Daily Image
01-09-2009
Twittering Antennas
| Submitter: | Stefan Wijnholds on behalf of the LOFAR beam team |
| Description: | Mutual coupling is one of the complicating effects in dense antenna arrays. In the case of LOFAR's LBA station arrays, this effect is hard to model using full electromagnetic simulations due to the complex geometry of the problem. Each antenna in this irregular array has its own ground plane, which only measures 3-by-3 meters. The LOFAR beam dream team, consisting of Johan Hamaker, Michiel Brentjens, Sarod Yatawatta and Stefan Wijnholds, are therefore endeavouring to measure it by different means. Direct measurements were done on August 12 on the old FTS-array in our backyard in Dwingeloo, and on August 18 on an actual LOFAR station (CS302). The setup consisted of a box containing a 12 V 8 Ah battery pack feeding an amplifier chain amplifying the thermal noise from a 50 Ohm load to -125 dBm/Hz. This signal was fed into a 180 degree splitter terminated by 1 dB attenuators. The two output signals were capacitively fed into a dipole using two joints (top right). The whole setup comes in a convenient package made and tested by Klaas Dijkstra (top left). The two plots at the bottom show the amplitude and phase of the coupled signal on the indicated baselines within the FTS-setup, which is a 3 meter spaced uniform linear array of parallel dipoles. These plots confirm two basic intuitions regarding mutual coupling:
We have also seen a number of more subtle effects, which are currently being investigated in more detail. |
| Copyright: | ASTRON / LOFAR |
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