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International Journal of Antennas and Propagation
Volume 2012 (2012), Article ID 187123, 9 pages
Can Radiators Be Really Isotropic?
1Department of Communication Engineering, Holon Academic Institute of Technology, 52 Golomb Street, Holon 58102, Israel
2AFEKA Tel Aviv Academic College of Engineering, 38 Mivza Kadesh, Tel Aviv 69107, Israel
Received 5 October 2011; Accepted 2 January 2012
Academic Editor: Dau-Chyrh Chang
Copyright © 2012 H. Matzner and E. Levine. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In search for isotropic radiators with reasonable quality Factor (Q), bandwidth, and efficiency, one looks for practical radiators with a typical resonant length of . We present here a Green's function analysis in Fourier of a microstrip element and a far-field integral method in configuration (real) space of single and dual U-shaped elements. Both solutions analytically prove that the power radiation patterns are isotropic in nature (while the thickness and the width tend to zero), although the polarizations are not symmetrical in all cuts. It is also shown that the power isotropic U-shaped radiator, for which the surface current density is infinite, can be replaced by another finite-size radiator, having finite-surface current density, such that its far-field is exactly the same as the far-field of the U-shaped isotropic radiator.
Accumulated experience, empirical and theoretical, with microstrip elements proved their efficiency as elements of antenna arrays, as can be shown, for example, in . The beamwidth of these elements is relatively broad, where the vertical part of the element contributes to the end-fire direction. The beamwidth becomes broader as the width and the height of the elements become narrower. This phenomenon has motivated us to reinvestigate the age-old problem of the unity gain antenna. Such an antenna would be considered as an achievement from the technological point of view: one is eager to have a multidirectional circularly polarized antenna for satellites and space vehicles in order to ensure communicability with Earth ground stations . Indeed,  states that the need for isotropic antennas not academic in that a radar designed with such an antenna and so interconnected that the polarization is reversed on reception, would receive equal signals from most reflecting objects, no matter where they were placed on a sphere centered at the antenna. In  it is shown for the so-called “ transmission line antenna” that as the distance between the conductors becomes smaller, the pattern tends to be more isotropic. A relatively recent summary of the search for an isotropically radiating source is given in , from which we learn that an infinite long source current distribution based on parallel to “turnstiles” distributed uniformly over the -axis radiated power isotropically. In this case the feed function is , where is the wavenumber and is the zero order of the modified Bessel function of the second kind. The pattern of one turnstile is shown, for example, in .
The peculiarity of the class of isotropic power radiators offered here is that the source has a finite size and that the far-field is analytically solvable. Moreover, it is shown [7, 8] that the origin of their phenomenon is the magnetoelectric symmetry of the radiator. These radiators are based on an infinite-surface current density, but this is not a serious impediment, because a finite-size, finite-surface current distribution radiator can be constructed such that the fields outside this radiator are the same as for the basic infinite-current isotropic power radiator, as will be shown in what follows.
The structure of the paper is as follows: in Section 2 we present the far-field calculation of an idealized element and present its radiation cuts for various values of width and height above ground. It is shown that as the width and the height above ground of the element become smaller relative to the wavelength, the power pattern becomes more and more isotropic. A U-shaped power isotropic radiator is presented in Section 3. The proof of the isotropic behavior of the far-field radiation power density of this radiator is described, as well as its polarization patterns. In Section 4 we present a double U radiator, for which the polarization patterns are more symmetric. In Section 5 we show how to replace the U-shaped radiator, for which its surface current density is infinite, by a spherical surface having radius and finite-surface current density, for which its far-field is exactly the same as the far-field of the U-shaped radiator. Conclusions are given in Section 6.
2. Patterns of an Idealized Microstrip Element
Consider the geometry of a element above an infinite ground plane as shown in Figure 1. For simplicity there is no dielectric substrate, although the general analysis takes such substrate of thickness and relative dielectric constant into consideration. The surface current density on the radiator is composed of horizontal and vertical component as follows: where bold letters are used to designate vectors hereafter. A detailed Fourier analysis of the radiated fields [9–11] can be summarized as follows.
The Fourier transform of the tangential electric field on the plane which includes the patch is given by where is the free space impedance, is the wavenumber, is the relative dielectric constant layer under the patch, and is the thickness of the dielectric layer, and is the surface current density on the patch in Fourier space. The other variables are The electric field in spatial coordinates can be calculated by taking the inverse Fourier transform of . However, this transform cannot be done analytically, so further results will be written in the Fourier domain.
