Top-hat Monopole Research Articles

A Wideband, Low-Profile Log-Periodic Monopole Array With End-Fire Scanning Beams

In this letter, we propose a wideband, low-profile phased array with vertically polarized end-fire radiation patterns. The array consists of four identical log-periodic top-hat monopole antennas (LPTMAs) and could be completely embedded within a shallow cavity for sight invisible integrated installation on metallic platforms. Simulation results show that the proposed array operates over a wide frequency band of 1.0–3.0 GHz for active voltage standing wave ratio <2.0. It also possesses end-fire radiation patterns with ±45° beam scanning capability in the horizontal plane. Prototypes of the proposed LPTMA and a uniformly spaced linear array (1 × 4) are fabricated and validated in experiments. Measured results are in a good agreement with the simulation results. The measured results demonstrate that the proposed end-fire scanning array is attractive for applications that require wideband beam scanning, excellent aerodynamic, and stealth performances.

Log-Periodic Monopole Array With Uniform Spacing and Uniform Height

This paper presents a log-periodic monopole array (LPMA) with uniform spacing and uniform height operating in a wideband of 2–6 GHz. Different from the conventional LPMA with logarithmic spacing and height, the proposed antenna with uniform spacing and uniform height simplifies the design procedure and retains a low profile. The top-hat monopoles and folded top-hat monopoles are used to reduce the height of the antenna. A maximum bandwidth of 3:1 is theoretically proposed by considering the maximum and minimum spacing factor of the array. A meandering microstrip line is employed as the feeding line. The compensation of the length of the feeding line is proposed for the uniformly spaced LPMA compared to the conventional array. A prototype is fabricated and tested. Measured results indicate that an impedance bandwidth from 2 to 6 GHz (3:1) with VSWR < 2 is achieved. The measured peak realized gain is in the range of 6.6–8.7 dBi, which has great potential in many communication systems such as missile and airborne applications.

2–18 GHz Conformal Low-Profile Log-Periodic Array on a Cylindrical Conductor

This paper presents a conformal log-periodic array covering an ultra-wideband of 2–18 GHz. It employs monopoles of different heights for the high band, top-hat monopoles for the middle band, and folded top-hat monopoles for the low band. The top hats help to achieve a low profile of the monopoles while the folding is to maintain a good input resistance at the low band. A meandering microstrip line on the bottom is used to feed the array. The proposed antenna is mounted on a conducting cylindrical surface with all the top hats conformal and aligned to the outline of the cylinder. A prototype is fabricated and tested. Measured results show that an impedance bandwidth from 2 to 18.3 GHz (9.15:1) for VSWR $0.053~\lambda _{L}$ at the lowest operating frequency, which should be useful in many communication systems such as missile and airborne applications.

Low-Profile Top-Hat Monopole Yagi Antenna for End-Fire Radiation

A top-hat monopole Yagi antenna is presented to produce an end-fire radiation beam. The antenna has an extremely low profile and wide operating bandwidth. It consists of a folded top-hat monopole as the driven element and four short-circuited top-hat monopoles as parasitic elements. A broad bandwidth can be achieved by adjusting the different resonances introduced by the driven and parasitic elements. A prototype operating at the UHF band ( ${\mathbf{f}_\mathbf{0}} = \mathbf{550}\;\mathbf{MHz}$ ) is fabricated and tested. Measured results show that a fractional bandwidth ( $\vert{\mathbf{S}_{\mathbf{11}}}\vert ) of 20.5% is obtained while the antenna height is only ${\mathbf{\lambda }_\mathbf{0}}/\mathbf{28}$ at the center frequency.

Top-Hat Monopole Antenna for Conical-Beam Radiation

This letter demonstrates that a top-hat monopole antenna can be an excellent candidate for generating conical radiation beam. Design equations are derived to obtain the size of the top disk loaded at the end of a short monopole for conical-beam radiation. Simulated and measured results of a fabricated prototype are presented and compared to verify the design equations and its conical-beam radiation performance.

