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Keywords:

  • monopole antennas;
  • antenna height reduction;
  • antenna arrays

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References

[1] Using a Radial Guide Field Matching Method, an investigation is performed into reducing the height of an electronically steered circular array of monopole antennas composed of a central active element surrounded by passive elements being either short- or open-circuited. It is shown that a considerable height reduction can be achieved using top hats attached to monopoles ends and by applying dielectric coating underneath the top hats. The trade-off in achieving height reduction is narrower impedance bandwidth.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References

[2] Monopole antennas are an important class of radiating elements whose applications have been well established in mobile cellular communications with regard to both base stations and handsets. Recently, a renewed interest in arrays of monopoles has been shown with regard to indoor and vehicle wireless communications. For examples, the work presented in Scott et al. [1999] and Yang and Ohira [2001] concerns arrays of monopoles whose beam can be steered using PIN or varactor diodes. With regard to these array configurations, there is a considerable interest in lowering their profile due to aesthetic and conformal reasons. When ordinary wire monopoles are used, the minimum height of the array is approximately one quarter wavelength. This height creates visual pollution when this type of array is aimed for use in mobile portable equipment. A method of reducing the height of a circular array of monopoles by embedding it in a dielectric material was demonstrated by Lu et al. [2001].

[3] In this paper, an alternative method of reducing the height of monopoles is presented. This is done by attaching hats to the top of monopoles and by applying dielectric coating from the base to the top hat. An investigation is performed into the behaviour of such dielectric coated top hat monopoles in a circular array configuration. The investigated array includes an active central element and N concentric peripheral elements, which are either short- or open-circuited to steer the array's beam.

[4] The initial analysis of the array is performed using a Radial Guide Field Matching Method (RGFMM) which is described in this paper. As this method was originally intended to be used for the analysis of probes in a radial waveguide, its results need to be verified. The required verification is done using commercial EM simulation software called FEKO, which is able to simulate radiating structures composed of wires and plates in free space. In this paper this method is called a Free Space Method of Moment (FSMoM) (FEKO - Comprehensive EM solution, available at http://www.feko.co.za).

2. Antenna Configuration and Analysis Method

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References

2.1. Antenna Configuration

[5] Figure 1 shows the investigated circular array of top hat monopoles covered in a dielectric material. This antenna array consists of 1 active central element and N passive peripheral elements. In order to direct the beam to a certain position, K out of N passive adjacent elements are short-circuited and act as a reflector. The remaining NK elements are open-circuited and are transparent to the produced wave. This type of antenna system resembles in its operation a steerable Yagi-Uda antenna and can be implemented for an outdoor or indoor wireless communication system. Using the beam steering method chosen here, it is able to steer its beam to N different positions around the azimuth.

image

Figure 1. Circular array of dielectric coated top hat monopoles on infinite ground plane.

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2.2. RGFMM Method

[6] The investigation is carried out using the RGFMM simulation method which is based on the theory initially described by Bialkowski [1984] and Morgan and Schwering [1994]. First, it is explained here for the case of a single monopole antenna. This method requires the presence of an upper ground plane for the simulated monopole as shown in Figure 2. When the upper ground plane is greater than one free space wavelength, the input impedance and radiation pattern of the monopole becomes approximately the same as in free space [Morgan and Schwering, 1994]. The reason is that a monopole antenna predominantly radiates in the direction towards the horizon. As a result, when the upper ground plane is moved up, its interaction with the monopole becomes negligible. The convenience of this approach is its short computation time for each frequency point. For an array of monopoles the computation time is increased but only by the factor of approximately of N, where N is the number of monopoles forming the array.

image

Figure 2. RGFMM structure of dielectric coated top hat monopole.

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[7] For the method as applied to an array, it is assumed that all the monopoles are excited from the gaps, g0 (central element) and g1 (peripheral elements), located at the base. The dimensions of the central element are as follows: total monopole height (l = B1 + B2), monopole wire radius (a), excitation gap height (g0), top hat radius (Ra), top hat thickness (B2). The peripheral elements are evenly positioned on a circular radius, ro, and the dimensions of excitation gaps (g1), top hat radius (Rc), top hat thickness (B5) are identical for all these elements, however they can differ from the central element. All these dimensions are shown in Figure 1. There are two extra parameters introduced to the structure when method is being employed. As shown in Figure 2, B is the distance between the bottom and top ground plane and B3 is the distance between the top of the monopole and the top ground plane.

