30 ottobre 2009

USA: la radioastronomia in banda televisiva

Sulla newsletter del NAB, RadioTechCheck, c'è un bellissimo articolo sulla radioastronomia nel contesto della regolamentazione delle bande VHF-UHF. Negli Stati Uniti e nel Canada, il canale 37 della televisione, intorno ai 600 MHz, è stato assegnato ai radioastronomi (con una moratoria della FCC che è stata convenzionalmente trasformata in una norma: nessun broadcaster analogico ha più utilizzato quel canale). Perché questa frequenza così bassa? La radioastronomia dell'idrogeno esplora lo spettro della banda L, intorno ai 1400 MHz, ma come ben sapete i corpi celesti non sono fermi, alla loro distanza e velocità relativa corrisponde un spostamento Doppler che trasporta le righe radio dell'idrogeno molto più in basso, proprio sui 600 MHz. In questa porzione di spettro si osservano tra l'altro oggetti molto distanti (dell'ordine dei 10 alla 22 chilometri) e anziani (9,5 miliardi di anni).
L'articolo fa riferimento, riassumendolo, a un intervento di Andrew Clegg, della National Science Foundation, in occasione del Broadcast Sympsium della iEEE (14-16 ottobre ad Alexandria, in Virginia), proprio sul tema del destino del canale 37 nell'era dello switchover televisivo al digitale.
La frequenza di 608 MHz del canale, è alla portata dei radioastronomi dilettanti. Un bellissimo sito di progetti autocostruiti, quello di MTM Scientific Inc. ha una una pagina su un tuner per radioastronomia (con tanto di indicazioni per la costruzione di apposite antenne a alto guadagno) che sembra davvero carino. Qui vi riporto un grafico con la gobba corrispondente alla ricezione, con il kit radioastronomico, di Cassiopea A, una potente fonte di energia nello spettro radio. Sullo stesso sito della MTM trovate fra l'altro anche degli ottimi kit per la costruzione di antenne loop, ad anello, ideali per le onde medie.


Broadcasting’s Neighbor – Radio Astronomy

The International Year of Astronomy 2009 (IYA2009) is an event initiated by the International Astronomical Union (IAU) and UNESCO to celebrate astronomy and its contributions to society and culture. Not coincidentally, 2009 also marks the 400th anniversary of the first use of an astronomical telescope by Galileo Galilei, the famous 17th century Italian philosopher and scientist. While it was 400 years ago that Galileo first pointed a telescope at the heavens, it wasn’t until the mid-1930’s that the science of radio astronomy was born.
Radio astronomy involves searching the heavens for objects which radiate electromagnetic radiation in the radio frequency bands as opposed to the optical frequency bands seen by telescopes such as those used by Galileo. An interesting talk on the relationship between radio astronomy and broadcasting was given at the recently-held IEEE Broadcast Symposium (Alexandria, Va., October 14-16, 2009, www.ieee.org/bts [qui il programma dettagliato del simposio]) by Dr. Andrew Clegg of the National Science Foundation entitled “The Co-existence of Broadcast and Radio Astronomy Services or What Ever Happened to TV Channel 37?” Portions of Dr. Clegg’s presentation are excerpted below.
As can be seen in the table at right, a significant number of frequencies have been allocated for radio astronomy in the VHF and UHF bands. Shown here (in gray shading) are all of the radio astronomy frequencies allocated in these bands along with the other services that share these frequencies with the radio astronomers.
Also shown (in orange shading) are the TV and FM radio frequency allocations in the VHF and UHF bands. Looking in the UHF band, the radio astronomy allocation from 608 to 614 MHz (which is shared with Land Mobile services) is 6 MHz wide and falls precisely between TV channels 36 and 38. In fact, these frequencies were originally allocated as TV channel 37 until 1963, when the FCC instituted a 10-year moratorium on use of this channel, to make these frequencies available instead to radio astronomers. Ultimately this ban became permanent, both in the U.S. and Canada, and in fact no local TV station has ever been allocated TV channel 37 in either country. In addition, TV channel 37 is allocated for radio astronomy in many other countries around the world.
The allocation of TV channel 37 for radio astronomy is a result of the need by astronomers for regularly-spaced spectrum bands in which to make observations. At the time of the initial allocation, astronomers had access to frequencies in the 1400-1600 MHz and 150-300 MHz bands but were lacking frequencies in between; the allocation of TV channel 37 at 608-614 MHz to radio astronomy resolved this problem. Observations at these frequencies are made primarily of hydrogen atoms, the most prevalent atoms in the universe.
Radio frequency emissions from hydrogen atoms fall nominally at 1420 MHz, but because of the expansion of the universe which creates a Doppler shift of spectral line emissions, astronomers observe hydrogen emissions at frequencies lower than this, as well. The farther away a hydrogen atom is, the greater the Doppler shift of its emissions and the greater the shift towards lower frequencies. It turns out that when astronomers look at hydrogen emissions around 600 MHz, they are looking at a distance of about 8x1022 km, and when the speed of light is factored in, this means that these emissions are about 9.5 billion years old.

