This work presents an analysis of the radiating properties of lossy antennas using the Spherical Wave Expansion (SWE) theory. A new formula to predict the gain of uniformlyspaced end-fire arrays based on the dissipation factor is proposed. The latter allows an analytical synthesis of the complex excitation coefficients associated with the array elements to achieve supergain characteristics. Similar studies have been previously dealt with (super)directivity optimization, characterized by high sensitivity and severe gain attenuations, when the dimensions of the array are considerably reduced. In this context, the proposed study shows a significant improvement of the antenna gain and efficiency compared to synthesis procedures having an objective of the directivity maximization.
The problem of gain estimation of a superdirective dipole-based end-fire array is discussed in this contribution. The current method to compute the gain, for a given element radiation efficiency, is based on the array factor (AF) theory. This work is intended to show that an equivalent formulation can be done using the Spherical Wave Expansion (SWE). Besides the interest in validating the theory, the main objective is a better understanding of the radiation and attenuation phenomena that occur in compact and superdirective arrays. The limits in their practical implementations are imposed by the high sensitivity of the system. The SWE theory provides more information in the expression of the radiated field, thus unfolding the possibility to address the problem with lower sensitive solutions.
End-fire arrays are among the best candidate for high directivity and compact dimensions. Several solutions for the synthesis of superdirective arrays have been proposed in the literature. However, what is called a "superdirective" array suffers inevitably high losses and very poor radiation efficiency. More appealing is the joint optimization of directivity and efficiency, or the intrinsic gain, of the antennas. This paper set side by side the directivity and gain optimization methods based on the Spherical Wave Expansion (SWE) theory. The synthesis of superdirective and supergain end-fire arrays is proposed when Huygens sources, which attain the highest level of directivity, or simple bent dipoles are selected as elements of the array. On behalf of the spherical modal expansions, the results obtained for different optimizations of the two arrays are examined.
Electrically Small Antennas (ESAs) are of high interest in applications where a compact size wants to be preserved. With the objective of the realization of a highly efficient and superdirective array within a very compact space, a new radiating element and array design is proposed, optimized for maximum attainable gain. A cosines-dipole printed antenna is designed by adjusting the amplitude and periodicity of the cosine microstrip, to resonate at the desired frequency of 916 MHz. Then, to achieve high gain two elements are closely spaced, mirrored to their excitation point, one fed and the other parasitic. An analytical method for gain optimization based on the Spherical Wave Expansion (SWE) theory is used to maximize the array gain. Two different array geometries are compared by looking at the power distribution of the spherical modes, and the configuration that returns the highest gain is chosen. A realized gain of 6.5 dBi is achieved in simulation, for a total size included in a radiansphere of ka=1, with the advantage of having a printed antenna, thus planar, easy to integrate, and low cost. The proposed antenna solution presents in theory an excellent trade-off between electrical size and realized gain.
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