Our goal is to not only make an inexpensive antenna array, but also a very sensitivity one. To get an idea of the sensitivity of our receivers, we measure the galactic hydrogen line at 21 cm wavelength. The tile was mounted on the roof of my mother-in-laws’ house, which is located in the Vredefort dome, a fairly radio quite enviroment. The reference transmitter was set to 1419.8 MHz as can be seen by the large spike right in the middel of the figure below. To normalise the receiver signal, the signal is compared to the expected HI signal. To calculate the expected signal, HI profiles was downloaded from here. Astropy was then used to calculate the movement of the earth and to calculate a sky[…]
In previous posts, an aperture array was described where each receiver has its own free running clock and an external reference transmitter is used for time and frequency calibration. But does this really work? Can such a tiles do coherent reception? As a first proof of concept, the signal from a GLONASS satellite (Kosmos 2459) was observed with three receivers as it passes over a tile. The satellite transmit at 1244.6875 MHz with a bandwidth of 500 kHz. The reference transmitter was set on 1244.5 MHz and the rtl-sdr dongles on 1244.75 MHz, sampling at 2.4 MHz. 20 seconds of data was captured at a time and the correlation between the signals was calculated with the Odroid C2. Each dot in the figure[…]
The proposed antenna array consists of sensitive receivers and a number of software-defined radio receivers. Because each receiver has it’s own clock, the output signals are incoherent. Is it possible to do beam forming with such an antenna array? Can the signals be made coherent, digitally? Theoretically the answer is yes, but this is going to be a bit technical … Frequency calibration What we do is add a reference transmitter, transmitting a constant frequency, which is received by all the receivers. The frequency offset of each channel can then be compensated for, as shown in the diagram below: Basically, each radio receiver has a different frequency (f1,f2,f3,…) with which the received signal is mixed. By doing an FFT, the Odroid can determine the offset of the reference[…]
The first post describes the challenge of designing a low-cost aperture array. In the next post, an low-noise active antenna receiver was described. The next step is to down converter, digitize and process the data. Down conversion and sampling Below the ground plane of the receiver, the output of each receiver is connected to a rtl-sdr dongle. These dongles cost about $16 when bought in large quantities, have a bandwidth of 24 – 1766 MHz, a stable clocks (<1 PPM), can power the receiver through its input, and a IQ sample rate of 2.4 MHz. Digital processing The rtl-sdr dongles are connected with USB cables to an Odroid C2. The Odroid has 4 cores which is able to process the 4 channels in real time.[…]
The first step in meeting the challenge of a good, low cost aperture array is to have good, low-cost receivers. North-West University has developed a 0.5-1.5 GHz active receiver elements consisting of a single PCB that acts as both the antenna and low noise amplifier. The output of the receivers are connected through a ground plane using coax cables: These receivers has very good sensitivity in the 0.6 – 1.6 GHz band. The noise temperature is close to that of the ATF35143 transistor used and the effective area of a single element almost follow that of a dipole. Each PCB cost about $30 and has 2 receivers, one for each polarisation direction. More detail about the receiver will be posted later.
How many radio pictures have you seen? We know about normal optical photos, X-ray photos, infrared photos, but radio photos are not very common. Why not? Because radio cameras, having many ‘pixels’ are big and expensive. For high resolution the camera need to be several wavelengths big and for many pixel, you need many coherent receivers. Antenna Arrays are basically a radio camera, where you have many small receivers. You can see it as having a pixel per receiver. A normal radio receiver is equivalent to a single pixel camera. However, when you have an array with 100 receivers, you can form a 100 pixel radio image. Astronomy example You can still make a picture with a single receiver. Below[…]