High Temperature Superconductivity and Superconducting Antennas



I. The History of Superconductivity



Superconductivity was discovered in 1911 by the Dutch physicist, Heike Kammerlingh Onnes. Onnes dedicated his science career to the studies of the properties of materials at extremely cold temperatures. In order to study materials at these very low temperatures, major advances in refrigeration had to be made. The first advance was the invention of a vacuum insulated cryostat- a cryostat is essentially a large "thermos bottle" designed for low temperature research. A few years later on July 10, 1908, Onnes liquefied a few drops of helium by cooling it to 452 degrees below zero Fahrenheit (or 4 Kelvin’s or 4 K); this was the start of his explorations in the properties of materials at low temperatures previously unreachable. Liquid helium enabled him and future scientists to cool materials to temperatures within 4 Kelvin’s of Absolute Zero (0 Kelvin’s), the coldest temperature physically possible. Absolute Zero is the temperature at which the thermal energy of atoms becomes as small as possible.



Later in 1911, Onnes began to study the electrical properties of metals at these newly accessible low temperatures. It was known that the resistance of metals dropped as the temperature was lowered, but, no one knew how much the resistance would drop as one approached Absolute Zero. Some scientists including William Kelvin, believed that the electrons would actually stop as temperature reached Absolute Zero causing the resistance to increase. Other scientists including Onnes, believed that the resistance would continue to decrease and disappear completely only at Absolute Zero. Still others felt that a plateau in resistance would be reached at some low temperature.



Onnes first decided to pass an electrical current through a pure wire of mercury while measuring its resistance as he steadily lowered the temperature. As to everyone’s surprise, there was total disappearance of resistance at 4.2 Kelvin’s. He called this newly discovered state "Superconductivity". In another experiment, Onnes started a current in a superconducting wire which was he kept cold in liquid helium for one year. After one year, Onnes found that the current was still flowing with no measurable losses without any assistance from a battery. He named these currents persistence currents. In a third experiment, Onnes measured the resistance of a lead wire in the superconducting state as he slowly increased the current. Also to his surprise, the wire showed no resistance initially but suddenly developed a high resistance as the current exceeded a critical value. This maximum current was termed the critical current density and it typically exceeds 10 million amps per square centimeter at 4 Kelvin’s! Due to these important discoveries, Kammerlingh Onnes was awarded the Nobel Prize in Physics in 1913.





Typical Superconductor Resistance vs Temperature





Critical Current Density vs Temperature of a High Quality YBa 2 Cu 3 O 7 Film



Whenever a new scientific discovery is made, scientists strive to explain their findings with a theory. In the 1920's, Albert Einstein and Onnes attempted to explain the phenomenon of superconductivity but Einstein soon decided that too little was known about these materials to develop a theory at that time. To gather more information on superconductors, W. Meissner and R. Ochsenfeld studied the magnetic properties of superconductors in the early 1930's. They discovered in 1933 that superconductors have another interesting property. Superconductors will not allow a magnetic field to penetrate into their interior. This finding is due to the generation of currents on the surface of the superconductor which exactly cancel the magnetic field in the superconductor's interior. This new property was called the Meissner Effect and it is this property that may someday allow the development of high speed levitating trains. Likewise, due to a limiting critical current density, Meissner discovered that if the magnetic field placed on a superconductor is increased beyond a critical value, the superconducting state suddenly disappears and resistance returns. This maximum magnetic field is now called the critical field.



These new findings along with the new theory of quantum mechanics allowed the development of a theory for superconductivity. In 1957, three American physicists at the University of Illinois, John Bardeen (inventor of the transistor), Leon Cooper, and Robert Schrieffer, described a model that has since stood as the theory describing the disappearance of resistance in these materials. Although the mathematics is very complicated, the model in simple terms shows that the motion of an electron through a wire causes a wave to form in the positively charged nuclei of the atoms which in turn attracts an oppositely charged second electron in such a way as to avoid colliding with any of the atoms. Restated, the electrons ride on each others "wakes". In 1972, Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize in Physics for their theory of superconductivity which is now known as the BCS theory. By the early 1980's, the first commercial applications of superconductivity began to emerge-MRI magnets used in numerous hospitals throughout the world.



In 1986, two IBM staff scientists Georg Bednorz and Alex Muller, studied the electrical properties of a new class of oxide ceramics known as perovskites. Working at IBM's Zurich Switzerland facility, Bednorz and Muller discovered superconductivity at a record high of 35 K in some LaBaCuO crystals. This was 12 K higher than any scientist believed possible. Soon research labs from around the world began to confirm the findings and nearly every country in the world has at least one person studying superconductivity in hope that it may someday save billions in lost energy due to resistive losses in transmission lines.



