LOFAR stations have two types of antenna. These are the Low-Band Antenna (LBA) array and the High-Band Antenna (HBA) array. The LBA and HBA antennas receive the frequency range of 10-90 and 110-270 MHz respectively. However, the signal processing is arranged slightly differently. Each “channel” in the signal processing system has three inputs into which the antennas are connected. From the receiver units (RCUs) there are several signal paths that can be used, each of which switch in different filters. Different antennas and frequency bands can be selected and sampled.
Each individual receiver unit has both an LBA and an HBA polarisation connected to it hence it is not possible to observe with both of these simultaneously. Furthermore, because of the filters used in the RCUs, it is not possible to observe across the entire frequency range of the HBA simultaneously either. One “RCU mode” must be selected for each given channel.
- RCU mode 3 = LBA input, 10-90 MHz filters
- RCU mode 5 = HBA input, 110-190 MHz filters
- RCU mode 7 = HBA input, 210-270 MHz filters
NOTE: Other modes are available which use different filters and/or clock rate
It appears that each RCU is limited to a single mode leading one to believe that a LOFAR station can not observe using the LBA and HBA antenna simultaneously.
Specialists at KAIRA developed software to combine the modes. The refer to it as RCU mode 357, as one can observe with RCU modes 3, 5 and 7 simultaneously. The modes are interleaved, so that there is still a distribution of antennas for each; this allows beam-forming to take place. Multiple beams across all bands can be formed giving frequency coverage over nearly the entire VHF band. See example below.