Radio antennas are a fundamental part of detecting any kind of radio signal. In order to choose the best type of antennas for the SURA experiment (considering different decisive factors including the capability of building the antenna structure, time, and final price of each radio station) different models of radio antennas were investigated. Particularly, we explored radio antennas that were used in various cosmic ray experiments with good results including Inverted-V and Short Aperiodic Loaded Loop and LPDA antennas and Butterfly antenna.
As a result of a comprehensive study on different types of radio antennas, we chose Inverted-V and LPDA antennas for the first phase of the SURA experiment. The Inverted-V type is rather easier to build and it is reasonably less expensive while the LPDA is one of the best types of radio antennas for detecting cosmic ray radio signals but is more expensive and difficult to build. After a successful design and computer simulation of the antennas, the process of making the antenna structure started, and the final product produced.
In March 2018 the first prototype of an Inverted-V antenna mounted on the roof of physics faculty and passed the initial tests successfully. The general framework of this antenna is very similar to a typical dipole antenna assembled in an Inverted-V shape.
This unit is sensitive to frequencies below 100 MHz which is the operative band of the SURA experiment. The structure of this unit is made of PVC which makes it light, easy to handle, transportable and makes the overall unit very cost-effective
The second type of antenna, designed and produced for the SURA experiment is a log-periodic dipole array antenna (LPDA). This type of antennas was proven to be one of the best designs for detecting radio pulses from extensive air showers. LPDA type of antennas is essentially a sequence of half-wave dipole antennas assembled together to provide a high bandwidth unit. This type of antenna is of special interest to detect cosmic ray radio signals since they are more sensitive towards the sky. Following a computer simulation, the first set of LPDA antennas including 4 units were designed, produced, assembled, and deployed in October 2018. The final product has extreme resistance against various weather conditions and has a nominal wind rating of 120 km/h to ensure proper operation of the antenna on the field.
These units are sensitive to the 30-80 MHz frequency band. Each unit has ten dipoles made of ultra corrosion-resistant anodized aluminum alloy connected to the central waveguide which is mounted on top of the LPDA unit. As a result of the antenna design, it can be assembled in less than 20 minutes by a single person and can be fitted on various telescopic masts.
In the current setup, radio signals from each radio station transmit using high quality coaxial cable. Despite its heavy weight compared to other possible candidates, this model is capable of transmitting signals to longer distances with less attenuation.
In order to preserve radio signals and prevent loss of any possible cosmic ray candidates, incoming radio pulses from each antenna are amplified and bandpass filtered immediately after they leave the antenna. Both devices are assembled in a waterproof box designed for this purpose and mounted under each radio antenna unit. The Low Noise Amplifier (LNA) is a DC to 500 MHz low power unit with a consistent gain of 25 dB over its operative range.
A second type of an LNA unit is also designed for the SURA experiment. This unit only needs a 5V DC power to operate and provides nearly 26 dB of gain in the frequency range of the SURA experiment. This device is passing its final tests and upon successful operation will be used in the next stages of the experiment.
After leaving the LNA, radio signals enter the bandpass filter (BPF) with stopbands from 44 to 79 MHz to eliminate any intermodulation from radio sources above 80 MHz especially the FM band. The bandpass filter has a low insertion loss (1.5 dB max) with good selectivity.
The brain of the Operation
The amplified, bandpass filtered radio signals transfer to the central receiver unit (CRU) for further analyses. This unit is currently located on the roof of the physics faculty. A waterproof box near the radio antennas houses the electronics.
The power needed for the electronics were provided by a photovoltaic panel connected
One of the most difficult challenges of detecting cosmic ray radio signals is their short lifetime which is around 10 to 100 ns. As a result, very fast and accurate digitizers are required to be able to properly capture and record these radio pulses and digitize them for future investigations. The SURA receiver consists of a 14-bit quad channel 160 MSPS analog to digital converter which results in 6.25 ns sampling resolution.
The digitized radio signals then transfer to a data capture card houses an Arria-V GZ Field Programmable Gate Array (FPGA) from Intel with 32 Gb DDR3 SDRAM which makes it capable of capturing a total number of 2Gigabyte of 16-bit samples.
The FPGA is connected to a computer using a USB3 cable in order to transmit the data with the highest possible speed. Both units are low power so the photovoltaic panel along with the batteries can power them for an entire day.
We're not stop there
We have big ambitions...
New antennas are coming soon
We are planning to add more than 10 new antennas on the next phase of the experiment. In that phase, the SURA will move to a new location and the experiment layout will change to have a more effective detection area.
New electronics in testing
In order to improve the quality of the results, new electronics have been designed for the SURA experiment. these units are under final testing and will be added to the electronic chain in the near future.
We're working on the software
The software engineering team of the SURA experiment is working hard to improve the efficiency of the SURA analyze program. In the next version of the code, the performance will increase and we will add new features to the package.
Since we are planning to move the entire SURA setup to a new location in near future, we are constantly monitoring the performance of the solar power to ensure proper operation of the units in the future.