The electronic warfare (EW) and radar communities expect to be major beneficiaries of the performance advantages high-temperature superconductivity (HTS) has to offer over conventional technology. Near term upgrades to system hardware can be envisioned using extremely small, high Q, microwave filters and resonators; compact, wideband, low loss, microwave delay and transmission lines; as well as, wideband, low loss, monolithic microwave integrated circuit phase shifters. The most dramatic impact will be in the far term, using HTS to develop new, real time threat identification and response strategy receiver/processing systems designed to utilize the unique high frequency properties of microwave and ultimately digital HTS.

This paper describes two applications for HTS technology on an airborne platform. The first application is a rf front-end for an 8 to 40 GHz microwave/millimeter-wave ESM system. The second application is a 2 to 4 GHz HTS Spiral Antenna Array System. The HTS antenna system maintains a wide field of view with a performance that approaches a steerable beam antenna. This is a cost effective approach for improving the collection capability of a system without the expense of developing a steerable beam antenna and the associated beam control hardware (tracker) and software.

The low loss and novel electronic properties of high temperature superconductors can impact satellite communication systems in many ways. Low loss beamforming networks, low loss broadband delay lines, phased array antennas, low noise receivers, digital signal processors, and other HTS components can benefit the Data Distribution Satellite, the Geostationary Earth Observatory, and the Deep Space Relay Satellite.

Low-resistance probe coils offer improved low-noise detection of rf magnetic fields in low- signal-level magnetic-resonance imaging and spectroscopy applications. Coils are presented that have been designed for: (1) low-field MRI machines; (2) NMR microscopes; and (3) NMR spectrometers. Quality factors (Q) exceeding 10⁴ have been achieved for all three applications by the use of thin films of the high-temperature superconductor YBa₂Cu₃O₇-(delta) . The first ever coil utilizing multilevel superconducting films is presented. Progress in the application of these coils to high-performance instruments is reviewed.

European activities on the optimization of microwave properties and applications of HTS for microwave components and subsystems are reviewed. For that purpose, the various contributions are discussed in terms of relevant material issues and are related to different classes of applications. Passive as well as active microwave devices based on both linear and nonlinear conductivity properties of HTS between 0.5 and 500 GHz are covered. Additionally, some application categories are discussed from a general point of view. It is shown that the present state of technology has made possible the successful demonstration of many single HTS components and of first super- semiconducting hybrid subsystems.

An optically controlled bandstop filter bank has been developed using High-Temperature Superconducting (HTS) thin films for the filter resonator elements and light-sensitive GaAs for switch control. In the non-illuminated (reject) state the maximum rejection for each filter was typically 60 dB, and the attenuation could be switched to less than 0.5 dB at 10 GHz in the illuminated (pass) state. The high Qs of HTS resonators enable three-section filters with superior shape factors to be realized. We report on the development, design and test of a five- channel filter bank divided into two bands. The filters were designed using a bidielectric `stripline' configuration to reduce unwanted resonator coupling and permit frequency setting by adjusting tuning screws above each resonator. The filter elements were rectangular hairpin- style resonators loaded with an interdigitated capacitor to form the switch topology. The HTS realization permits switch banks with a large number of channels to be constructed with low insertion loss. Such units could be used to protect receivers while maintaining a low noise figure. This architecture allows for low loss protection against multiple signals, superior to existing YIG bandstop filter technology.

First we report on the microwave characteristics of the third order intermodulation distortion (IMD3) in a High-Tc Superconductor (HTS) Josephson Junction (JJ) Downconverter at 12 GHz. We have successfully developed high quality nonlinear YBCO microbridge Josephson junctions for such an active MMIC as a mixer with RF, LO, IF and bias filters, which have been fabricated on (100) MgO substrates with 20 mm X 20 mm X 0.5 mm dimensions. The minimum conversion loss of the JJ mixer is 11 dB at very small local microwave input power LO equals -20 dBm which is two order less than a Schottky diode mixer. Consequently, this small optimum LO power gives the small rf input power at which the output IF power of the YBCO mixer saturates. Two-tone third-order intercept point (IP3) performance is a significantly important figure of merit typically used to define linearity of devices and circuits. The rf input power equals -15 dBm at the IP3 point is obtained for the YBCO mixer at 15 K and LO equals 10.935 GHz with -22 dBm. We have successfully measured the dependence of IMD3 on temperature, bias current, and LO power.

A receiver with an rf band of 34.6 GHz to 35.4 GHz, an integral local oscillator with a frequency of 36.5 GHz, and an IF frequency of 1.1 GHz to 1.9 GHz was designed and fabricated. The rf filter was a three pole distributed design fabricated with High Temperature Superconductor (HTS) films. The IF filter was a 9 pole lumped element design fabricated with HTS films. The local oscillator was a parallel feedback type design using a sapphire resonator shielded by two HTS ground planes. The rest of the elements used normal conductors or semiconductors operated at 77 K. The entire subassembly was integrated into a liquid nitrogen dewar and its performance is reported in the text, with the important figures of merit being low noise figure and low phase noise.

