New sbn Tx horn
Links to plots:
performance with all of the data combined (old, new horns, all sources)
the gain and beam width of the horns (.ps) (.pdf)
the pointing errors for the runs (.ps) (.pdf)
the focus curves for the two horns (.ps) (.pdf)
A new sbn transmit horn was installed on 04may06. It was designed
to have a smaller edge taper on the tertiary (to increase the gain). The
increase in temperature from the spill over does not matter for the transmit
horn (although you don't want to melt anything with the spill over of the
transmitter!). To test the horn, a room temperature receiver was placed
on the sbntx system before and after the horn was replaced. A cal noise
diode was included for calibration. Data was taken on the following dates:
Data was taken on 3 sources. The data taking started
at 16:00 to 16:30 and went to about 20:00.
The noise diode for the sbnTx measurements had an
ENR of 15.6 db. It was coupled into the system with a 22.4 db coupler.
These number would give a cal temperature of 62K. For the computations
i used 60 K. Since we were comparing before an after measurements
the important thing was that the cal was constant between the measurements
(it's absolute value was not that important). The receiver for sbtx had
to be installed and removed a number of times during this measurement period
(there were ongoing sband runs). During the removal, the cal cable was
The new horn was designed to decrease the edge tape
of the illumination pattern on the tertiary. This would increase the gain,
decrease the beam width, and increase the Tsys (more spill over for the
receiver). The gain should increase by the square of the beam width decrease
(g proportional to 1/beamwidth^2). The system temperature change would
not be obvious with the receiver we were using. The Tsys was about 110K
while the temperature in the dome is 250-300 K. With a cooled receiver
this would stand out better.
The plots show the results of the measurements:
The first set of plots show the
system performance with all of the data combined (old, new horns, all sources)
The second set of plots compare
the gain and beam width of the horns (.ps) (.pdf):
Page 1: gain, Tsys, Sefd, and Beamwidth. The sbTx (black,grey +, purple
box, -) gain and Tsys may be a little high (since the cal value is
not that accurate). The Tsys for sbTx is about 105K. The sefd's do not
depend on the cals.
Page 2: The beam efficiencies depend on the cals so the sbtx values can
Page 3: The pointing errors.
The gain should have increased as the square of the
beam width. It looks like it is linear.
Page 1 Gain: Each panel is a separate source. Black is rcv Horn, red is
old tx horn, green is new tx horn. The offset between the tx and rcv horns
is probably coming from the tx cal value being a little off. The increase
is gain between txOld and txNew is real (since the same cal used for both).
The may05 txNew data was not included since the cal diode didn't fire.
Page 2 beamWidths: This includes the 05may txNew horn data. The beam width
decreased by about 5 asecs from the oldHorn to the new horn. This is the
average beam width in the az, za direction. I checked the beamwidth difference
(in the az,za directions). This remained the same before and after the
Page 3 Ratios: The txNew horn data was interpolated to the azimuth positions
of the txOld horn points and then the ratios were computed. The top plot
is the ratio GainNewTxHorn/GainOldTxHorn. The increase is 3 to 4 %. The
bottom plot is the inverse ratio of the beam widths (BeamWidthOldHorn/BeamWidthNewHorn).
There is a 3-4% change in the beam width. The 04may05 txNewHorn measurements
were repeated on 05may05. The beam widths from the two txNew horn days
track pretty well.
The 3rd set of plots compare
the pointing errors for the runs (.ps) (.pdf):
Page 1-2 ZaErr: page 1 is the za Error vs za while page 2 is za Error vs
The Txnew horn has a 10 asec offset from the txOld horn for B0758+143 rising.
They agree pretty well setting. The second source B1040+123 only had a
5 asec difference. The difference was also there on the 2nd day of the
sbTxNew measurements. I checked the tiedown tensions and they didn't look
like they lost tensions. It may be that the txOld horn data is the one
that stands out (even if it is closer to 0).
There is a ramp in za error vs azimuth. This could be caused by a platform
tilt or a time error. I checked the clocks and they looked ok.
Page 3-4 azErr: The azimuth error vs za and azimuth.
Page 5-6: A vector plot of pointing errors on the dish. The length of each
arrow is proportional to the pointing error (1 tick mark is 5 asecs). The
direction is the direction of the error. Each circle is a 5 degree step
Focus curves were done using the old
sbn tx horn (28apr06) and with the new
sbtx horn (10may06).
The plots compare
the focus curves for the two horns (.ps) (.pdf):
Top: Platform focus height for the old horn (red) and the new horn
(black). The sold lines are the source rising. The dashed lines are the
source setting. The vertical scale is in feet.
Bottom: The newHornFocus - oldHornFocus distance. The vertical scale
is in inches. The focus height for the new horn was interpolated to the
measured azimuths of the old horn and then the difference was taken.
The large difference at za=2 is probably due to the rapidly changing focus
error as the azimuth swings around at transit. The interpolation probably
didn't work too well here. The new horn looks like it positioned .2 to
.4 inches closer to the dish than the old horn (it needs to move farther
from the dish to focus).
The average beam width decreased by 3-4 % when the new txHorn was installed.
The gain increased by 3-4% when the new horn was installed. If the sidelobes
remained constant then the gain increase should have been closer
to 10 %.
The first sidelobe height did look a little larger for the new tx horn.
The new Tx pointing error for the source B0758+143 rising had a za error
of about 10-15 asecs. This was not seen in the Tx old horn. It was also
not this large for the rcv horn.
We may have a platform tilt (relative to when the last model was made).
The new horn is focussing .2 to .4 inches farther from the dish than the
old horn. This means that it may be positioned closer to the dish than
the old horn.