ASSA Radio Astronomy Group

Operation

Radio power signals arriving from directions close to the axis of the parabolic antenna are focused by reflection to an antenna feedhorn optimised for operation at 1420MHz. Antenna gain is assumed to be approx. +30dBi. Signals then pass through two L band probes phased 90 degrees apart and coupled to a pair of low noise amplifiers (LNA) with noise figures of ≤ 0.2dB at 1420MHz. The LNAs were built, adjusted and tested as a matched pair by Dominique Faessler, HB9BBD - www.hb9bbd.ch. Each LNA provides an input 40MHz bandpass filter and two amplification stages producing 40dB of gain. The bandpass filters prevent out of band signals such as those generated by geostationary satellites and other man made interference sources from producing intermodulation products in the SDR's image rejection mixers.

In the current configuration, the two signals are then combined with a 90° quadrature hybrid coupler and fed via low loss cable to one SDR. In order to minimise cable losses, the SDR is housed directly underneath the antenna. Sometime in the future I plan a dual SDR configuration based on the USRP II which will allow each LNA output to be fed via low loss cable to each SDR, the incident & quadrature (I & Q) signals allowing more complex DSP to further reduce noise effects and enable other enhanced signal detection techniques.

The current SDR is a dual conversion superhetrodyne type with demodulation modes for amplitude modulation (AM) and narrow band frequency modulation (FM). Specified sensitivity at 1420MHz for 500Hz IF bandwidth in CW mode for 10dB S/N is -119dBm (250nV).