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: 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). 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
(Tsys/Gain New)/(Tsys/Gain Ch) (486/8)
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/