The complex input power at the antenna terminals is
This expression has real and imaginary parts. The contribution to the real part comes from the radiation into free space and from surface waves excitation effects. In the case of air as a dielectric substrate surface waves do not exist. The contribution to free space radiation comes from integration in the last equation over the range which is called the “visible range.”
Transforming the equation for into spherical coordinates and restrict it to the visible range gives the following result: where is the power radiation of the antenna that is expressed in terms of the Fourier transforms of the surface current density components and :
In our case , hence
Return back to the radiator, one can take an educated model for the horizontal current (near resonance) as and the vertical current is the current (in Amperes). Taking Fourier transform of
The vertical current can be replaced by two horizontal contributions . This replacement is exact for the relevant fields on and above the plane . The replacement is as follows: where is the Fourier transform of the surface current density which replaces the vertical surface current density: where .
Substituting the surface current densities in the pattern formula, we can calculate the far-field pattern of the idealized microstrip element as function of and . Figure 2(a) presents the -plane power pattern cut and Figure 2(b) shows the -plane power pattern cut. It is clearly seen that as and tend to zero, the pattern cuts become more and more isotropic.
3. The U-Shaped Power Isotropic Radiator
We present here another current source which radiated its power isotropically. This time the calculations are done in configuration (or real) space. The geometry of the current source is shown in Figure 3, where the element is placed in a different position for convenience. The relevant current density is given by The relation between the wavenumber and the length is given by Following Elliott’s procedure , the electric far-field is given by where is the directional weighting function given by The and components are given in our case by hence, we have for the polar components , and finally
Hence, the U-shaped radiator whose thickness and width tend to zero radiates its power isotropically in the far-field.
The polarization patterns of the radiator are presented is what. Figures 4(a), 4(b), and 4(c) present a view from directions Z, −Y, and X, respectively. The radiator is drawn at the center of the plots. It is shown that the polarization of the radiator strongly dependents on the direction.
4. The Double U-Shaped Power Isotropic Radiator
We have found the electric far-fields of the U-shaped radiator and have proved that the total power radiated is isotropic. However, this element shows different polarizations (linear, elliptical, and circular) at different cuts. In case that much more “regular” polarizations are required, we propose to use a double U-shaped radiator whose geometry is shown in Figure 5. The current density of this radiator is given by
The electric far-field components are given by
Figures 6(a) and 6(b) describe the view from directions Z and X, respectively. It is shown clearly that the polarization patterns are more symmetric than in the case of the U-shaped radiator. It can be shown that the symmetry of the radiation patterns can be described by known symmetry groups .
5. A Finite-Size Finite-Current Density Source Which Radiates Power Isotropically
We have seen that the U-shaped and the double U-shaped power isotropic radiators have infinite-surface current density. In order to remove this infinity, it is possible to replace the infinite-current density source by a finite-equivalent-current density source. A way to do this is to calculate an equivalent spherical surface current density, where the radius of the sphere is (see Figure 7), where the fields outside the sphere are the same as for the infinite-current density radiator. For convenience, this calculation is performed using Gaussian units. Following , we apply the multipole procedure. Let I be the region inside the spherical surface of radius , and II be the region outside the spherical surface, the fields in region II are given by where are the spherical Bessel and Neumann functions . The coefficients can be calculated for a given source current by
Inserting the current density of the U-shaped power isotropic radiator we have the expressions for the coefficients and , where
The details of the calculations are given in . In order to find the surface current density on the surface of the sphere of radius such that the fields outside the sphere will be the same as the fields of the isotropic radiator, we expand the fields inside the sphere:
We can find the coefficients from the equation then is known, and the surface current density on the sphere is given by where the coefficient in region I has been expressed in terms of the coefficients in region II .
Figure 8 describes constant surface current density curves on the sphere surface.
The far-field can be expressed in terms of the multipole expansion by
Figure 9 shows the first 6 terms in the former expansion are enough for an excellent convergence. In the graphs we see the far-field power density as function of for as an example.
The microstrip element has been investigated in the limit case where its width and its height above ground tend to zero. We have shown that in this case the far-field power density of the radiator tends to be isotropic in half space. Adding the image of this element, we have the U-shaped power isotropic radiator. The polarization pattern of the U-shaped radiator has low symmetry; hence, we have presented the double U-shaped power isotropic radiator, for which the polarization pattern is better. In order to remove the infinity in the current density of the radiator, we change the U-shaped radiator by a power isotropic spherical surface, having finite-surface current. However, we have to learn how to control, in practice, the details of the current density of the antenna.
This paper is devoted to the memory of our great teacher Professore Shmuel Shtrikman of the Department of Complex Systems, Weizmann Institute of Science, Rehovot, Israel, Life Fellow IEEE, who passed away on November 2003.
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