[8] A single monopole (such as the central monopole) is excited at the base using voltage, V0, which result in an electric field in the gap that can be written as:

  • equation image

where g0 is the height of the gap. This equation assumes that the electric field is uniform and oriented in the y direction. The field variation due to edges is ignored. This assumption is used in many other similar problems concerning excitation of wire antennas and generates accurate results for the input admittance/impedance.

[9] The input admittance of the monopole looking from the gap is given by

  • equation image

where Sa is the surface surrounding the gap aperture, Ho is the magnetic field in the aperture, and equation image is a unit vector normal to the aperture surface.

[10] The Yin can be determined once the magnetic field Ho is known. Here the task of determining Ho is accomplished as follows. First, the field in the close proximity of the monopole is assumed axially symmetric. Second, for a single monopole, the volume inside the parallel plate guide is divided into cylindrical regions, I, II, III. Next, the electric and magnetic fields in each region is expressed in terms of cylindrical functions with unknown expansion coefficients. Since the electric currents flowing along the probe have principally y and r components, it is expected that the produced field will have an Ey component, but no Hy component (Hy = 0). The fields around the probe are then TMy.

[11] In region III, the electric field should satisfy the boundary condition ∂Ey/∂y = 0 at y = 0 and at y = B since those points are at the surfaces of the ground plane. The electric field at region III due to a single monopole is given by

  • equation image

where H0(2)nr) is the zeroth order Hankel function of the second kind, which represents outwardly traveling radial wave. This field is normalized to a radius of r = Ra, and ɛ0n is the Neumann factor (ɛ0n = 1 if n = 0, ɛ0n = 2 if n > 0) which is introduced to make the equation easier to solve. Fn is a set of unknowns which must be determined. Γn is the propagation constant and is given by

  • equation image

where

  • equation image

are the eigenvalues for region III, and

  • equation image

where k is the free space wave number for region III, ω is the radian frequency, ɛ is the permittivity and μ is the permeability of region III.

[12] For the electric field in region II, the boundary conditions that must be observed are ∂Ey/∂y = 0 at y = BB3 and at y = B, as well as being finite at r = 0. Thus it can be written as

  • equation image

where Dn is a set of unknowns to be determined and the propagation constant Γ2n are given by

  • equation image

where

  • equation image

are the eigenvalues for region II, and

  • equation image

where k2 is the free space wave number, and ɛ2 is the permittivity of region II.

[13] The electric field in region I is slightly different from the fields from other regions. It is a superposition of the field from the voltage source applied to the gap and the field which is induced due to secondary sources which are generated by reflections of the primary wave from the surrounding structure.

[14] The electric field induced due to the other sources can be written in the form

  • equation image

which satisfies the boundary condition that Ey1i = 0 on the conducting surface of the probe in region I. The An is a set of unknowns to be determined, and the propagation constant Γ1n is given by

  • equation image

where

  • equation image

are the eigenvalues for region I, and

  • equation image

where k1 is the free space wave number, and ɛ1 is the permittivity of region I.

[15] The electric field due to the gap source has a similar form to that of Ey3 in terms of the outward traveling wave and can be written as

  • equation image

Its exact form may be determined by performing a Fourier analysis of the electric field in the gap along the surface at radius r = a. After accomplishing a Fourier analysis, the electric field due to the voltage in the gap may then be written as

  • equation image

where

  • equation image

The final superposition of the electric fields in region I yields the equation

  • equation image

For the field matching analysis, expressions are required for the Hϕ magnetic field components. These may be determined by applying the relationship for the TMy modes [Harrington, 1961], which for the nth y harmonic is given by

  • equation image

Using this approach, the ϕ component of the magnetic field is given in the following expression:

  • equation image

Applying the same approach to equation (4), the ϕ-component of the magnetic field in region II may be written as

  • equation image

Similarly for region III

  • equation image

Now, all the electric and magnetic field components for each region have been obtained. They can now be matched at the common interface r = Ra. The matching conditions are

  • equation image
  • equation image

[16] Determining the unknown field expansion coefficients can be accomplished by using the Galerkin procedure, which produces a set of linear equations for the unknown field expansion coefficients. Gaussian elimination or matrix inversion can then be used to solve the set of linear algebraic equations. Once the unknown coefficients are found, the Yin can be determined by substituting them into equation (Yin).