A prototype 3-GHz-bandwidth real-time spectral analysis receiver has been demonstrated by Lincoln Laboratory. This receiver is based on high-T_c tapped-delay-line chirp filters and uses high-speed semiconductor circuits to perform the output processing. The receiver is presently being space-qualified for flight on the Navy's Second High-Temperature Superconductor Space Experiment. This receiver is being developed for near-term use as a cuing receiver to enhance the signal processing capability of existing receivers. Possible applications for the receiver include both electronic warfare and remote sensing. The ability to perform the output processing required by this system is an important issue. This prototype receiver makes use of commercially available semiconductor components. The prospect of using semiconductor circuits presently under development to enhance output processing capability is described. The trade-offs between output processing complexity and the information provided by the receiver also are outlined.

A five-bit high-temperature-superconductive digital instantaneous frequency-measurement (DIFM) subsystem has been constructed for the determination of the frequency of unknown signals over a 500 MHz bandwidth, centered on 4 GHz, with a resolution of +/- 7.8 MHz. The subsystem contains a cryogenic section with five discriminator modules utilizing superconductive delay lines, GaAs mixers, and power dividers. With a single-tone cw input between -40 dBm and +10 dBm, the frequency quantization boundaries of the subsystem are, on average, 3.1 MHz from their design values. This system demonstrates the potential system-level application of high-temperature superconductive electronics in instrumentation, communication, radar, and electronic warfare.

A low-noise microwave receiver downconverter utilizing thin-film high-critical-temperature superconducting (HTS) passive circuitry and semiconductor active devices has been developed for use in space. It consists of an HTS pre-select filter, a cryogenic low-noise amplifier, a cryogenic mixer, and a cryogenic oscillator with an HTS resonator. The downconverter converts a 200 MHz wide band centered around 7.35 GHz to a band centered around 1.0 GHz. When cooled to 77 K, the downconverter plus cables inside a cryogenic refrigerator produced a noise temperature measured at the refrigerator port of approximately 50 K with conversion gain of 18 dB.

High temperature superconductors, because of their extremely low loss at high frequencies and their high current handling capability, have the potential for use in computer interconnect boards. They offer the potential advantages of high interconnect density, reduced interconnect delays, and higher data rate. Because silicon CMOS circuits dramatically improve in performance at low temperatures, cooled computers may become attractive in the future to capture both the improved interconnect and circuit benefits.

Two tone intermodulation distortion (IMD) in High Tc YBCO superconducting films on LaAl₂O₃ substrates are experimentally studied. Experiments performed on straight through microstrip transmission lines indicate highly non-linear signal transmission characteristics for these films. In addition to the third harmonic, higher order intermodulation products are also observed even without using any resonant structures. The observed intermodulation distortion shows a minimum as a function of temperature for a given input power level. Close to Tc, in addition to a large increase in IMD, higher order intermodulation products also become appreciable. SEM photographs are used to relate the surface topology of the films and the grain boundaries to the observed IMD. Thin films deposited by various techniques are investigated.

The influence of small external magnetic field on microwave properties of superconductor thick film ring resonators has been studied at various cryogenic temperatures. The ring resonators made of (formula available in paper) materials were realized by screen printing on yttrium stabilized ZrO2(YSZ) and (100) oriented single crystal MgO substrates. External magnetic fields were found to have a strong effect on the microwave reflection of the superconducting ring resonators. These magnetic field-controllable microwave resonators may offer (1) a new method to improve the properties of microwave resonators and other microwave devices and (2) a new method to construct new types of devices (e.g., magnetic field sensors of high sensitivity).

We have fabricated and characterized YBCO (YBa₂Cu₃O_7-x) microstrip resonators on LAO (LaAlO₃) substrates that include thin film STO (SrTiO₃) coplanar capacitors to study the dielectric properties of thin film STO. The low frequency capacitance of the STO/LAO capacitor is measured as a function of temperature and dc bias. We use the observed resonant frequencies to extract the microwave frequency capacitance of the structure and the Qs to determine the microwave losses. A conformal map is developed and used to transform the observed capacitances into dielectric constant values for the thin film STO.

Flux flow devices are three terminal active superconducting elements. Among the circuits demonstrated are mm-wave amplifiers, a fairly complete logic family and multiplexers/demultiplexers. Before it can be decided if this technology is viable for circuits of sufficient complexity to be commercially interesting, many manufacturability and materials questions must be answered. Work pertaining to these questions as well as demonstrations to date are discussed.

Two 100 nanosecond long delay lines have been built. Bandwidths of up to 7 GHz have been demonstrated, with insertion losses of less than 0.08 dB/nanosecond at 6 GHz. Each 100 nanosecond long delay line is contained in a package less than 4 inches by 4 inches by 0.5 inch. The delay lines use a meandered coplanar waveguide line built on a simple, easy to model unit cell. Coupling within individual cells was modeled using `Tranline,' a 2D EM simulator. Cell-to-cell interactions were modeled using NODAL, an in-house simulator that allows for coupling between non-adjacent lines. In the design of the unit cell and the layout of cells on the wafer, there is a tradeoff between coupling and delay density affecting size and performance. Since these delay lines are passive and patterned using photo-lithographic techniques, they exhibit good unit-to-unit phase repeatability. Phase repeatability is of particular interest in applications where multiple signal paths need to have the same transfer characteristics. Data on phase matching is shown between two 100 nanosecond long assemblies.

