Smirnov V.U., Nikitin O.R.
Vladimir State University named after A.G. & N.G. Stoletov, Russia
Formation of local areas of electromagnetic energy concentration for diagnostics and hypertermia treatment
In many target of medical diagnostics and hyperthermia there necessitate the creation of narrow local region of space with the focus of the high energy in it. Creation of such field is non-trivial problem, because the size of this region must be less then wavelength in diameter and attenuate to half power at a distance not exceeding the wavelength. A universal tool for creation focused electromagnetic fields at a fixed region of space are phased antenna arrays. The focus of the antenna array is best accomplished in the near field of an antenna array that will reduce the loss in free space and biological tissues. Note, however, that for a sufficiently narrow to the focal spots need to use arrays with a large number of radiators. This circumstance leads to a significant rise in the cost of the antenna system. In this paper proposes the use of phased antenna arrays with a partially filled aperture, which will allow to substantially reduce their cost.
A schematic model of a flat phased arrays with full filling of aperture radiators is shown in Fig. 1.
Fig. 1. Schematic image flat phased array.
Radiators are located at the nodes of square grid in X0Y plane of the Cartesian system with coordinate , ; , where M and N – number of radiators on x and y direction respectively, - step of a grid.
Focal axis, including the point of focus, coincides with the axis of z: . The distance from the emitter to the point of focusing is equal: , where . In case of spherical phase distribution in aperture with the center in the point , the required phase of excitation current are determined by the expression: . The total value of the at the point of focus under the uniform amplitude of excitation of radiators is determined by the ratio:
For an random point in the near field of the phased antenna array with coordinates x, y, z, does not coincide with the release, the resulting field is:
where , - coordinate of the center.
On the basis of the formula (3) was carried out computer simulations of antenna arrays with different degree of filling, the solid line corresponds to an array with the full filling of the radiating aperture, dashed - grate with a cross-form filling and dotted - an array with the contour filling. The results of the simulation are shown in figures 2 and 3.
Fig.2. The normalized distribution of the field in the plane of polarization , , .
Fig.3. A normed field distribution in a plane perpendicular to the focal axis and passing through the point of focus , , .
The calculations show, that the use of the phased antenna arrays allows to achieve a very narrow the local areas of concentration of the electromagnetic field. It is clear that in the case of cross-filling radiating aperture length and width of the focal areas at the level-3dB is almost identical to the same values for full array antenna. The number of emitters decreased in of 8.75 times. If the circulating filling a very big length of the smallest width of the focal area with the decline in the number of radiators in 4.5 times.