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An effective compensation method to compensate for the mutual coupling effect in magnetic resonance imaging (MRI) phased arrays is introduced. This method uses the knowledge of the position of the signal source in MRI, i.e., the active slice, to define a new mutual impedance that accurately quantifies the coupled voltages and enables them to be removed from the terminal voltages almost completely. Numerical results using the method of moments show that the percentage errors in the compensated voltage are at least on the order of 10-5% and the isolations between two coils are more than 120 dB even at a low-field case of 0.5 T ([f.sub.o] = 21.3 MHz). This method can be implemented by either software or hardware. Index Terms--Compensation method, magnetic resonance imaging (MRI), mutual coupling, mutual impedance, phased array.

An effective method is introduced to compensate for the mutual coupling effect in the estimation of signal parameter via rotational invariance techniques (ESPRIT) direction finding algorithm. The compensation method seeks to quantify the mutual coupling more accurately by using a new mutual impedance. Study of two closely spaced arriving signals shows that the high-resolution capability of ESPRIT can be achieved only when the mutual coupling effect is compensated for by using our new compensation method. Critical situations with a large signal level difference, with an increased mutual coupling effect resulting from a more compact-size antenna array, and with signals coming from a nonhorizontal elevation angle are also studied using the ESPRIT algorithm. Results show that the new compensation method, when applied to ESPRIT, is more accurate, more robust, and more flexible than the previous open-circuit voltage method. Index Terms--Direction finding, estimation of signal parameter via rotational invariance techniques (ESPRIT), mutual coupling, mutual impedance.

The performance of the maximum ratio combining method for the combining of antenna-diversity signals in correlated Rician-fading channels is rigorously studied. The distribution function of the normalized signal-to-noise ratio (SNR) is expanded in terms of a power series and calculated numerically. This power series can easily take into account the signal correlations and antenna gains and can be applied to any number of receiving antennas. An application of the method to dual-antenna diversity systems produces useful distribution curves for the normalized SNR which can be used to find the diversity gain. It is revealed that signal correlation in Rician-fading channels helps to increase the diversity gain rather than to decrease it as in the Rayleigh fading channels. It is also shown that with a relative strong direct signal component, the diversity gain can be much higher than that without a direct signal component. Index Terms--Antenna diversity, maximum ratio combining, Rician channel, signal-to-noise ratio (SNR), multipath fading.

A new method is introduced for the compensation of the mutual coupling effect of a linear adaptive dipole array employed in adaptive nulling of interference signals. The new method adopts a practical approach in that it needs only the measured voltages across the antenna terminal loads and an estimated current distribution for the calculation of the mutual impedances. The mutual impedance is defined and calculated differently from the conventional method and the results are more effective to remove the mutual coupling effect. The new method does not require the knowledge of the elevation angles of the signal of interest (SOI) and the interferences and still works if the elevation angles of the SOI and the interferences do not deviate too much from the horizontal direction. This increases the capability of the array to work in three-dimensional signal environments. Computer simulations for a number of rather extreme signal environments have been carried out to testify the robustness and the capability of the new method. Index Terms--Adaptive array, adaptive nulling, mutual coupling, mutual impedance.

The spatial correlation coefficient of signals received by two normal-mode helical antennas in a multipath environment characterized by Clarke's model is rigorously obtained. A new formula relating the signal correlation coefficient to the mutual impedance is derived. The mutual impedance is defined and calculated in a new manner. Our findings show that: 1) the envelope correlation coefficient of two helical antennas is greater than that of two monopole antennas; 2) the envelope correlation coefficient is dependent on the antenna terminal load and the mutual impedance and an optimal antenna load can be determined to produce the smallest correlation coefficient; and 3) the frequency response of the envelope correlation coefficient resembles that of the voltage standing-wave ratio. Index Terms--Antenna diversity, helical antennas, mutual impedance, signal correlation.

A practical normal-mode helical antenna array together with a new method for the compensation of mutual coupling effect is proposed for the efficient adaptive nulling of interferences in a mobile communication environment. The helical antenna array is shown to be able to generate deeper nulls than an equivalent monopole antenna array in the presence of mutual coupling effect. The helical antenna elements are smaller in size and less susceptible to the mutual coupling effect than the monopole antenna elements. The new method for compensation of mutual coupling effect is shown to have a greater ability and flexibility than previous methods. A new definition of mutual impedance is introduced and used to find the coupling-free signal voltages across the antenna terminal loads. Numerical examples demonstrate the superior performance of the proposed adaptive array and the new compensation method. Index Terms--Adaptive hulling, helical antenna array, mutual coupling effect, mutual impedance.

A general method of formulating eigenfunction expansion of dyadic Green's functions in lossless, reciprocal and homogeneous chirowaveguides is presented. Bohren's decomposition of the electromagnetic field is used to obtain the vector wave functions. The method of [Mathematical Expression Omitted] is used to rigorously derive the magnetic and electric dyadic Green's functions. A specific application to the cylindrical chirowaveguide illustrates the method.

In this paper, the complete set of four spatial-domain electromagnetic dyadic Green's functions are rigorously derived for the parallel-plate chirowaveguide. These dyadic Green's functions are presented in the cylindrical coordinates, which are found to facilitate numerical calculations. An electric-field integral equation for the coaxial-probe excitation problem is formulated using the dyadic Green's functions, and the moment-method solution is sought. The probe admittance and current distribution along the probe at different chiral levels are obtained. Results show that a substantially higher admittance level is obtained, but the admittance bandwidth decreases with the 455 parameter. Stopbands at which no net power input into the waveguide are observed. This characteristic is found to have no match in the nonchiral waveguide. The computed current distribution along the probe shows a greater current magnitude than that of the nonchiral waveguide. The validity of the numerical solution is checked with the measured values for the nonchiral case. Index Terms--Chirowaveguide, dyadic Green's function, excitation method.