Preamplifiers and band-pass filters

With a 10 m large diameter the parabolic antenna reflector gives a theoretical gain of 40 dB on 1,4 GHz band. The parabola prime focus is located 5 metre distant from the grid reflector. The source cavity is handled by a mast boom and holding a dipole that is the receiving part of the antenna. Signals captured by the dipole are first amplified by a transistor MOSFET preamplifier that is at least 33 years old as it has been put there when the telescope was built. This preamplifier characteristics are the best that could be achieved in the 1980’s. For the present time we have no original document about it but we have been given a copy of a paper where it was described to have a 1.5 dB noise factor and a system temperature of 150° K. Then a 15 m long coaxial cable connects the preamplifier to an SDR receiver. Despite this cable is a low loss one, a certain level of signals could be lost and some noise added due to the length of the cable. Recently we have been extremely surprised that when applying 12-15 V to the coaxial cable via a Bias-Tee did awake the old preamplifier after 33 years ! Using a RIGOL DSA815 9KHz-1.5GHz spectrum analyzer we estimated the preamplifier gain at 1400 MHz to be around 20 dB.

-64 dB signal level from radiotelescope source with preamplifier not powered
20 dB preamplifier gain around 1400 MHz ( -40 dB versus noise level -60 dB)

IMG_1893

It was necessary to add another preamplifier 15 m from the source in front of the SDR. (it is a KU LNA 133BH). Its characteristics are displayed on next figure (+28 dB and 0.6 dB noise factor around 1420 MHz).

IMG_1896

Intrinsic preamplifier noise factor, important for overall radiotelescope receiving performances is displayed on the left scale. Gain is given on the right scale.

Gain&Facteur_Bruit
Deuxième_preampli

When antenna motorisation is repaired we will have access to the source cavity. At this time the first MOSFET transistor preamplifier will be replaced by a better one dedicated to radioastronomy. HP LNA with CAVITY BAND PASS FILTER. Specifications: The filter section, will give the observer better selectivity (High Q) and will ward off some types of annoying interference, such as radar. The filter will pass the 1420 MHz signal and effectively attenuate unwanted signals. Manufacturer provides LNA characteristics: LNA, N/F of 0.29 +/- 0.15 dB, gain of 35 dB +/- 3 dB. With N male connector.

DSC_0062
SKU: RAS-1420LNA-CBPF
F5DQK LNA actual measurements

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