Moving the lbn horn to the correct phase center


link to plots: system performance before and after the horn move.


 The lband narrow (lbn) horn is a ying-kildal horn. The phase center for all of the kildal horns were scaled from kildal's paper  describing the horn. Focus curves done on each receiver  showed that  all of the kildal horns had a platform focus position about .5 lambda below the non-kildal horns (and below the expected position). This implied that the ray path for these horns was too long (so the platform focused lower to compensate for this).  German Cortes modeled the lband horn on HFSS and found that the phase center of the horn was closer to the horn than the kildal paper stated. The corrections for the various horns were then scaled using the wave guide diameter.

Measurements before and after the horn move.

    On 19aug02 the source J1344+141 was tracked rise to set using the heiles calibration scans. On 22aug02 the lbn horn was moved down (toward the tertiary) 3.84 inches and then the same source was tracked rise to set. The value used for the source flux was 1.3 Jy at 1415 Mhz. The plots show  the system performance before and after the horn move.
  • Fig 1 top is the gain [K/Jy] before (black)  and after (red) the move. This includes rise and set.

  • Fig 1 bottom is the fractional gain change (new/old)-1.
  • Fig 2 shows the gain,Tsys,Sefd, and average beam widths before (black) and after (red).
  • Fig 3 has the coma, first sidelobe height, Main beam efficiency, and main beam + 1st sidelobe efficiency.
  • Fig 4 plots the pointing error before (black) and after (red). The bottom plot shows the change in pointing error.
  •     The new gain is now 10.5 K/Jy flat out to 15 degrees and then starts dropping because of the za spill over (actually it should start dropping a bit higher in za). The absolute gain value is as certain as the flux and cals. The important change is that the gain is no longer a function of za below za=15. Prior to the move the gain had a  linear  za dependence all the way down to za=0. This was also seen in the gain curve measurement done for lbw back in sep01 (-.1 K/Jy per deg za see figure 3). The linear dependence was probably being caused by the focus error in the rails (green lines). The  0 to 2" rail error vs za should not affect lbn too much. Adding 3.84" would make the 0 to 2" ramp an appreciable error (this sounds good, but our current model of pitch, roll, and focus does not place the rail ramp at the azimuth of this source !).

        The increase in gain must come from somewhere. Looking at fig 2, the average beam width actually increased a bit after the focus move. On figure 3, the coma parameter has not changed a lot. The only difference seems to be that the first sidelobes after the move have decreased.

        The system temperature also looks like it has increased by a bit (although we probably need more measurements to verify this since the weather conditions were not identical on the two days).

        The pointing error for azimuth and setting za have change  by up to 10 asecs. If we had just moved the horn, you would not expect an az,za dependence to the pointing errors. If the fitting is working differently because the beam shape has changed as a function of az,za then this could explain it. The other possibility is that the before data was taken in  intermittent light rain (although we never lost the distomats and tiedown average height tracking). It may be that the moisture may have caused the cables to stretch unevenly. For lband this pointing error is probably not significant.

        The other kildal horns should probably also be moved. The cband receiver also shows a linear za dependence with gain. The only question with cband is that the cband focus curve (red line bottom plot) shows that its current focus is close to 1256.35 feet. This is the location where we are currently keeping the average height of the platform. Moving the horn lambda/2. may make things worse.

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