Investigation of the special features of the coupling coefficients for microstrip asymmetric hairpin resonators at frequencies of the second passband

Abstract

3 UDC 537.621; 621.317.023 The behavior of the frequency-dependent coupling coefficients is investigated for a pair of microstrip resonators formed by conductors shaped as an asymmetrical hairpin. It is demonstrated that the total coupling coefficient of the resonators in a two-pole filter becomes close to zero in the region of the frequency of the second resonance due to mutual compensation for inductive and capacitive interactions. This allows a device for protection against high-power radio pulse to be developed based on such resonators with additional hairpin resonator containing control element manufactured from high-temperature superconductor (HTS) film. High-speed power limiters - protection devices (PD) - are necessary to protect input receiver circuits from high-power radio pulses. It is obvious that the electric PD strength is determined by the portion of power absorbed by the device from that incident on its input. Therefore, in addition to high speed, such devices must also possess high reflectivity in the off-state of the device. Microstrip resonant structures whose control element is manufactured from a high-temperature superconductor (HTS) film are promising for such devices actively investigated nowadays (1-3). The high speed of these protection devices is caused by small times of phase transition of the HTS material from the superconducting to normal state when current exceeding the critical threshold flows through it. The high reflectivity of the device in the off-state is caused practically by broken coupling between its input and output when the state of the HTS film changes from superconducting to normal one. The principle of PD operation consists in the following. The third resonator with the strip conductor completely or partially manufactured from the HTS material film is added to two microstrip resonators (MSR) with configurations of the conductor strips that provide mutual compensation for the inductive and capacitive interactions near resonant frequencies (4). This resonator provides required coupling between the MSR, thereby forming the desired passband of the device. When the HTS film goes to the normal (high-resistance) state under the influence of a radio pulse of sufficient power, coupling between the resonators is broken, and the device operates in the regime of limitation. The level of signal suppression in the limitation regime is one of the main specifications of protection devices. As demonstrated investigations (3), this level decreased with increasing central frequency of the working band of the device. In many respects, this is due to a decrease in sizes of resonators and devices as a whole accompanied by a decrease of the distance between the input and output of the device and by the corresponding increase of the directly transmitted signal.