Receiving antennas for electromagnetic pulses and transients.

Abstract

The pulse receiving characteristics of several thin-wire antennas have been investigated in this thesis. First, a loop antenna with a uniformly distributed impedance has been investigated, analytically, in the frequency domain. An equivalent circuit has been developed for the impedance-loaded loop antenna receiving an arbitrary steady-state electromagnetic field. The frequency- and time-domain receiving characteristics of a resistively loaded loop antenna have been obtained. It has been found that the loop antenna with a uniform distributed resistance has a broad receiving bandwidth. Nonetheless, the antenna is dispersive and is therefore not suitable for the reception of electromagnetic pulses or transients. Second, pulse-receiving characteristics of resistively loaded straight dipole antennas have been investigated in the time domain. Frequency-domain receiving characteristics have been obtained using the Fast Fourier Transform method. Two types of resistively loaded straight dipole antenna designs have been considered. One design has a high distributed resistance and a low terminal resistance, and gives a broad bandwidth at the expense of the sensitivity. The other design has a low distributed resistance and a high terminal resistance. It does not sacrifice the sensitivity but requires an active high-impedance receiver circuit. It has been found that the Wu-King resistively loaded dipole has the maximally flat frequency response close to the upper roll-off frequency, when terminated with a high-impedance load. Third, a time-domain stepping-in-time procedure has been developed for the analysis of unloaded and resistively loaded V-antennas. V-antennas with various terminations have also been analyzed using the Numerical Electromagnetic Code. After identifying the limitations of resistively loaded straight and V-antennas in receiving fast pulses and transients, a passive compensation technique has been developed to significantly increase the upper frequency limit of the V-antenna. The new V-antenna consists of a resistively-loaded thin-wire V-dipole or monopole, a Response Equalizer Circuit and an oscilloscope probe (or other termination). For a given termination, the new antenna has a higher sensitivity and a much broader bandwidth compared with the Wu-King antenna. It is estimated to have a 3-dB bandwidth from 25 MHz to 10 GHz when used with a commercially available 500 $\Omega$ passive probe. The sensitivity is approximately 1 mV/(Vm$\sp{-1})$, taking into account the 20 dB attenuation of the probe. With a 50 $\Omega$ probe, this antenna has a 3-dB bandwidth from 110 MHz to 10 GHz, and a sensitivity better than 4 mV/(Vm$\sp{-1}$). A V-antenna and a Wu-King monopole antenna have been fabricated using thin-film deposition technology. A time-domain antenna range has been built and used to measure the time-domain response of the experimental antennas. It has been found that the experimental V-antenna is capable of receiving a 520-ps approximately Gaussian pulse, without significant distortions. The frequency-domain receiving characteristics of the two antennas have been obtained using the Fast Fourier Transform method. The receiving response of the V-antenna has been measured in the frequency domain, using a TEM cell and network analyzers. The experimental thin-film V-antenna has a sensitivity of 1.12 mV/(Vm$\sp{-1}$) and a flat receiving response from 35 MHz to 3 GHz. It was not possible to measure the response beyond 3 GHz, due to limitations of the pulse generators available. The experimental thin-film Wu-King antenna has a sensitivity of 22 mV/(Vm$\sp{-1}$) and an upper roll-off frequency of about 300 MHz, which is limited by the bandwidth of the active receiver circuit.