A new reverberation chamber has been installed at the Cape Town offices of the SKA (Square Kilometre Array) project. Designed to test for radio emissions from the equipment intended for use at the MeerKAT site, the chamber will be used for testing of computers, network switches, cameras, power supplies, shielded enclosures and, most importantly, the telescope subsystems.
Chamber components were acquired from Comtest NL and installed by Interference Testing and Consultancy Services (ITC) South Africa. The SKA SA team worked with Stellenbosch-based electromagnetic compatibility specialists, MESA Solutions, to characterise the chamber.
“We cannot have the ultra-sensitive MeerKAT receivers detecting radio emissions from nearby subsystems and site equipment because it will degrade the telescope’s performance and, therefore, the astronomical observations,” explains Simon Norval, RFI (radio frequency interference) manager at SKA SA.
“Equipment found to emit radiation levels in excess of that prescribed by the South African Radio Astronomy Service (SARAS), will either be electromagnetically shielded or, where feasible, located far enough from the MeerKAT. Shielding of ‘key performance’ equipment to levels below the required radiation standard is one of the biggest engineering challenges of the project,” Norval adds.
The walls of the reverb chamber are made of specially-treated galvanised steel to create surfaces that fully reflect the radiation emitted from equipment being tested. Inside the chamber, a multi-plated structure, called the stirrer, rotates and ensures that any radiation inside the chamber is uniformly distributed, to make the measurements more accurate. “It works something like a microwave oven in terms of field distribution,” says Norval.
The design of the door of the chamber was critical, because it is from here that signals could escape. For this reason, an extra metal-laminated ribbon seal was added to ensure that the chamber is completely leak-free. Similarly, the external ports (for power and data signal throughput) have in-line filters which prevent external radio signals from entering the chamber.
Following the physical installation of the chamber, testing confirmed that the shielding effectiveness is of the order of 120 dB, which means that radiation inside the chamber is attenuated by one billion times, before it reaches the outside world (similarly, radiation in the environment is weakened by the same amount, before it reaches the inside of the chamber).