error (red) 4 the 4 crosses. The top plot is versus azimuth position (of
the ch) and the bottom plot is versus za (of the ch). The combined pointing
error of about 3 Amin is 15% of a beam (looks like those offsets were pretty
good for this za).
The feed is located uphill from the line feed and at a lower azimuth (on
the left looking uphill). The offsets for the feed should be in azimuth
and zenith angle. I used great circle az, za so the azimuth encoder value
used was larger by 1./sin(za). This worked ok because the za did not change
much during the measurements.
The great circle offsets can be used to compute the
physical location of the antenna relative to the line feed. On the sky
theta degrees of motion is : Radius*Theta=arcdistance. Using the radius
for the rails of 420 feet this gives:
3.66 feet in azimuth and
11.73 feet in za
The fit results were averaged for the 4 crosses and are shown in the
table below. The sefd is the System equivalent flux density. It is the
strength of a source in Jy that you need to equal the system temerature
(Tsys/Gain).
| srcDeflection (24.3 Jy) |
.05 Tsys |
| SEFD (JySrc/srcDeflection[Tsys units]) |
486 Jy |
sefdNewFd/sefdCh
(Tsys/Gain New)/(Tsys/Gain Ch) (486/8) |
60.75
18Db |
| beamWidth Az (fwhm) |
20.9 Amin |
| beam Width Za (fwhm) |
23.2 Amin |
| pntErrAz |
2.2 Amin |
| pntErrZa |
1.9 Amin |
| azOffset used (great circle:za=11) |
.5 deg |
| zaOffset used |
-1.6 deg |
| azimuth offset from line feed |
-3.66 ft |
| za offset from line feed |
11.73 ft |
The linefeed has a gain of about 15 Kelvins per Jy at 11 degrees za.
The warm radar receiver temp is about 120 so the SEFD of the line feed
is probably 8 Jy. So the test feed is has a sefd 18 db worse than the line
feed (both with warm receivers).
processing: x101/040912/doit.pro
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