Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.naic.edu/alfa/ealfa/meeting1/minutes/optics.html
Дата изменения: Mon May 8 23:01:36 2006 Дата индексирования: Sun Dec 23 01:56:30 2007 Кодировка: Поисковые слова: iceland |
A layout scheme figure was shown. Also Gregorian focal plane scanning losses. The asymmetries between feeds could be overcome by shifting feeds by a small offset (2.7 cm) in the X-direction.
Now assume the 25cm x 26cm, 2.7cm X-offset configuration
Polarization Characteristics: Pol Probes Orientation in OMT. The calculated Cross Polarization is better than 20 dB in most horns across the frequency range.
Lister S-S | The system temp seems disappointingly high especially at high frequency; can you explain why it is so high? |
German | The answer is spillover. This type of horn has a lot of spillover and it is worse at lower frequencies than at higher ones. |
Lister | Will the tertiary skirt help? |
German | The tertiary skirt will help more at the lower frequency. Its details are currently being designed; the limitation is its weight right now. There are still some problems with design. The timescale is a few months for definite design. |
Bob B. | Can you estimate how much Tsys will go down? |
German | Hopefully, 5K across band. Hopefully down to about 30K at 1225 MHz; 25K or better at 1525 MHz. |
Riccardo G. | How will the skirt affect the different beams? |
German | Slide 1.9 shows calculated Tsys with 2 points for each frequency; one is for the center beam and other is for the outside. About 2K difference at outer freq, 1K difference in center and same at high freq. |
Bob B. | Is Tsys measured with OMT? What else adds? |
Ganesh | Yes it is measured with OMT. The rest is spillover, and power scattered into main beam from platform. The tertiary skirt will help some. |
Steve S. | Back to the overall beam map. It says -8.5dB for outer sidelobes. Is that relative to the center beam or relative to their own beam. |
German | See Fig: 1.24, which is the calculated beam comatic level. Again, there are 2 points wrt respective main beams. Clearly the center pixel is much nicer. In Fig 1.23: Everything is relative to the central pixel. |
Riccardo | What are individual contributions to the Tsys? |
German |
See Fig 1.9: T![]() ![]() ![]() |
Bob B. | That suggests that the tertiary skirt is really important. |
Lister | When calculation of beam is done, are the beams in isolation or is cross coupling included? |
German | In isolation. Coupling -50 to -60 dB down in terms of noise. The amplifiers in general only go in one direction (amplification) but in terms of noise, there is an equivalent temperature looking out of the amplifier. The energy coming out is in general very low, so I didn't include that. |