|Session:||Session 3P2A - Invited Papers (09b)|
|Date:||Wednesday, November 08, 2006|
|Time:||14:00 - 15:30|
Antennas for Radio Astronomy
van Ardenne, A.
Astronomy is a fundamental, curiosity driven research aiming at improving our understanding of the physics that govern the universe and its making. The radio branch of astronomy called radio astronomy covers over 5 decades of observing frequency from MHz up into the TeraHerz domain with a multitude of observing techniques. Not only offers this wide range of frequencies an incredibly rich view of the universe and hence underlying physical phenomena, but more interesting to this presentation is that its users, the astronomers are pushing the limits of their observing suit in which antennas play a key role. A major challenge then is on the antenna engineering community to respond in an equally ambitious way to satisfy the often ill posed demand. Over the last decade, the radio astronomical community has been remarkably active in preparing itself for major new observing instruments redefining the way radio astronomy will be done for decades to come. Typically, the lead time toward realization is order 15-20 years over which time the technological and engineering readiness has to be proven. Such timescales are long enough to accommodate new insights to be absorbed and to educate several generations of young academic researchers! An example of a new instrument at the high frequency end and operational for full scientific exploitation around the end of this decade, is the Atacama Large Millimetre Array ("ALMA", see e.g.: www.eso.org/projects/alma). In the process of being build now by a consortium of European, North American and Japanese institutions, the 50 ALMA antennas will be located at a high altitude plateau of the Atacama desert at 5000m. The 50 telescopes will operate in unison as a so-called aperture synthesis array, allowing precise and good quality images of radio sources to obtain. The observing frequency range of this array runs in several frequency bands from below 100 GHz up to close to a Teraherz i.e. from wavelength less than a millimetre to the sub-millimetre domain. This places extreme requirements on the surface and pointing accuracy of the dual reflector dishes which primaries are 12 m in diameter, located in the open i.e. no protective radome, as well as on the quasi-optical cryogenically cooled receiving systems. At the very low frequency end and also at the southern hemisphere, the Indian Giant Metre wave Radio Telescope ("GMRT") located in Ooty, consists of 30, 45 metre diameter telescopes and started full scientific operation around 2000. Its receiver suit covers the range from 150 MHz to over 1 GHz that is from decametre to meter wavelengths and in this wavelength regime is the largest synthesis array telescope worldwide. At still lower frequencies, northern Europe and most pronouncedly the Netherlands, houses the Low Frequency Array ("LOFAR", see: http://www.astron.nl/ska/ and http://www.lofar.org) which plans to become operational in a few years. Its frequency range of operation ranges (in two bands excluding the FM receiving band) from 30MHz to 250MHz. Contrasting the above mentioned arrays and in particular as compared to ALMA, there are hardly any requirements on the mechanical accuracy of the receiving antennas as they are electrically small albeit very wide band dipoles, fixed mounted with almost all sky coverage. Future research needs of astronomy have led radio astronomers worldwide, to study the concept of a next generation of radio telescopes for what has been dubbed the Square Kilometre Array ("SKA", see http://www.skatelescope.org). Planned for operation in the early part of the next decade it will outperform today's radio telescope instruments in many aspects. In particular a three-decade frequency range, greatly improved sensitivity (in radio astronomy characterized as the effective collecting area over the systemtemperature i.e. Aeff/Tsys ) and angular resolution that matches and even outperforms, the capabilities of planned instruments in other frequency domains, are challenges that are now being investigated. As in parallel, several other new radio telescope projects like the many small dishes concept of the US SETI (see: http://www.seti.org/science/ata.html ) are taking shape aimed likewise as the ones mentioned above, to be ready at shorter timescales, there are key elements in these which could serve as guides for the SKA. In Europe emphasis is placed on studying a low cost planar phased array concept in the context of a European program called SKADS that started last year (see: http://www.skads-eu.org and e.g.  for earlier work). This presentation aims to inform and update on these exciting developments, to glimpse into the scientific rationales for building these and to present the concepts and the technologies that are being examined around the globe, emphasizing the key role that antenna engineers are playing in concert with other disciplines.  A. van Ardenne, B. Smolders, G. Hampson; "Active Adaptive Antennas for Radio Astronomy; results of initial R&D toward the Square Kilometre Array", Proc. SPIE Conf. 4015 Radio Telescopes. Ed. H. Butcher, Munich, March 2000.
Terahertz Technology for Space and Earth Applications
De Maagt, P.; De Maagt, P.
The terahertz (THz) part of the electromagnetic spectrum falls between the lower frequency millimetre wave region and, at higher frequencies, the far-infrared region. The frequency range extends from 0.1 THz to 10 THz, where both these limits are rather loose. As the THz region separates the more established domains of microwaves and optics, a typical THz technique will incorporate aspects of both realms, and may even draw on the best of both. The two bounding parts of the spectrum also yield distinct sets of methods of generating and detecting THz waves. These approaches can thus be categorised as having either microwave or optical/photonic origins. As a result of breakthroughs in technology, the THz region is finally finding applications outside its traditional heartlands of remote sensing and radio astronomy. Extensive research has identified many attractive uses and has paved the technological path towards flexible and accessible THz systems. Examples of novel applications include medical and dental imaging, gene theory, communications and detecting the DNA sequence of virus and bacteria. The presentation will discuss the range of THz applications and will present the components and systems that are utilised for the frequency region.