Techniques Developed for Low-Profile Printed Antennas

In the last decade, the use of parasitic patches in printed antennas gained huge attention for numerous applications in communication and navigation systems. To achieve a narrow bandwidth (>5%) and moderate gain from 15 to 25 dB, a microstrip patch antenna is the best choice. However, the necessary massive production of printed antennas for wide bandwidth and better control of radiation characteristics requires divergence in substrate parameters and manufacturing tolerances. Hence, to achieve the design specifications for an antenna, a simple and accurate analysis of radiating elements is required. The theoretical explanation of the basic-shaped patches and slots is done by the spectral domain approach, while the cavity or transmission line model is used for the analysis purpose. To design planar arrays, a detailed knowledge of various properties such as mutual coupling effect, directivity and losses of the linear sub-arrays of printed antennas is required. For designing the array of printed antennas, easy formula and analysis are explored. 2D arrays having a sub-array with non-identical elements, for example cross-fed geometry, are appropriate for cheaper antennas.

Features of Printed Antenna Technology

The printed antenna is the latest candidate in the field of antenna engineering. The printed antenna is fit for the latest applications because of its advanced features. Table 1.4 [12] illustrates the various performance factors that are significant and essential for designing an antenna for some specified applications. Table 1.5 illustrates all the operational and manufacturing considerations [12] which are in the equal demand, and these considerations are broadly relevant to the application. It is understandable that a new feature, i.e. production of thermal noise at the receiver- side antenna, related mainly to large lossy microstrip arrays is not appropriate for the basic antennas. Similarly, power handling, material effects, such as the use of new materials, mechanical and electrical stability of materials, and inter-modulation effects are basically related to the microstrip patch antennas.

Basic Issues and Design Limitations

To realize the basic issues and design limitations of printed antennas, a study of qualitative outline without examining specific designs in detail has been performed. A slotted triplate and cavity-backed printed antennas define the relation between the

TABLE 1.4

Checklist of Performance Factors for Antenna Design [12]

TABLE 1.5

Manufacturing and Development Considerations [12]

bandwidth and the height of the substrate. It defines that the bandwidth decreases with the decrease in the distance of separation between the radiating patch and the ground plane. Therefore, thick antennas have a large bandwidth. An antenna bandwidth is stated as the frequency range in which the condition of matching input is suitable, but the performance limitations of radiation pattern may also dictate the antenna bandwidth. On this basis, one may expect the bandwidth of the very thin printed antennas to be further reduced and this is definitely the case. An explanation is provided by the well-known super-gain concepts [54], which relate the antenna size to its bandwidth. The printed antenna occupies less volume than the cavity-backed antennas, and hence, a less bandwidth can be expected.

Printing of the feed structure on the substrate combined with the radiating element is the next concerning issue. A small amount of power which is coupled through one feeder to another due to surface wave action in the dielectric substrate and some additional losses are been introduced by the feeder lines. Due to this, controlling the distribution of antenna aperture and therefore side lobe level becomes difficult. However, the direct radiation from the feeders on the microstrip substrate results in the further degradation of radiation pattern. Therefore, there is a need for various screened feeder structures to be used in the requirement of modest side lobe levels until other methods are found. A substrate surrounds the microstrip radiating element by which the surface weaves are supported, and a little amount of power at each radiator is injected into the substrate, which is different from that of a slot antenna. The control on the side lobes can be further exaggerated by scattering of these surface waves at the boarder of the substrate.

At last, the mechanical tolerances are the major factors in limiting the precision of the lean microstrip structures due to which the amplitude distribution and aperture phase are controlled during manufacturing. The tolerance on the electrical and mechanical parameters of the material such as substrate ageing, temperature, etc are been added to the later part. The antennas backed with a thicker cavity having air gaps require a highly intricate assembly during manufacturing, but are hardly affected by the electrical and mechanical tolerances.