This work directly compares coplanar superconducting transmission lines and single-pole resonators patterned from YBCO to aluminum structures for use in GaAs/YBCO hybrid circuitry. A cryogenic on-wafer station was used to make s-parameter measurements of passive coplanar circuits as well as to characterize the performance of GaAs MESFETs at 80 K. Comparisons were made between measured data and theoretical results for passive YBCO and aluminum structures. The YBCO film was also measured using a parallel plate technique to determine microwave surface resistance to establish a correlation between patterned film and thin film microwave properties. Small-signal models were constructed to accurately predict the operation of 0.25 micrometers gate length GaAs MESFETs at 80 K under a variety of bias conditions. The cutoff frequency and maximum frequency of operation of the GaAs MESFETs increased by 29% and 13% respectively under a drain-source voltage of 2.0 V (Id equals 100% Idss) as the temperature was lowered from 300 K to 80 K.

In this paper, we analyze electrical small HTS (High-Tc Superconductors) patch antennas using the modified spectral domain moment method (SDMM). This paper presents the numerical results for a 2.8 GHz HTS patch antenna with 12 mm length and 1.5 mm width on the grounded dielectric substrate with thickness d equals 0.5 mm, (epsilon) _r equals 25 and tan (delta) equals 0. As the result, we can see that efficiency of the HTS patch is improved, for example, 3.8 times at T equals 0(K) and 2.9 times at T equals 77(K) improvement are obtained from our analysis. We also show that the resonance frequency of the HTS patch is shifted as temperature is changing.

We report on the microwave characteristics of coplanar strip transmission line devices fabricated in epitaxial YBaCuO thin films deposited on LaAlO₃ substrates using the novel laser-writing patterning technique. This technique uses laser heating to selectively anneal regions of an oxygen depleted YBaCuO thin film to form local superconducting phases. In this manner, superconductive coplanar strip transmission line structures surrounded by the semiconducting YBaCuO phase were patterned with no photomasks, surface contamination or edge degradation.

The microstrip filters, such as multipole lowpass filter and multipole bandpass filter, have been designed, fabricated, and characterized. The YBa₂Cu₃O_7-(delta ) (YBCO) epitaxial thin films on MgO substrates were grown in situ at 760 degree(s)C by pulsed laser deposition. The design of filters in a microstrip configuration and their microwave responses were simulated by microwave design simulator, respectively. The filter patterns on the laser ablated YBCO/MgO film were generated using conventional photolithography and ECR-etching process. The performance of filters was measured as a function of temperature and frequency. Microwave loss properties were superior for the filters patterned from HTS films to the filters fabricated from Au-metal films of the same dimensions.

Millimeter-wave confocal resonators are used in a new, commercially available instrument to map the surface resistance of large area (2 - 4 inch diameter) superconducting thin films. Q-factors are measured from the reflection coefficient of the cavity formed by a spherical aluminum mirror and a planar conductor sitting at half the radius of curvature of the mirror. The surface resistance of the superconducting film is extracted from the measured Q values. Typical R_s values of 20 - 40 m(Omega) are measured for high-quality 2' high-T_c superconducting thin films at 94 GHz and 77 K.

A superconducting antenna array with a proximity coupled feed network operating at 20 GHz has been developed. The antenna is a 4 X 4 array and its performance was measured from 18 GHz to 22 GHz. At temperatures below 80 K, there was a 15 dBi gain measured at several frequencies in this range. The design of a low loss superconducting phase shifter monolithically incorporated into the YBCO feed network is indicated.

Epitaxial YBa₂Cu₃O_7-x (YBCO) superconducting thin films have been grown on (100) MgO substrates by pulsed laser deposition. The characterization of YBCO superconducting thin films grown on MgO under the optimized condition has been performed with x-ray diffraction (XRD), Atomic Force Microscopy (AFM), magnetic property measurement in SQUID magnetometer and resistivity measurement. The fabrication and characterization of dual mode disc resonators (DMDRs) have been discussed with superconducting thin films grown under optimized deposition conditions.

For the purpose of studying the microwave parameters of the thermonuclear plasma, radioastronomic applications, etc., wide-band high-sensitive high-speed spectrum analyzers, radiometers, and receivers are required which operate in the frequency range of 50 - 300 GHz and higher. The existing techniques have a number of disadvantages. Thus, the high-speed heterodyne receivers are rather narrow-band ones, therefore, to cover the whole millimeter- wave band, complex multichannel systems should be designed. On the other hand, the wide- band Fourier spectrometers are very inertial and complex systems and have great losses in the long-wave portion of the millimeter-wave band.