2.3. Analysis Method for Circular Array of Monopoles

[17] The field matching method derivations for a single element, described above, can be extended to the case of a circular array. For the case when a single monopole is excited and the remaining monopoles are terminated (for example, short-circuited), the problem can be resolved into N + 1 canonical problems, each featuring a circular symmetry. Using this decomposition, it is sufficient to consider only the regions of the central active element (I below the top hat, II above the top hat), only one of the peripheral elements' regions (IV below the top hat, V above the top hat) and the region external to the monopoles (region III).

[18] If the central monopole is labelled #0, and the peripheral monopoles as 1, 2, …, N, the Y-matrix equation of the array can be written as

  • equation image

where V0,V1,…, VN are the voltages of the monopoles 0, 1, 2,…, N and I0, I1,…, IN are the resulting currents at the feeding points.

[19] In order to obtain Y00 and Yi0, an excitation of the central monopole is applied while the remaining monopoles are short-circuited. Under such symmetrical excitation, the voltages Vi = V1 and the currents Ii = I1, where i = 1, 2, …, N. Thus the admittance-matrix equation becomes

  • equation image

which can also be written as

  • equation image

Using the fact that Y0i = Y01 for i = 1, 2,…, N, Y00 and Yi0 can be obtained:

  • equation image

[20] Equations (16)–(17) indicate that in order to determine Y00 and Yi0 only currents on two monopoles, the central and the peripheral, need to be determined. This requires considering the electromagnetic field solution involving 2 × M unknowns, where M is the number of modes (and field expansion coefficients) required to describe the field surrounding both the central and one of the peripheral antenna elements.

[21] When one peripheral monopole, for example m = 1 (due to symmetry, for the remaining peripheral probes the solution is obtained by re-numbering them), is excited and the remaining peripheral monopoles m = 2, 3,…, N are short-circuited, the solution to this problem can be obtained through the decomposition of the original excitation into symmetric excitations of the peripheral elements. This decomposition explores the symmetry of the array. The following set of excitation voltages that are applied to peripheral monopoles (while the central monopole is short-circuited) accomplishes this task:

  • equation image

where s = 1, 2,…, N − 1. Owing to symmetry and the form of excitation, only currents on the central monopole and only on one peripheral monopole need to be determined.

[22] When all these excitations are summed up (using the principle of superposition), then

  • equation image

The above result implies that the superposition of such N (s = 1, 2,…, N − 1) symmetric excitations is equivalent to the excitation when antenna m = 1 is excited with the voltage of 1 while the remaining N − 1 antennas are short-circuited (their excitation voltages are 0).

[23] Assuming that Is1 is the current calculated for the peripheral probe number 1 when the s-type of voltage excitation Vsm is applied, the mutual admittance Ym1 is given by

  • equation image

It is apparent that using the method of decomposing a voltage excitation into N symmetrical excitations, N problems need to be solved each featuring only 2 × M unknowns, where M is the number of harmonics describing the field in the y direction. The resulting problem of the two probes is solved by forming field equations similar to those for a single probe. The difference lies in the inclusion of an extra term which describes interactions between the two probes. This term is given in terms of outgoing cylindrical wave harmonics and concerns region III.

[24] Using a computer algorithm, which is developed based on this theory, investigations into the effect of top hat and coating dielectric on the resonant frequency and input impedance of top-hat monopoles can be performed. In particular, an insight into reducing the height of the monopole array can be established.

3. Results for Circular Array

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References

[25] The effects of using top-hat and dielectric coating for a monopole in the stand-alone configuration have been described previously by Morgan and Schwering [1994]. Morgan and Schwering have shown that the introduction of top-hat and/or dielectric coating results in in lowering the resonant frequency of the monopole. This is because the total effective electric height of the monopole increases due to the top-hat and the dielectric coating. Thus, in order to shift the resonant frequency to its original value, the height of the monopole must be decreased as discussed in Gangi et al. [1965]. It has to be noted that the use of high dielectric constant material can result in narrowband operation of monopole. Because of this reason, embedding with high dielectric constant is not explored in this paper.