The antenna designers are well known that the performance achieved from an array of waveguide travelling waves is broadly dependent on the accuracy with which the waveguide itself is designed and manufactured. A parallel feeding method is not so much critical for use in design, but at that time, some factors such as cost, size and weight are adversely affected. It is obvious that the accuracy to which the aperture distribution of all the flat-plate antennas can be designed is also dictated by the design of their respective transmission line structures. The design of printed antennas therefore largely centres on the transmission line properties of microstrip lines. However, there is a main problem with microstrip in as much that no exact design equations exist in simple closed mathematical form. For instance, the field behaviour within a rectangular metal waveguide can be rigorously expressed in terms of simple trigonometric formula; the more difficult waveguide problem of the effect of rough metal surfaces, deformed sides, the presence of holes and obstacles, etc., can be analysed with a degree of accuracy sufficient for component design [55]. To obtain the field structure in the microstrip line with a precision demanded by most design specifications, it is necessary to compute equations involving extensive mathematical series, but this in turn leaves some doubt about the resulting numerical accuracy and somewhat defeats the object from a precision design standpoint. The characterization of discontinuities in microstrip lines such as steps, tapers and bends can be assessed at a low frequency using quasi-static techniques which embody the assumption that the radiation effects can be completely neglected. Consequently, the resulting data are of very limited use in printed antenna design, giving at best some indication of how to shorten the line lengths to tune up radiating elements.

The purpose of this segment is to elaborate the existing design issues and limitations for printed antenna analysis. To our knowledge, still certain basic properties of printed antennas are concentrating on their merits and demerits, and therefore, the design challenges of the future are represented as follows.

As compared to the conventional antennas, the printed antennas consist of many differences. Some of them are having two degrees of freedom, being used in very thin topology and the ability to be designed in any shape inside the boundary of x- and у-axes. One of the most wearisome properties is the ‘loss’, which is encountered in the connecting elements of feeders of thin conducting strip applied in large arrays. The loss that occurs in the radiating elements is also a problem for various applications. The result of using a thin substrate is the limited bandwidth achieved by radiating elements. Actually, a thin substrate used in printed antennas acts as an intrinsic high-Q resonator. Another problem is the generation of surface waves, which is not ignored by deploying foam-type substrates. In demand of low side lobe levels and cross-polarization, the presence of surface waves distorts the radiation pattern. Some problems are generated by compromising the manufacturing of assembly of simple and single со-planar structure of printed antennas. The last one is the high cost of the substrate materials that provide mechanical and electrical stability during operation. Otherwise, the cost of the substrate is an inherent feature of a printed antenna manufacturing process. To understand the importance of future advancement scope of printed antennas, the above basic issues and limitations are highlighted.

Analysis Methods for Some Common Patches

To realize the different antenna radiation characteristics such as input impedance, gain, efficiency, radiation pattern and polarization, the analysis becomes complicated due to the inherent characteristics of the printed antennas, such as heterogeneous dielectric medium, inhomogeneous boundary conditions, narrow frequency band and different types of feed technique. Thus, a trade-off is created between the complexity of the methods and the accuracy of the result. The analysis of printed antennas and consequences of its physical insight are explained by various methods. The antenna analysis may be performed by using different analysis models such as transmission line model (TLM), generalized TLM, lossy TLM, cavity model, generalized cavity model and multi-port network model (MNM). The transmission line model is the simplest technique to give a better physical insight into a designed antenna. It has been mostly useful for designing rectangular shapes of patches. It has also some fundamental drawbacks of neglecting field deviations along the radiating edges. By using the cavity model which is more complicated than the transmission line model, these fundamental drawbacks may be overcome. But it is not easy for the modelling of coupling. However, it provides an exact solution with a great accuracy.

A detailed comparison between different models is illustrated in Table 1.6 [56]. As depicted in the table, it can be easily summarized that the cavity model is the most prominent analytical method for basic configurations.

Analysis of Rectangular Patch Antenna by Transmission Line Model

The transmission line model is the best suited method for analysing the rectangular- or square-shaped patch antennas and will be discussed in this segment. In this method, an antenna with a basic structure including a radiating patch, a dielectric substrate and a metallic ground plane is required. This method is applied on the microstrip line-fed or probe-fed printed antennas as illustrated in Figures 1.17 and 1.18, respectively [57]. Assumed dimensions of the conducting patch for the fundamental mode are a length of ‘L’, a width of 'W’ and a thickness of ‘f, as shown in figures. The patch conductor has a conductivity e_p. This analysis method considers infinite dimensions for the dielectric substrate of length L_s, width W_s, and thickness h, which is placed in the plane of the radiating patch. The relative permittivity of the dielectric substrate and the loss tangent are denoted by e_r and S, respectively. The configuration of the substrate contains one homogeneous layer or multiple layers with different characteristics. Dimensions of the ground plane named with L_g and W_g may extend up to infinity for the analysis purpose. However, it has a thickness t_g with conductivity a and rms surface error e. Figure 1.17 shows an antenna fed by a microstrip line of width W, and length L_f.