Dr. Clegg indicated in his talk that radio astronomers appreciate the efforts of the broadcast industry in coordinating possible interference mitigation due to adjacent-channel TV stations. Some other interesting facts presented by Dr. Clegg include the following:

* The science of radio astronomy was discovered by accident back in the 1930’s when a Bell Laboratories engineer, Karl Jansky, was trying to identify sources of interference to long-distance wireless telephone circuits. Using the rotatable antenna shown at right, Jansky identified three sources of interference: local lightning storms, the aggregate of distant lightning storms, and a persistent but variable source that came and went on a slightly-less-than daily rate. When Jansky plotted the direction of this variable interference, it coincided with the plane of our Milky Way galaxy. Observations over several months in 1932-1933 confirmed the result, and radio astronomy was born.

* While Jansky’s result was of academic interest, there was no commercial reason to pursue further observation, and the “science” of radio astronomy languished – even astronomers didn’t follow up. Grote Reber, an amateur radio operator and an employee of a radio company, had read about Jansky’s discovery, and, beginning in 1937, took it upon himself to further investigate the nature of cosmic radio emissions, purely as a personal effort.

* Jansky and Reber’s observations were only the “tip of the iceberg” as they were able to detect only the very strongest astronomical radio emissions. The bulk of radio astronomy observing deals with much, much weaker emissions. In honor of Karl Jansky, the basic unit of radio astronomy signal strength is the jansky (Jy) = 10-26W/m2/Hz. A 1 Jy signal is equivalent to -176 dBm received signal strength for a 6 MHz bandwidth TV channel, using a 0 dBi receive antenna (-46 dBµV/m). Signals a million times weaker (1 µJy) are routinely observed and studied, corresponding to a -236 dBm TV signal. As a result, radio telescopes are extremely sensitive to RF interference (RFI) from adjacent channels.

* Radio telescopes measure the amount of radio power coming from a source by using very careful differential noise measurements (on-source vs. off-source). This is necessary because system noise, even when using cryogenically cooled receivers, often dominates the total noise power. The accuracy of differential noise measurements (assuming Gaussian statistics) improves with the square root of the product of the observing bandwidth and the averaging time. Therefore, most radio astronomical observing makes use of the widest bandwidth and longest possible observing time. Bandwidth is often limited only by RFI, feed horn response, or other fundamental limitations.

Additional information on the International Year of Astronomy is available on the Internet at www.astronomy2009.org.

2 commenti:

r3fan ha detto...

Al di la delle interessanti informazioni sulle bande radio dedicate alla radioastronomia fornite dall'articolo citato, non ho potuto fare a meno di dare un'occhiata al sito MTM da te citato e dopo aver letto le specifiche del "tuner per radioastronomia" mi permetto sommessamente di sollevare un sopracciglio dubbioso sull'adeguatezza di tale ricevitore per gli scopi sia pur amatoriali o dilettanteschi in ambito radioastronomico: cifra di rumore ~10dB; mi sembra un'enormità, in radioastronomia la cifra di rumore è tutto, si può amplificare quanto si vuole ma se il segnale è affogato dal rumore proprio del ricevitore non si cava un ragno dal buco. Si devono usare amplificatori RF dal rumore più basso possibile, NF < 1 dB, sul tipo di questo ad es (http://www.radioastronomysupplies.com/show_detail.php?item_id=14). Anche l'antenna proposta non mi sembra all'altezza quanto a guadagno e direttività e temo che un appassionato che volesse cimentarsi in questa sperimentazione con le apparecchiature proposte andrebbe incontro a delusione.
cordialmente, luciano

Andrea ha detto...

Una NF intorno ai 10 dB è certamente molto elevata, ma il punto era proprio quello di realizzare un semplice detector di sorgenti radioastronomiche in banda televisiva con attrezzature off the shelf, come suggerito nell'introduzione del sito MTM. Altrove su Web si trovano riferimenti a semplici radiotelescopi per frequenze superiori, anche se in genere in quei casi si costruisce un downconverter amplificato a bassa cifra di rumore nel front end e si utilizzano tuner commerciali alle medie frequenze. Ripeto, stiamo parlando di una primissima introduzione alla materia, può effettivamente darsi che 10 dB siano troppi, come potrebbe essere che sorgenti intense come Cassiopea A riescano a passare. Per una analisi più tecnica del kit MTM e dei limiti messi giustamente in risalto da Luciano, potete fare riferimento a questo recente thread della lista della Society of Amateur Radioastronomers.