[26] In order to verify the results from RGFMM, the FSMoM method, as offered by commercially available software FEKO, is used. For the case of an infinite ground plane, utilises top-hat dipole according to the image theory so that effect of finite ground plane can be omitted from the analysis. Impedances from the dipole are then halved to make them equal to the monopole's impedance. Here, only FSMoM for top-hat dipoles without any dielectric are considered because of the difficulty of inputting the dielectric embedded monopoles in the current version of FEKO.

[27] The next section of this paper shows the effects of coating the top hat monopole with dielectric. Figures 3–7 provide the graphical results for these effects. In each of these figures, there are four graphs shown. “Ordinary” refers to ordinary monopole simulated using FEKO, “FSMoM” refers to top hat monopole simulated using FEKO, “1” refers to top hat monopole without dielectric coating simulated using and “2” refers to top hat monopole with dielectric coating simulated using RGFMM.

image

Figure 3. Mutual coupling effect on impedances of dielectric coated top hat monopole.

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image

Figure 4. Impedances of dielectric coated top hat monopole as a function of circular array radius when the individual monopole resistance is fixed at 37.5Ω at 2 GHz.

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image

Figure 5. Impedances of dielectric coated top hat monopole as a function of circular array radius when the individual monopole reactance is fixed at 0Ω at 2 GHz.

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image

Figure 6. Impedances of dielectric coated top hat monopole when resistance at 2 GHz is fixed at 37.5Ω.

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image

Figure 7. Impedances of dielectric coated top hat monopole when reactance at 2 GHz is fixed at 0Ω.

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[28] Before studying the circular array configuration, examination of mutual coupling effect between two top hat monopoles with and without dielectric coating is done. Figure 3 demonstrates the impedance of a dielectric coated top hat monopole at 2 GHz when another identical short-circuited monopole is placed at a range of distances from it. The monopoles used have the parameters: wire radius (a) = 0.5 mm, excitation gap height (g0) = 3 mm, top hat radius (Ra) = 10.5 mm, top hat thickness (B2) = 2 mm. Dielectric coating of permittivity 2 was also used in this simulation. The monopole height, l, varies according to the dielectric constant used to maintain a resistance of 37.5Ω at 2 GHz. It is seen in the figure that the impedance of the active top hat monopole converges to 37.5Ω faster than for ordinary monopoles.

[29] The next study involves six peripheral elements placed on a concentric ring around an active central monopole. Three of these peripheral monopoles are assumed to be short-circuited to act as reflector, and the remaining three are open-circuited so that they are transparent to the EM field produced by the central element. For this experiment, two types of reduced height monopoles are considered. All the elements in the array are assumed to be identical in size. In the first simulation, the size of monopole is selected so in a stand-alone configuration its input resistance is selected so its input resistance is kept at 37.5Ω at 2 GHz. In the second simulation, the size of monopole is selected so in stand-alone configuration its input reactance is kept at 0Ω at 2 GHz. The parameters of the antenna to keep the resistance at 37.5Ω at 2 GHz are as follows: wire radius (a) = 0.5 mm, excitation gap height (g0) = 3 mm, top hat radius (Ra) = 7.5 mm, top hat thickness (B2) = 2 mm, monopole total height (l) = 22.5 mm with no dielectric coating and 21.25 mm with dielectric coating of permittivity of 2. The parameters of the antenna to keep the reactance at 0Ω at 2 GHz are as follows: wire radius (a) = 0.5 mm, excitation gap height (g0) = 3 mm, top hat radius (Ra) = 7.5 mm, top hat thickness (B2) = 2 mm, monopole total height (l) = 13.5 mm with no dielectric coating and 12.25 mm with dielectric coating of permittivity of 2. simulations were done with no dielectric coating.

[30] Figure 4 demonstrates that as the radius of the circular array, ro is increased, the central element input resistance converges to 37.5Ω. It also has to be noted that top hat monopole resistance stabilises faster than for an ordinary monopole. The reactance of the top hat monopole can also be seen to be higher than for an ordinary monopole, which follows the same observation when only 1 short-circuited monopole is present. Figure 5 shows a similar experiment but for the reactance to converge to 0Ω instead. It can be observed that the reactance of the top hat monopole is not affected very much by the presence of parasitic elements. The resistance of this type of monopole can be seen as much lower than ordinary monopoles.

