Multiplexer filters for the Korean Very Long Baseline Interferometer (VLBI)
Recent advances in mesh filter design and assembly technology at our production site in Cardiff now enable us to offer filters with a clear aperture diameter of up to 300mm. Twelve filters (four sets of three different designs) were delivered to the Korean Astronomy Obseratory, now known as the Korean Astronomy and Space Science Institute for their Very Large Baseline Interferometer.
The three filter specifications are as follows:
• 1.0cm-1 (30GHz) low pass. 231mm clear aperture
• 2.4cm-1 (70GHz) low pass. 220mm clear aperture
• 3.5cm-1 (105GHz) low pass. 150mm clear aperture
Filters under test at NPL
The three filters are used to separate the following four frequency bands:
The minimum requirement for both low frequency transmission efficiency and high frequency reflection is 90%, with a target specification of 95%. The filters are used at fifteen to twenty degrees away from normal.
The performance of the filters was tested using:
The photographs below show some of the filters undergoing testing on the VNA at NPL
The following graph shows the measured transmission profile of the 1cm-1 LPE filters. Thick vertical lines mark the frequency bands that are to be separated. The requirement of a minimum T of 90% is certainly met and, with less noisy data, we are confident that the target of 95% has been met. Below 48GHz the Cardiff polarizing FTS is an inappropriate instrument to use because its source, an Hg arc lamp, provides very little power at these low frequencies. However extrapolation from the data measured at higher frequencies indicates an excellent agreement with theoretical predictions.
Our filter technology is remarkably versatile and covers a wide frequency range. We have low-pass filters up to 650cm-1 (20THz) and band-pass filters centred as high as 300cm-1 (10THz).