The Radiation System Synthesis by the Power Criterion as a Problem of Optimization with Restrictions

A nonlinear synthesis problem of antennas according to the prescribed power (squared amplitude) radiation pattern (RP) is considered in the variational statement that yields in the possibility to take into account an additional restriction to the synthesized power RP. The problem of synthesis consists of finding such currents in antenna, which generates the RP with the best approximation to the given one. The respective Euler’s equation is reduced on the basis of used functional. This is nonlinear integral equation of Hammerstein’s type. The effective numerical methods are elaborated and applied for its solving. The computational results verify the effectiveness of approach proposed.


Introduction
In the process of design the various radiating systems, in particular antennas and arrays, requirements are imposed mainly on the amplitude or power RP; the phase characteristic (the argument of complex RP) remains free of any requirements. The antenna synthesis problems according to amplitude characteristics are extensively investigating in the recent decade for plane arrays with different geometries [1] [2] [3], for the case of specific form of the given RP [4], as wells as due to moving the antenna techniques to new perspective THz range of frequencies [5].
In the problems of optimization, as well as in related to this area the synthesis problems according to the amplitude or power RP [6], the phase RP can be used as an additional optimization parameter, which allows obtain the better approximation to the prescribed RPs.
The first works in this area were [7] [8], in which the synthesis problem of a linear antenna by a given amplitude RP was formulated as a variational problem of minimizing the mean square deviation of the given (real positive function) and amplitude of the synthesized RPs. The obtained Euler's equation for a minimizing functional was a nonlinear integral one. Since such equation describes not only the extreme points of the considered functional, but also all its possible stationary points, its solution may be ambiguous. Moreover, the number of solutions can vary with the change of physical parameters of problem (process of branching the solutions). This property was described and discussed firstly in [9]. The approach proposed was applied for different types of antennas [10] and generalized in [11]. The theoretical investigations related to study of branching process for one-and two-dimensional cases were conducted in [12] [13] [14] [15] [16].
The formulation of the synthesis problem of antennas according to the power RP was apparently proposed for the first time in [17] as a modification of the synthesis problem by amplitude RP. In the later works [18] [19] [20] [21] such statement of problem was described and applied for one-dimensional and two-dimensional antennas. Recently such a statement was supplemented with the condition of the norms' equality of synthesized and given power RPs [22].
The corresponding nonlinear integral equation was obtained; it was solved by the modified Newton's method. Since this equation has a non-unique solution, not all of these solutions have been obtained.
The above disadvantage can be overcome using a polynomial approach to synthesis problems according to the amplitude RP [23] [24]. This approach is based on the exact representation of solutions to the respective nonlinear integral equation by complex polynomials of low degree; it was generalized and described in detail in [6]. Its application to the problem of antenna synthesis by the power criterion is given in [25] [26] [27].
In this paper, using the variational statement of synthesis problem, we elaborate the numerical methods for the linear antenna and equidistant array. The additional restriction on the norm of synthesized power RP is taken into account in the presented functional, that is, the synthesis problem is formulated as a problem of conditional minimization of used functional with constraints. The formulation of problem uses a general operator representation of the synthesized RPs by the currents in antenna. Study of properties of solutions to the respective Euler's equations for linear antenna is focused more on the theoretical investigation related to branching process, because the properties of direct and adjoint operators are simpler in this case that allow deal with some analytical transformations. Since the above operators for plane array are more complicate, then for this antenna we deal mainly with synthesis of the specific prescribed power RPs.

Formulation of the Problem
It is known that the RP of the radiation system is a vector complex-valued function and its dependence on the distribution of currents in system has a linear character. Abstracting from the specific type of the radiation system, this dependence can be written with the help of some linear operator A where f is RP created (synthesized) by radiation system, u is current distribution in system.
The form and properties of the operator A are determined by the type and geometry of the radiating system.
The synthesis problem according to the power RP is formulated as a problem of minimizing the functional ( ) Here 1 ⋅ , 2 ⋅ are the mean-square norms in the spaces of functions u and f, respectively; 2 0 F ≥ is the given power RP within a certain area Ω , and 0 α > is the given real parameter. The first term in (2) ensures the proximity of the synthesized power RP to a given one in the main lobe (region Ω ), while the second term imposes restrictions on the norm of exciting currents. We obtain the equations for the synthesized power RP ( ) from the necessary condition of functional (2) minimum, here A * is operator adjoint to A.
Note that Equation (3) has a obvious trivial solution 0 f ≡ always. To avoid this undesirable property, as well as to limit the trend of proportional reduction of the RP f with increasing in the α coefficient, the functional to be supplemented by the additional condition 2 2 This means that we will consider the problem of minimizing the functional with constraints on the RP in the region Ω . The application of the Lagrange multiplier method to the optimization problem (2), (4) leads to a minimization of functional where λ is the Lagrange multiplier, which along with f is also be determined from the minimum of the functional (5).
The Euler equation of functional (5) can be written in form If f and λ are found from (6), then the optimal current u is calculated by Journal of Applied Mathematics and Physics Equation (6) can be supplemented by condition (4), then they are considered as a system of equations for determining f and λ .

