The operating frequency range of LOFAR the full 10 MHz to 240 MHz, while the antennas are optimized for the ranges of 30-80 MHz and 120-240 MHz. Because the optimized bandwidth of the operating frequency range spans 8 octaves, at least two types of antenna are necessary: the Low Band Antenna (LBA) and the High Band Antenna (HBA).
In the original design, to accommodate science below 30 MHz an extra provision is made in the electronics for a third antenna type, also referred to in the LOFAR nomenclature as the Low Band Low (LBL) antenna. In that context the LBA is also referred to as the Low Band High (LBH) antenna.
At the moment the LBL inputs in stations in the Netherlands are connected to LBA antennas (known as the LBA Inner configuration).
Low Band Antennas (LBA)
LBAs are designed to operate from the atmospheric spectral cutoff of the "radio window" at 10 MHz up to the onset of the commercial FM radio band at 90 MHz. Due to the presence of strong RFI, this range is operationally limited to 30-80 MHz by default.
A digital filter is employed to suppress the response below 30 MHz. However, there is the option to de-select this filter and observe down to 10 MHz.
The LBA's consist of simple dual linear polarization (X and Y) droop dipoles above a conducting ground plane with the wires at an angle of 45 degrees with the ground.
The dipole wires are connected to a molded head containing a low-noise amplifier (LNA). The molded head of the LBA rests on a vertical shaft of PVC pipe.
The tension of the springs and the ground anchor hold the antenna upright and also minimize vibrations in the wires due to wind loading. The dipole itself rests on a ground plane consisting of a metal mesh constructed from steel concrete reinforcement rods. A foil sheet is used to minimize vegetation growth underneath the antenna. Each polarization has its own input and hence two coaxial cables per LBA element run through the vertical PVC pipe. Power is supplied to the LNA over these same coaxial cables.
Examples of the LBA response curve are shown in the following figure:
Figure: The frequency response for several LBA dipoles of a LOFAR station. Note the strong RFI below 20MHz, partly due to ionospheric reﬂection of sub-horizon RFI back towards the ground, and above 80 MHz, due to the FM band.
Using digital beam-forming techniques, the omnidirectional response of the LBA antennae allows for the simultaneous monitoring of the entire visible sky.
High Band Antennas (HBA)
HBA's have been designed to operate in the frequency range between 120 and 240 MHz.
At these frequencies, sky noise no longer dominates the total system noise as is the case for the LBA's. Consequently a different design was required to minimize contributions to the system noise due to the electronics.
Each HBA "tile" is an assembly of 16 bow tie shaped dual dipole antennas arranged in a 4x4 grid with a spacing of 1.25 m between the dipoles (measuring 5x5 m). Each HBA tile is equipped with an analogue radio frequency (RF) beamformer, which forms a single "tile beam" by combining the signal from these 16 antenna elements in phase for a given direction in the sky. This in turn limits the field of view of an individual tile to approximately 30 degrees at full-width half-maximum (FWHM) at a frequency of 150 MHz.
Tiles are then arranged in fields with three different layouts (see "Station Description and Configuration").
Note that the regular spacing of the HBA dipoles gives rise to grating lobes, with a strength comparable to or larger than the main beam.
Figure: The frequency response for several HBA tiles of a LOFAR station.