La Villette radio-telescope finally awaked!

In Paris Parc de La Villette, the radiotelescope has been activated Sunday July 21, 2019, more than thirty years after it was built.

While the radio-telescope was settled in Paris La Villette Parc 33 years ago it had never been used and all its electronic parts dismantled and electric main power removed soon after. Read the full story about the « Stars song » project on the introduction page of this site describing the effort of rehabilitation accomplished by Dimension Parabole association since 2018. Based on a paper from 1984 published in an astronomy journal (Ciel et Espace) describing the new radiotelescope characteristics, we read about an existing transistor MOS-FET preamplifier located near the antenna dipolar source. We thus tentatively applied a 10-15 Volts continuous voltage to the coaxial cable descending from the source cavity in order to try to power it on. In the 80’s a MOS-FET transistor preamplifier was the most performing device available. We did not have informations about its gain performance. We only had an information about its 1.5 dB noise figure.

As soon as we applied more than 10 V DC (with a protected variable voltage power supply 10-18 V) we happily observed that the meter displayed a 100 mA current was delivered. This proved that an electronic device was indeed connected to the coaxial cable seven metres above the insertion point.

Then Rémi F6CNB/N5CNB used a 500 KHz-1,6 GHz spectrum analyzer connected to the antenna in order to measure the signal power received by the dipole antenna through the coaxial cable. A test pulse signal was sent at the same time by the analyzer through a second coaxial cable to a dipole located at the center of the parabolic dish reflector. This dipole had been designed for receiving calibration purpose.

The following picture displays the level of received signal while the remote preamplifier was not powered by a continuous voltage.

Signal level was at -63 dB between 1000 and 1500 MHz while preamplifier is not powered
Preamplifier measured gain is around 20 dB at 1400 MHz (60 – 40 dB))

Power supply voltage was then injected into the coaxial cable through a special device (Bias-Tee) for applying the voltage to the remote MOS-FET preamplifier and at the same time let the radio signal get down to the receiver located 15 meter down. We have set at this location an SDR receiver behind a second preamplifier with a gain of 25 dB and 0.7 dB better noise figure. Resulting signal level measured by SDR was -77,3 dB as shown on next picture when remote MOS-FET preamplifier was not powered.

Alternately, when DC voltage was applied to remote preamplifier, received signal raised to a level of -64,6 dB. The difference (77,3 – 64,6) translates the remaining gain of 12,7 dB due to conducting loss into the 15 m coaxial cable. This loss is probably greater than 12 dB for if we do not insert the supplementary local preamplifier, we do not observe any change in the signal level measured by SDR receiver while remote preamplifier was powered. There is no doubt that a better result will be achieved when the second preamplifier is installed closer to receiving dipole. Lately the MOS-FET preamplifier will also be replaced by a one designed for radio-astronomy with a better noise figure.

On-line WebSDR does not display any specific signal but it is useful for signal level measurements.

We will report our progress in reception performance and results when preamplifiers are set closer to source antenna.

Up to now the radiotelescope antenna is fixed in vertical position while declination motor is being repaired. However we took the opportunity of Earth rotation to record twice per day signals from our Galaxy passing through the very narrow antenna beam (1 degree angle). Read the report of first « light » captured by the radiotelescope.