Microwave Loss Reduction in Cryogenically Cooled Conductors
- PDF / 314,588 Bytes
- 9 Pages / 439.37 x 666.142 pts Page_size
- 56 Downloads / 194 Views
Microwave Loss Reduction in Cryogenically Cooled Conductors R. Finger & A. R. Kerr
Received: 23 April 2008 / Accepted: 30 June 2008 / Published online: 26 July 2008 # The Author(s) 2008
Abstract Measurements of microwave attenuation at room temperature and 4.2 K have been performed on some conductors commonly used in receiver input circuits. The reduction in loss on cooling is substantial, particularly for copper and plated gold, both of which showed a factor of 3 loss reduction. Copper passivated with benzotriazole shows the same loss as without passivation. The residual resistivity ratio between room temperature and 4.2 K, deduced from the measurements using the classical skin effect formula, was smaller than the measured DC value to a degree consistent with conduction in the extreme anomalous skin effect regime at cryogenic temperatures. The measurements were made in the 5–10 GHz range. The materials tested were: aluminum alloys 1100-T6 and 6061-O, C101 copper, benzotriazole treated C101 copper, and brass plated with electroformed copper, Pur-A-Gold 125-Au soft gold, and BDT200 bright gold. Keywords Cryogenic electronics . Conductor loss . Skin effect . Copper . Gold . Aluminum . Benzotriazole
1 Introduction Input circuit loss is an important factor limiting the sensitivity of microwave and millimeter-wave receivers. Recently, there has been interest in the possibility of reducing the loss of cryogenic microwave components by using conductors whose resistivity is known to decrease substantially on cooling, and even high temperature superconductors. High temperature superconductors are used in some very low loss microwave circuits, but it is not clear whether, in a typical receiver application, they would have a real
R. Finger (*) : A. R. Kerr National Radio Astronomy Observatory, 1180 Boxwood Estate Rd, Charlottesville, VA 22903, USA e-mail: [email protected]
Int J Infrared Milli Waves (2008) 29:924–932
925
advantage over pure metals whose DC resistivity at low temperatures can be more than an order of magnitude smaller than at room temperature. For high-Q circuits, such as narrow-band filters, superconductors have an advantage over normal metals, but in the usual cryogenic input circuitry of a radio astronomy receiver—mode transducers, polarizers, transmission lines, hybrids, couplers, all of which have low Q—the low resistivity of pure normal metals may result in loss of an acceptably low level. The practical advantages of using plated or machined metal conductors, rather than high temperature superconducting ceramic materials which must be deposited on high dielectric constant substrates, are apparent. While aluminum, copper and gold-plated conductors are commonly used in the input circuits of receivers, the improvement in performance due to the reduction in loss at cryogenic temperatures is not well documented. DC measurements [1] indicate that the change of resistivity of these metals on cooling is strongly dependent on material purity and the internal stress resulting from machining and thermal treatm
Data Loading...