In February 1987, a still higher superconducting record of 92 K was made by scientists at the University of Houston and the University of Alabama in Huntsville who substituted yttrium for lanthanum bringing superconductivity into the liquid nitrogen range. This increase in temperature is significant because liquid nitrogen (which boils at 77 K) is as cheap as coffee. Because of this large increase in operating temperature, these new materials are now called "High Temperature Superconductors". Since then scientists have found additional materials that superconduct at temperatures exceeding 133 K-nearly half way to room temperature (or 290 K)! Currently, many governments, corporations and universities are investing huge sums of money in the study of High Temperature Superconductivity, particularly in the development of commercial applications. The higher operating range of these new materials has influenced vast efforts in the development of these compounds, and changing the theory of the behavior of superconductors at these relatively higher temperatures. Most important, electrical power applications for these new superconductors are expected to be practical in the next decade, thanks to the efforts of so many scientists and countries. It is likely that we will see unbelievable improvements to the electronics and power industry over the next few decades, not to forget the improvements to the communications industry.







II. Superconducting Antennas



The recent discovery of high-temperature superconductivity at liquid nitrogen temperatures (77 Kelvin’s) brings us a giant step closer to the vision of early scientists. Applications currently being pursued are mostly extensions of current technology used with the low-temperature superconductors such as powerful magnets used in MRI scanners. Additional applications include magnetic shielding devices, extremely sensitive medical imaging systems, infrared sensors, analog signal devices, and microwave communication devices, and waveguides. As our knowledge of the properties of high-temperature superconducting materials increase, more efficient power transmission lines, smaller and more efficient generators, energy storage devices, particle accelerators, and levitating trains will become more practical.



The ability of superconductors to conduct electricity with zero resistance can be exploited in the use of many electronic applications. Currently, a substantial fraction of electricity is lost in the form of heat through resistance associated with the traditional conductors such as copper or aluminum. More than 15% of the world's generated electricity is lost as heat-this corresponds to trillions of US dollars that could be saved! At present, the development of these larger scale applications has primarily been slowed by the difficulty of preparing wires from these novel materials since these superconductors are brittle ceramics. Fortunately, major advancements were made in the past decade in making flexible high-temperature superconducting wires using fine superconducting filaments embedded in silver tapes.



High-temperature superconductors also hold promise with the miniaturization and increased speed of computer chips. Computer chip speeds are primarily limited by the generation of heat due to resistance of the interconnects and the charging time of capacitors. The use of new superconductive films could be employed to produce more densely packed chips that could transmit information more rapidly by several orders of magnitude. To date superconducting electronics have had impressive accomplishments in the field of digital electronics. Logic delays of 13 picoseconds and switching times of 9 picoseconds have been experimentally demonstrated. However, the primary difficulty has been producing chips having a useful number of transistors. With present technology, 1 out of 3000 transistors has a defect when superconducting interconnects are employed. However, three years ago, 1 out of 100 had a defect. As the manufacturing reproducibility approaches that of the current microprocessors, new and extremely powerful digital electronic devices will become a reality.



The use of superconductors in transportation has already been proven using liquid helium as a refrigerant. Prototype levitating trains have been constructed in Japan using low-temperature superconducting magnets. Superconducting magnets are already crucial components of several technologies. Magnetic resonance imaging (MRI) is playing an increasing important role in medicine. Furthermore, particle accelerators used in high energy physics studies are very dependent on the use of high field superconducting magnets. The recent controversy surrounding the continued funding for the Superconducting Super Collider (SSC) illustrates the political ramifications of the applications of these new technologies. As liquid nitrogen based higher- temperature superconducting magnets become available, more industries will be able to afford these magnets because the cheaper cost of the higher operating temperatures.



New commercial innovations usually begin with the existing technological knowledge generated by the industrial research scientist. The work of commercialization centers on the development of new products and the engineering needed to implement the new technology. Through the collaborative efforts of government funded research, international research groups, and commercial industries, applications of new high-temperature superconductors will be in the not so distant future. Time lags, however, between any new discovery and the introduction of a new device. For instance, the discovery of the laser in the early 1960's has only recently been appreciated today through applications such as laser surgery, laser optical communication, and compact disc players. The rapid progress in the field of superconductivity leads one to believe that applications of superconductors is limited only by one's time and imagination.



2. Superconducting Antennas



By now, you are probably wondering how can these new high- temperature superconductors could improve the communications industry. Only the mind can set limits to the potential number of improvements that can be envisioned. The telecommunications industry already uses high-temperature superconducting films to coat the inside of their microwave waveguides to reduce losses in their system. Furthermore, as superconducting transistors are developed, perhaps longer lasting and smaller "finals" could be developed for transceivers.



