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Ultra wide band low noise amplifier for low frequency applications in radio astronomy

Now it is well recognized that although Jansky discovered radio astronomy at decametric wavelengths, the quest for higher angular resolution and the difficulties in handling ionospheric effects have left the low frequency (below 150 MHz) spectrum poorly explored. This is despite many unique astrophysical questions that can be better addressed at low frequencies, and problems requiring accurate source spectra at low frequency end as well. However the recent advancement in technology and better understanding of imaging algorithm, interest has been rekindled in low frequency astronomy which requires a high dynamic range imaging in the presence of man made interferences. With this the usage of short dipoles (whose length is much smaller than wavelength) as feeds for parabolic dishes and as stand alone antenna elements for arrays has taken the centre stage. Using short dipoles a compact array could be constructed with less mutual coupling effects. If the dipole is electrically small in size, then its radiation pattern is shown to be almost independent of frequency allowing for broad band operation. One of the main disadvantages of a short dipole is that it offers a very high reactive impedance to the sky radiation as a result of which the latter gets poorly coupled to the receiver. The poor coupling can result in a system in which instead of the sky noise the receiver noise may dominate the system temperature. To ensure a sky noise dominated system even with poor coupling, one requires low noise amplifier even at low frequencies. The amplifier should also have a high dynamic range since the radio frequency interference in the low frequency regime is very high.