SEFD vs freq for new lbw receiver

april 2003

     On 17apr03 1 minute on/off position switching was done on 3C138. The frequency stepped through the range 1100 to 1700 in 100 Mhz junks (4*25Mhz bands). This was repeated a second time going from 1110 to 1710 Mhz in 100 Mhz junks. The System Equivalent Flux Density (SEFD) was computed using (polA + polB)*.5 (since 3C138 is 7.5% polarized at 1400 Mhz).  This uses the flux density of the source (taken from chris salter's fits). It does not use the cal values.
    The first set of data covered za 10.7 to 6.5 degrees. The second set covered 5.7 to 2.0 degrees za. The plots shows the SEFD vs frequency for the two frequency passes. Any pointing errors will increase the sefd (since i did not search for the peak). With the current model they are probably small. The two passed repeated pretty well.

    The SEFD is 2.5 Jy/Tsys down to about 1450 Mhz. It then begins rising until it reaches 4+ at 1100 Mhz. It is better than the old lbw receiver down to about 1180 Mhz. The lower plot shows that the system temperature rise is the major cause in the SEFD increase (and not a loss of gain). The Tsys values use the measured cal values, so you can look at it the other way and say that the cal values are probably pretty good since they cause the Tsys to follow the SEFD increase (this then assumes that the gain is constant).

    The sefd fit should work pretty good for data below 15 degrees za (no spill over and no gain loss).

The coefficients from the sefd fit (with freq in Ghz) are:

sefd(fGhz)= 45.800962 -77.522389*f + 46.350673*f^2 - 9.2439278*f^3

processing: x101/030417/