[31] Figure 6 shows the impedance of the central monopole over a frequency range when the circular array radius is at 75 mm(λ/2) and a resistance of 37.5Ω is being kept at 2 GHz. It is observed that the resistance value is more stable across the frequency range and as expected the reactance values of top hat monopole are higher than ordinary monopole. However, with relative dielectric permittivity of 2 underneath the top hat, a sharp increase in resistance can be seen as the frequency increases. In this figure, a slight discrepancy between FSMoM and RGFMM can be seen. This discrepancy can be attributed to different forms of excitations of the monopoles in the two types of simulations. In FEKO, a monopole is divided into segments and one of the segments is used for applying voltage. The length of this segment may differ from the gap height used in RGFMM.

[32] Results achieved in Figure 7 were obtained under similar conditions as in Figure 6 but with the aim of keeping the reactance at 0Ω at 2 GHz. From this figure, it can be seen that the resistance is lower when reactance of 0Ω is being kept at 2 GHz. Also, the reactance varies less with frequency. In many applications, it is desirable to have the input impedance of the array at 50Ω without using any external matching circuits. The presented results from the array indicate that this can be achieved by properly choosing the array's dimensions.

[33] By manually changing the height and radius of the array, an input impedance of 50 + j0Ω is maintained at 2 GHz for the active central monopole. Figure 8 shows the S11 parameters of the simulated and measured results for both the ordinary and top hat monopoles. The dimensions of the elements and radius of the array of ordinary monopoles are as follows: wire radius Ra = a = 1.5 mm, gap height g0 = 5 mm and its location h0 = g0/2 = 2.5 mm, total length l = B1 + B2 = 32.0 mm, and array radius of r0 = 67 mm.

image

Figure 8. S11 parameters of circular array formed by (a) ordinary monopoles of 32 mm height and (b) top hat monopoles of 25 mm height.

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[34] The dimensions for the array of top-hat monopoles (the central and peripheral elements are identical) are as follows: top hat radius Ra = 5 mm, top hat thickness B2 = 5 mm, wire radius a = 1.5 mm, gap height g0 = 5 mm, total length l = B1 + B2 = 25.0 mm and array radius of r0 = 72 mm.

[35] In the measured array, short-circuits were accomplished by introducing metal strips in the coaxial apertures in the ground plane. An analogous arrangement was practically impossible for non-short-circuited monopoles. This is because open-circuited coaxial connectors of SMA type were introducing residual reactance loading non-short-circuited monopoles making them not truly open-circuited. In order to overcome this practical difficulty, further FEKO simulations were performed for the case when open-circuited monopoles were removed. It was found from the simulations that the input impedance and radiation pattern of the array were approximately the same as those when the truly open-circuited elements were present. Thus, measurements of the input impedance and far-field radiation pattern of the array were obtained when the SMA open-circuited elements were removed.

[36] In practice, the required open-circuit conditions could be realized using virtual open circuits. These could be realized using short-circuited quarter-wave transmission lines. Practical designs of RF switches realizing such conditions have already been covered in the following works: Karmakar and Bialkowski [2002, 2001] and Bialkowski et al. [1996]. Therefore these designs are not repeated here.

[37] By comparing the results shown in Figure 8, one can see that both arrays (with ordinary and top-hat monopoles) show good impedance match of the central element at 2 GHz. It is also observed from the simulated results that the ordinary monopoles have wider 10 dB return loss bandwidth (0.6 GHz) compared to the top hat monopoles (0.4 GHz). The measured results confirm this as the ordinary monopole 10 dB RL bandwidth is 0.55 GHz. The impedance characteristics also follow those from simulations. The top hat monopole array measured 10 dB RL bandwidth is 0.4 GHz with a slight shift of resonance to the higher frequency. This discrepancy between simulations and measurements for both the ordinary and top hat monopoles can be caused by the manufacturing imperfections.

