A study on direction‐of‐arrival estimation methods using cosθ amplitude response antenna

At present, when the direction of arrival of a low-frequency electromagnetic wave such as one in the HF band is measured, loop antennas are used. However, in low-frequency measurement such as is used in underwater communications, subsurface resource detection, volcanic activity observation, and natural electromagnetic wave observation, the receiving power sensitivity decreases as the frequency is decreased. The purpose of this paper is to use a SQUID (Superconducting QUantum Interference Device), known as an ultrasensitive magnetic sensor, for construction of a small magnetic antenna for estimating the direction of arrival of low-frequency electromagnetic waves. As a first step, the antenna configuration and direction of arrival estimation method shown below are proposed. As a low-frequency broadband antenna with an amplitude response type, we consider a SQUID array, which has a cosθ mode vector consisting of SQUID elements on a circle with a small radius, which is negligible in comparison with the wavelength of the incident wave. For an antenna with a real-valued cosθ mode vector, we consider a direction-of-arrival estimation method within the framework of the estimation principle of ESPRIT and MUSIC, known as high-resolution direction-of-arrival estimation methods, and derive an estimation method. The obtained estimation method is shown to be ultimately reducible to a simple estimation equation for deriving the phase difference between the −1st and 1st fundamental waves after beam subspace transformation of the correlation function without eigenfunction expansion. By computer simulation, it is shown that an estimation accuracy of 0.2 to 2° is obtained by this estimation equation for the finitely many elements and snapshots that are actually anticipated. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(10): 8–22, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10009