High SNR flexible top hat monopole probe for 1.5 T MRI

In this paper, design principles and application of a thin and flexible intravascular top hat monopole probe with increased signal-to-noise ratio (SNR) and improved longitudinal and radial coverage are described and compared with the conventional monopole probe. The top hat probe is constructed by removing a segment of the shield of a coaxial cable. A flexible thin copper foil in contact with the inner conductor of the cable is added to the end section of the cable. The copper foil acts as an antenna top hat load increasing the uniformity of the current distribution on the probe and establishes a uniform region of SNR distribution around the probe. Improvement of the SNR distribution of the top hat probe relative to the conventional monopole probe is evaluated using quantitative indices. Experimental results show that for the top hat probe, the ratio of the maximum image SNR to its relative standard deviation (SR) is 2.73 times higher than that of a conventional probe.

Evaluation of the input impedance of a top-loaded monopole in a parallel-plate waveguide by the MoM/Green's function method

An accurate modeling of a top-hat monopole transition in a parallel-plate waveguide is performed using the method of moments/Green's function method. The selection of the appropriate source-field relationship to override divergence series is discussed in detail. Numerical results are given for the input impedance of a top-loaded coaxial transition. A mode-matching solution is used as a reference to validate the results.

A Systematic Analysis of Top-Hat Monopole Antennas Loaded with Radially Layered Materials

Top-hat monopole antennas have many desirable characteristics that include high polarization purity and low fabrication cost. A major drawback of such antennas is their inherently narrow bandwidth. Among many techniques developed to overcome this problem is to load the antenna with dielectric materials. Loaded top-hat monopole antennas have been analyzed numerically only via a mode-matching technique that is both limited and mathematically involved. In this research, we present a systematic analysis and an optimization process based on finite-difference time-domain (FDTD) method for top-hat monopole antennas loaded with layered dielectric material and radiating above an infinite and finite ground plane. Both lossless and lossy dielectric loadings are used. Results are verified by comparison with corresponding published data. New results are presented which show wider matching bandwidths and higher antenna efficiencies.

On the link between top-hat monopole antennas, disk-resonator diode mounts, and coaxial-to-waveguide transitions [and reply

Francavilla et al. (1999) and Morgan and Schwering (1994) presented an analysis of a top-hat monopole antenna loaded with dielectric layers radiating above a conducting ground plane. In order to obtain a simplified solution to this rather complicated EM field problem, they introduce an upper ground plane that converts the original problem to the one of a monopole radiating in a parallel-plate radial guide, which is easier computationally to handle, provided that the upper ground plane is located a sufficient distance from the lower ground plane, the currents on the monopole and consequently the input impedance are approximately the same as for the original problem (with no upper conducting plane). The authors apply a field-matching method, in terms of radial harmonics in conjunction with a method of moments, to obtain field expansion coefficients in radial regions of the parallel-plate guide structure. Subsequently they obtain computer algorithms for calculating the input impedance of a top-hat monopole antenna in a parallel-plate waveguide. The present author notes that the problem of a top-hat monopole, as considered in Francavilla et al. and Morgan and Schwering is very similar to the problem of a disk-resonator diode mount as well as to the problem of a coaxial-to-waveguide transition with a disk-ended probe. Due to their different contexts, many antenna researchers overlooked the similarity of these problems. Hence the purpose of this communication is to highlight this oversight. Replies are given by Schwering and Morgan, and by Francavilla Bodner et al.

Mode-matching analysis of top-hat monopole antennas loaded with radially layered dielectric

A top-hat monopole antenna with homogeneous or inhomogeneous dielectric loading over a ground plane is considered. Mode-matching analysis with proper enforcement of edge conditions is applied to the problem. The results of this technique are verified through independent calculations of admittance as well as by comparison of fields across matching regions. Measurements were also taken for comparison with the results from the model. It is shown that dielectric loading can reduce the electrical size necessary for self resonance, but only at the expense of a large increase in radiation Q.

Eigenmode analysis of dielectric loaded top-hat monopole antennas

Eigenmode expansions are used for computing the currents on a dielectric loaded top-hat monopole radiating above an infinite conducting ground plane. The approach is facilitated by adding a parallel ground plane above the antenna, thus allowing use of cylindrical harmonic field expansions in each of three regions. Expansion coefficients are found by enforcing boundary and continuity conditions at conducting surfaces and regional interfaces. Convergence and accuracy are assessed using comparisons with three other methodologies, namely an integral equation solution, a quasi-static analysis, and multi-octave experimental measurements. >