A more immediate application could perhaps be in the antenna system. Theoretically, superconductors could be employed to reduce the resistive losses in an antenna. However, since one "S" unit of signal strength corresponds to a change of 6 dB, a substantial increase in efficiency will be required for a target station to notice any improvement. Although less likely at the short wavelengths used by many world wide broadcast stations, dramatic improvements are more likely at very long wavelengths because of the severe space limitations of the antenna. It is well known that an antenna needs to be a minimum of 1/8 wavelength in length to be reasonably efficient. Unlike the short wave frequencies employed in most world wide communications, this constraint is not severe. Due to salt water penetrating ability, submarines utilize 40 km wavelengths; therefore, an efficient antenna needs to be several miles long in order to have a reasonable efficiency. These long wires do pose obvious difficulties in the operation of submarines; it will be shown below how superconductivity could provide significant reductions in the antenna length while keeping nearly a 100% radiation efficiency.



3. Antenna Efficiency



Antenna efficiency E(%) is calculated by the equation E(%) = 100% x R r / (R r + R g + R c cos2 h), where R r = radiation resistance, R g = ground-loss resistance (approx. 0 in horiz. dipoles), R c = loading coil resistance (~0 in superconducting coils assuming the superconducting ac-losses are negligible), and h = distance between the loading coil and the feed point in fractions of wavelength expressed in degrees. For example, if the loading coil placement is 1/8 wave from the feedpoint, h = 360 degrees/8 or 45 degrees.



To perform this calculation, the radiation resistance must be known. From electromagnetic theory (given in many physics texts), the radiation resistance (Rr) of a center-fed dipole is given by R r = 197 D2 / L2, where D = antenna length and L = wavelength. From this equation, it can be seen that a half-wave antenna has a radiation resistance of approximately 50 ohms. Most important, we see that as an antenna becomes short compared to the wave length, the coil resistance exceeds the radiation resistance thereby dissipating the power in the form of heat. Superconductivity will eliminate this coil loss while allowing the desired 100% radiation efficiencies in extremely short antenna systems.



4. Antenna Q-Factor



Loading coils are often described by their Q-factors or "quality factors". The Q of a coil is given by Q = X1 / R c where Xl = loading coil reactance; R c = loading coil resistance. As the Q-factor increases, the resonance band width becomes narrower. In a superconducting antenna, one might expect the Q- factor to approach infinity implying it will only resonate at one specific frequency. Fortunately, this is not the case because all materials contain microscopic defects; in superconductors, these microscopic defects cause each electron to follow a slightly different path in order to avoid these defective, non- superconducting, regions. As a result, each electron has its own specific electrical length from one tip to the other tip of the antenna and this phenomenon, also known as percolation, prevents the antenna from developing the undesired infinite Q-factor while maintaining the desired zero resistance.



5. World's First Superconducting Antennas



In the spring of 1995, the Fusion Energy Division of the Oak Ridge National Laboratory built a 2m VHF BSCCO antenna. Using a Hewlett-Packard 8753A Network Analyzer, the principle investigators, E.C. Jones and D.O. Sparks, discovered that the resonance frequency dropped by approximately 5% as the superconducting tape was cooled below the superconducting transition temperature. In addition, the Q-factor increased only slightly as already discussed above. The change in resonance frequency was believed to be the result of the rf current redistributing from the silver matrix in the normal state to the superconducting filaments as the tapes were cooled to their superconducting state with liquid nitrogen. Since these tapes had twisted filaments, the current had a 5% longer conduction path, i.e., "longer effective wavelength", at these superconducting temperatures. To the best of our knowledge, this was the first VHF antenna of its kind to have ever been built and to my disappointment, the Oak Ridge National Laboratory (who owned my superconductor patent rights) and the Department of Energy decided not to pursue this line of work or file any patents. In 1997, on my personal time, I built a 2 foot tall 160m superconducting antenna for use near 1.86 MHz. I found that as the antenna was cooled with liquid nitrogen, the signal strength meter of the transceiver used to test the antenna increased from an S1 to S9 indicating the clear feasibility of these materials for long-wavelength communications. Also, to the best of my knowledge, I am unaware of any one else who has ever built a superconducting antenna for long-wave communications. In contrary, other research groups have used superconducting antennas to reduce the ac-losses found at the higher microwave frequencies. The first microwave superconducting antenna is credited to the Electronic Materials and Devices Research Group at the University of Birmingham (United Kingdom) and this group was recently presented with the International IEE Premium Award for their work.

6. Closing Remarks

It is hoped that this bulletin will raise the awareness of the developments in this new field; it is likely that superconductivity will change the communications industry in some now unimaginable ways. Imagine holding a superconducting rubber ducky capable of working the recently publicized 1.750 km band without the need of stringing up an outdoor antenna! Better yet, imagine a short rubber ducky antenna capable of working wavelengths longer than 40 km - wavelengths well known for their ocean and/or underground cave penetrating capabilities!