TABLE 1.6

Evaluation of Different Analysis Models for Printed Antennas [56]

FIGURE 1.17 Detailed configuration of a rectangular patch antenna.

The aspect ratio W/h elaborates the cross-sectional geometry of the microstrip line for feed purpose. Furthermore, a high value of aspect ratio W/h considers a patch antenna as a microstrip line.

Figure 1.18 illustrates a transmission line analysis model for a printed antenna with rectangular patch. The cross sections AA' and BB' as shown in Figure 1.18

FIGURE 1.18 Rectangular patch antenna with transmission line model.

FIGURE 1.19 An equivalent transmission line model for a probe-fed rectangular patch antenna.

correspond to an open-ended termination with aspect ratio W/h of microstrip line of feed. Figure 1.19 represents a modified equivalent transmission line model in case of a coaxial probe feed.

Analysis of Circular Patch Antenna by Cavity Model

The basic structure of a circular printed antenna is shown in Figure 1.20. In the structure, a circular patch of radius a is supported by a substrate of height h. The

FIGURE 1.20 Geometry of a circular patch antenna.

feed is provided at (/>„, 0). p and cp are the radial and angular coordinates, respectively. The solution to the wave equation for a circular disc is calculated by using the polar coordinate system. Because the substrate height h is very less than Л₀, the waves do not travel towards the z-direction. As a result, the substrate consists of only the z-component of the electric field and the fundamental p and components of the magnetic field. The current vectors are normal to the edge of the circular patch and tend to zero at the edge. Therefore, the tangential component of the magnetic field vectors at the edge of the circular patch is minor. With these assumptions, the microstrip disc is modelled as a cylindrical cavity, bounded at its top and bottom by electric walls and on its edge by a cylindrical magnetic wall. Thus, the fields within the dielectric region of the microstrip cavity, corresponding to TM„„, modes, can be determined by solving the wave equation for a cavity.

The wave equation for a circular printed antenna without any current source is written as follows:

£

where к = 2.n.-^~

A,,

The electric field in the cylindrical cavity must satisfy the above wave equation and the magnetic wall boundary condition. The solution of the wave equation in cylindrical coordinates is as follows:

where J_nk are the Bessel functions of order n. Because E has only the z-component and d/dz=0, the magnetic field components become

where the prime sign denotes differentiation with respect to k_p, the argument. The other field components E , E and H. are zero inside the cavity.

The magnetic field boundary condition at the wall is defined as

At the edge of the circular disc, the surface current J must vanish; that is, H_lp is zero at p = a.

Hence,

where X_nm is the m'^h zero of the derivatives of the Bessel function of order n. Therefore, to configure every mode, a radius is defined to give a result in a resonance equivalent to the zeros of the derivatives of the Bessel function. Table 1.7 lists a few values of the lower-order modes of X_nm in ascending order. From the value of X_nm for the various modes given in the table, one infers that the mode for the value of n—m = has the smallest resonance frequency and is named as the dominant mode.

The modal expansion method is used for calculating the input impedance of the antenna as presented in [58].

The configuration of a circular-shaped patch antenna with their radiation properties among the basic printed antennas is extensively examined by designers. Assumptions for analysing circular patch antennas by this method are infinite dimensions of the substrate and the ground plane. Therefore, the obtained results are approximated and not influenced by the size of substrate and the dimension of the ground plane. The accuracy of the outcomes totally depends on the type of application. The impedance characteristic of the radiating patch depends on the parameters of the patch because of its highly resonant features. The infinite size of the ground plane has a negligible effect on the patch. Therefore, the patch itself determines the near-field radiation. On the other hand, at the back side of the antenna, manipulation in the radiation at broad angles is realized due to the fixed dimensions of the substrate and the ground plane.