Antenna and Analog Beamformer

Transcription

1 Antenna and Analog Beamformer Requirements The antenna system is responsible for collecting radiation from the sky and presenting a suitably conditioned MHz RF signal to the receiver node. Because a single antenna with the desired point-source sensitivity and wide-field coverage is not feasible, a phased-array design for the antenna was selected, with a single, electronically steerable beam. The desired properties of the antenna include: Zenith collecting area > 10 m 2 over as much of operating frequency range as possible Field of view from zenith to 60º ZA with <6 db gain variation with beam pointing direction Minimal antenna gain at the horizon, to reduce terrestrial RFI System temperature dominated by sky noise Dual linear polarization reception Low manufacturing cost Active Dipole Design The basic antenna element in the phased array is a dual-polarization active dipole over a conductive groundscreen as shown in Figure 1. The requirements outlined above are met by using a vertical bowtie element that is symmetrical about the horizontal centerline with an integrated LNA/balun at the juncture between the two arms of the bowtie. A photograph of a prototype LNA circuit board is shown in Figure 2. Each antenna has two LNA boards, one per polarization. 1

2 Figure 1. Prototype MWA-LFD antenna element. The total span across a bowtie is 64 cm, and the top of a bowtie is 65 cm above the groundscreen. The four 5-cm-high dielectric feet support the bowtie and securely attach it to the groundscreen. Figure 2. Prototype LNA board. The board is 3.8 cm square. E- and H-plane power patterns for a single antenna element above a groundscreen are shown in Figure 3 at three frequencies. A dip in the antenna gain at the zenith develops above ~250 MHz. This behavior is to be expected at the higher frequencies and is 2

3 characteristic of the pattern of a horizontal linear dipole positioned > ~3λ/8 above the groundscreen. The bowtie patterns are nonetheless superior to those of a simple dipole and some of its variants, such as a fat dipole or droopy dipole. Single element: 110 MHz, E-plane Single element: 110 MHz, H-plane Single element: 200 MHz, E-plane Single element: 200 MHz, H-plane Single element: 300 MHz, E-plane Single element: 300 MHz, H-plane Figure 2. Representative simulated and measured power patterns for a single antenna element above a groundscreen. E-plane on the left, H-plane on the right. Top to bottom: 110, 200, and 300 MHz. Tile Layout The term tile refers to multiple active dipole antennas above a ground screen. The principal design challenge here is to devise a suitable geometric arrangement of the dipoles such that adequate performance of the tile is achieved with a beamforming operation, while maintaining simplicity and low cost. The chosen solution is a regular 4x4 square grid with 1.07 meter spacing. The spacing corresponds to λ/2 at 140 MHz 3

4 and was chosen to optimize the sensitivity in the frequency range thought likely to contain the EOR redshifted-hi signal. The groundscreen is constructed of three panels of wire mesh with a 5x5-cm square grid and wire diameter of 3-4 mm. The panels are approximately 2x5 m in size and are overlapped to form a 5x5-m groundscreen. Figure 4 shows an assembled tile in Western Australia. Analog Beamformer The analog beamformer (BF) receives dual polarization signals from all 16 crossed dipoles in a tile, and applies independent delays to each signal in a manner appropriate to form a tile beam in a particular direction on the sky. True delay steering is employed, rather than phase steering, in order to point the beam properly over the full operating frequency range. The delayed signals are combined, amplified, and sent over coaxial cable to the node receiver for digitization. Figure 5 shows a simplified block diagram of the electronics for a single polarization signal from one dipole. X Y BPF 5 delay line sections Σ all X s 180 identical circuit for Y signals One section of 5 sections of switchable delay line lengths differ by factors of 2 PC board delay line attenuator Figure 5. Simplified block diagram of antenna + beamformer electronics. The first-stage LNAs are integrated into the antenna structure, while the rest of the electronics is housed in the beamformer chassis. 4

5 The beamformer is housed in a chassis, inside which are the delay line boards, a digital interface board, and a power supply to convert incoming 48 vac to 5 vdc for the LNAs and BF electronics. There is no active cooling or fan in the chassis. The chassis sits slightly elevated above the ground approximately 1 m from the edge of the tile. It is painted with infrared-reflective paint and shielded under a sunshade. A photograph of the prototype beamformer used in the early deployment tests in Western Australia is shown in Figure 6. 5