| Description: | The AJDI of August 15 2011 showed the scan angle dependent noise coupling contribution to the system noise temperature of an AA at a frequency of 1400 MHz. It is interesting to see how this contribution varies as a function of frequency. As the simulated noise coupling contributions at 1400 MHz are in good agreement with the model used to analyze the measurement results, we will assume that the simulations also accurately predict the noise coupling contribution as a function of frequency. The results are presented in a series of eight two-dimensional plots of the noise coupling contributions to the system temperature, illustrating the expected variation in system temperature as a function of scan angle and frequency for a large aperture array with a small beam width. Shown are the simulated noise coupling contributions on a 50 K temperature scale, in steps of 100 MHz from 1000 MHz to 1600 MHz, with the addition of the frequency point at 1375 MHz, the design frequency for half lambda spacing (11 cm) between the individual array elements. The stronger coupling between individual antenna elements at the low frequency end of the band results in larger noise coupling contributions, compared to the center near 1375 MHz. There is a remarkable radial symmetry in the noise coupling at 1200 MHz and 1300 MHz, with noise temperatures increasing in a similar way for increasing scan angle with respect to broadside, for all azimuth directions. At higher frequencies the coupling between elements becomes smaller, due to the larger electrical distance, however the noise coupling becomes less symmetrical and also shows larger variations as a function of scan angle. The noise coupling gives the highest noise temperatures for large scan angles, near the horizon, in some cases rising above 50 K. |