[38] Figure 9 shows the normalized far field patterns of the top hat monopole circular array when the impedance of the active central element is being kept at 50 + j0Ω at 2 GHz. Directional character is observed in both the azimuth and elevation planes. There are 3 types of graphs being shown in this figure. First, the term “simulated” refers to results from FEKO simulation. Second, the term “measured pattern with SMA open-circuited elements” refers to the measured results when the SMA open-circuited elements were present in the array. Third, the term “measured w/o open-circuited elements” refers to the situation when measurements were performed for the SMA open-circuited elements being removed from the array. The “measured pattern with SMA open-circuited elements” results are included in the graph to show that the open-circuited coaxial connectors of SMA type adversely influenced the radiation pattern of the array. In Figure 9a, which shows the far field pattern of array in the elevation(θ) plane, the 0° is taken from the underside of the ground plane. Thus, the 120° directional beam is actually a beam squint of 30° from the ground plane due to the ground plane. In Figure 9b, the 0° is taken from the short-circuited elements. Hence, the beam is directed to the direction of 180°. The corresponding simulated azimuth array gain is found to be 4.7 dB higher in comparison with the gain for a single monopole. As seen in Figure 9, good agreement between measurements and simulations is observed. There are slight discrepancies such as slightly lower sidelobes observed in the azimuthal plane (Figure 9a) or higher sidelobes level detected in the elevation plane (Figure 9b). These discrepancies could be caused by manufacturing imperfections of the antenna array. The other reason for the observed discrepancies could be due to the anechoic chamber, in which measurements took place. Its size and absorbing material were more appropriate for operation above 3 GHz frequency range.

image

Figure 9. Far field pattern of circular array when the active central element impedance is 50 + j0Ω at 2 GHz. (a) θ plane and (b) ϕ plane.

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4. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References

[39] This paper has presented methods for lowering the profile of a circular array of monopoles. The methods are based on the use of top hat on monopole's end and dielectric coating on the monopole from its base to its top hat.

[40] By using the dielectric coating and top hats, mutual coupling effect between monopoles is reduced. This finding has been confirmed when the active element is surrounded by 6 peripheral elements in a circular array configuration. For this array, in order to keep the impedance of the active central monopole at 50 + j0Ω at 2 GHz, a height reduction of 27.5% has been demonstrated. This height reduction has been achieved at the expense of a slight impedance bandwidth reduction as compared with the array of ordinary monopoles.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Antenna Configuration and Analysis Method
  5. 3. Results for Circular Array
  6. 4. Conclusion
  7. References
  • Bialkowski, M. (1984), Analysis of disc-type resonator mounts in parallel plate and rectangular waveguides, Arch. Elektron. Übertrag., 38, 306310.
  • Bialkowski, M., S. Jellett, and R. Varnes (1996), An electronically steered antenna system for the Australian MobilesatTM, IEE Proc. Microwaves Antennas Propag., 143(4), 347352.
  • Gangi, A., S. Sensiper, and G. Dunn (1965), The characteristics of electrically short, umbrella top-loaded antennas, IEEE Trans. Antennas Propag., 13(6), 864871.
  • Harrington, R. (1961), Time-Harmonic Electromagnetic Fields, McGraw-Hill, New York.
  • Karmakar, N., and M. Bialkowski (2001), A beamforming network for a circular switched-beam phased array antenna, IEEE Microwave Wireless Compon. Lett., 11(1), 79.
  • Karmakar, N., and M. Bialkowski (2002), High performance L-band series and parallel switches, Microwave Opt. Technol. Lett., 32(5), 367370.
  • Lu, J., et al. (2001), Multi-beam switched parasitic antenna embedded in dielectric for wireless communications systems, Electron. Lett., 37, 871872.
  • Morgan, M., and F. Schwering (1994), Eigenmode analysis of dielectric loaded top-hat monopole antennas, IEEE Trans. Antennas Propag., 42, 5461.
  • Scott, N., M. Leonard-Taylor, and R. Vaughan (1999), Diversity gain from a single-port adaptive antenna using switched parasitic elements illustrated with a wire and monopole prototype, IEEE Trans. Antennas Propag., 47, 10661070.
  • Yang, K., and T. Ohira (2001), Single-port electronically steerable passive array radiator antenna based space-time adaptive filtering, IEEE Antennas Propag. Symp., 4, 1417.