Usual Iterative Method
We propose different methods for solving the problem of antenna synthesis, which are reduced to the search for a stationary point of functional (5). For example, it is naturally to apply the iterative process of the gradient method for both the unknowns u and λ : , while the value of the step k γ in (8), (9) can be chosen for the same reasons as for unconditional optimization.
Instead of (8) one can use other iterative processes, in particular the Newton's method, which is described in the next subsection. In our case, the problem of minimizing function ( ) , L u λ by variable u is solved quite simply, so we offer the following iteration process:

Modified Newton's Method
In general, Equation (6) can be written in form where Φ is nonlinear operator-function, f is synthesized RP, c is numerical parameter characterizing the physical performances of antenna (as a rule it is value that combines the size of antenna and frequency of excitation).
If for a certain c there is a function f satisfying Equation (12), then we call the f c a solution of this equation. If f continuously depends on c, then we assume that the solution of Equation (12) belongs to a certain branch of the solutions, or is a "point" on this branch. Below, we propose such a modification of the Newton's method, which will allow to find such branches, as well as points of branching on them, if they exist.
Let the pair ( ) , n n f c be n-th approximation to the desired point on the branch of the solutions. The next n + 1-th approximation is found from the condition that the Equation (12) is satisfied in the first order by increments , , where n c c c and and (13) is linear with respect to ε and δ . If it is solved, the next approximation of solution is determined by formula Obviously, the Equation (12) is undetermined. If the iterative process converges to some values, then its limit, generally speaking, is an arbitrary point on the branch. The Equation (13) is also undetermined and it can have a certain set of solutions. If no additional conditions are imposed on the solution f, it is possible, in particular, to choose a solution with a minimum norm among the set of solutions, that is, the "point" of this set closest to the previous approximation will be selected for the next approximation. This choice avoids a situation where a new point on a branch is either too far or very close to the last point that has already been found. One can also require that the distance between the points on a branch during computations be kept constant by prescribing this distance, for example, by means of equality , and indices n − 1 and n meaning affiliation of the value to the previous step and to the desired point on the branch in the current step. The above equality complements the Equation (12). In this case, the Equation (13) must be supplemented by the equation which follows from the perturbation (16). As a result, the system of Equation (13), (18) becomes defined. In general, this system is equivalent to initial system of Equation (4), Equation (6).
Without decreasing universality, the condition (4) for the effective use of the generalized Newton's method can be written in the form.
In order to solve this system we use the modified Newton's method proposed in [6]. For this goal, we rewrite Equation (6), Equation (19) in form The next approximation to unknowns f and λ in this method is calculated as 1 n n n λ λ δλ In the case if parameter λ is fixed then Equation (20) is not in use in the system, therefore the last row and column in (24) are non-available.

Application to Specific Antennas and Arrays
The iterative procedures, elaborated in the previous Section are applied here for solving the synthesis problems for the specific antennas. Both the linear antenna and plane equidistant array are considered. The modified Newton's method is applied for the synthesis of linear antenna. This is because of fact that the operators A and * A for the plane array need more calculations that the respective operators for linear antenna. Because the usual iterative procedure (8), (9) is simpler, the calculations can be applied for more complicate antennas.

Linear Antenna
The proposed approach is applied in this subsection to synthesis of linear antenna. In this case, operators A, * A , and kernel ( ) where x is normalized coordinate in antenna,   (5)), the rest of terms have the auxiliary importance. In Figure 1, the dependence of 0 σ value on parameter c is shown for the different solutions to Equation (28). One can see that the quality of approximation depends essentially on α parameter (weight multiplier in functional (2)). The branching of solutions appears at the specific val-  At the fixed c, the current norm is almost independent on α for the solutions of all types. This is explained by the fact that this norm is essentially determined by the norm of the synthesized RP, which is fixed in our statement.
In Figure 2

Plane Array
Consider a plane array consisting of ( ) ( ) where nm I is the distribution of the complex currents in nm-th radiator, , θ ϕ are the angular coordinates of the spherical coordinate system ( ) , , R θ ϕ whose centre coincides with the centre of the Cartesian coordinate system xOy. It is assumed in (29) that the RPs of separate radiators do not depend on the coordinates θ and ϕ , that is they are constant (omnidirectional).  , F ξ ξ   is given in some area We obtain a nonlinear system of equations for finding the current distribution on radiators from the necessary condition of minimum of (32). The respective Euler's equation (6), for the functional (32) is where the kernel K is a real and it is calculated by The Lagrange multiplier λ is determined by a coordinate descent method while the value k γ at each step will be calculated using the relation: Consequently, the iterative process (8) Table 1, and the optimal solutions (synthesized RPs) for some set of the parameters c 1 and c 2 are shown in  One can see that such prescribed power RP F 2 can be not approximated with sufficient accuracy at the given number of radiators don't hurt the frequency (parameter c and wave number k respectively). In order to achieve better approximation for such RP, there is necessary to increase the number of radiators (and size of array respectively) in a great extent.
Even more effective solutions exist for RPs that do not have central symmetry. In Figure 6, the results of synthesis are presented for given two-lobe power RP Table 1

Conclusions
A generalized statement of the synthesis problem according to a given power RP In the case of plane equidistant array, for which the operator for calculations of RP by the currents in array radiators is more complicate, the numerical procedures were applied for solving the synthesis problems for the specific given RPs. The approach proposed foresees the application for other types of antenna systems, for which the RP is calculated by the currents (fields) in antenna elements by known linear operator.