Cage antennas optimised for bandwidth

Cage antennas are optimised via a genetic algorithm (GA) for operation over a wide band with low voltage standing wave ratio (VSWR). Numerical results are compared with those of other dual band and broadband antennas. Measured results for one cage antenna are presented.     

Wide bandwidth microstrip antennas for circuit integration

Novel multilayer concepts for the formation of efficient micro-strip antennas on alumina substrates that allow simple antenna/circuit integration to be achieved are described. Experimental results show that, by using these techniques, bandwidth in excess of 16 times those of alumina microstrip antennas can be obtained, at the expense of an increase in the overall antenna height.     

A bandwidth enhancement method for microstrip antennas

Bandwidth enhancement methods for electromagnetically coupled microstrip dipoles are discussed. It is demonstrated that if parasitic metallic strips are incorporated in the structure either co-planar and parallel to the embedded microstrip transmission line open end, or between the transmission line and the microstrip dipole, then substantial bandwidth enhancement results. Experimental verification of this model is introduced for a bandwidth definition based on the frequency range which satisfies a voltage standing-wave ratio (VSWR) < 2 criterion. Also, experimentalbar{E}- andbar{H}-plane patterns verify the theoretical model which accounts for radiation from the microstrip dipole, the parasitics, and the transmission line.     

Impedance characteristics of planar bow-tie-like monopole antennas

The impedance characteristics of planar bow-tie-like monopole antennas have been studied experimentally. The planar bow-tie-like monopole antenna achieves a broad bandwidth, typically of >75%. A modified formula is suggested to accurately evaluate the frequency corresponding to the lower edge of the impedance bandwidth     

Impedance stability and bandwidth considerations forelevated-radial antenna systems

An examination is made of the variations in feed-point impedance that occur in an elevated antenna system due to changes in the electrical characteristics of the underlying soil. The bandwidth of such antennas is also investigated by looking at deviations in the input impedance as the operating frequency is varied about its nominal value. In all cases, results for the elevated antenna are compared with those of the conventional ground-mounted system. The computer program used for this work was NEC (Numerical Electromagnetics Code), a method-of-moments software package     

New bandwidth extension technique for conformal microstrip antennas

A broadband microstrip patch concept is presented. Fed using electromagnetic coupling, but over a single layer of dielectric substrate, this simple mechanical structure permits antenna applications in structurally and thermally severe environments. One S-band application is presented for such elementary patch and for a cylindrical 'belt' microstrip antenna of omnidirectional pattern built around this elementary principle.<>     

Impedance and radiation pattern of antennas above flat discs

Antenna imaging by a perfectly conducting finite flat disc is investigated analytically and numerically as a function of the radius of the disc. A conical monopole of variable length is considered above the screen and the normalized terminal admittance of the antenna is plotted versus the disc radius. The radiation pattern is also plotted and the current distribution above and below the screen is evaluated. The results provide a quantitative measure of the effectiveness of finite screens in imaging antennas and bring to light the persistent distortion of the pattern caused by the finite size of the screen in contrast to the behavior of the impedance.     

Impedance matching arrangement for quadrifilar helix antennas

A ring arrangement of four wires near to the excitation point of a quadrifilar helix antenna can be used to achieve an impedance match. The matching technique is applicable to resonant, fractional turn antennas, and to shaped conical beam backfire mode antennas     

Stacking resonators to increase the bandwidth of low-profile antennas

A method of increasing the bandwidth of low-profile cavity-hacked slot and microstrip patch antennas without appreciably adding to the antenna dimensions is presented. This idea incorporates the stacking of structures which have close resonant frequencies. Energy is coupled between cavities via carefully placed slots in the common wall. An approximate two-and-a-half fold increase in input impedance bandwidth has been observed for a certain cavity-backed slot antenna.     

Impedance control of microstrip antennas using reactive loading

It is shown that it is possible to change the input impedance of a microstrip antenna over a wide range without affecting its resonant frequency by moving short circuits along prescribed loci. This theory has been verified by experiment; specifically, it is shown that the resonant frequency of the mode is left relatively unchanged by the appropriate placement of the short circuit loads. The experiment also shows that the agreement between the predicted and observed input impedance is quite reasonable in most cases. There was qualitative agreement between theoretical antenna patterns and measured antenna patterns although the measured cross-polarized components in the double-loaded element were higher than expected from the theory. A single load, varying in position, was seen to provide a reasonable range of impedance variation while retaining a very nearly constant resonant frequency, although the resulting asymmetries in the magnetic current distribution caused a high level of cross-polarized fields. The use of two loads, instead, produced an even greater range of possible impedances, with reduced cross-polarization levels as well     

Impedance and polarization characteristics of H and IHI slot antennas

Slot antennas can be end loaded to tune input impedance for use with active devices and to decrease overall slot length at a given resonant frequency. Cross polarization and impedance of end-loaded slots in an H configuration are analyzed. For an H antenna of approximately equal height and width, D-plane cross polarization was found to be -20 dB, and the resonant frequency decreased by 34% over a slot with a length equal to the width of the H. The cross polarization can be reduced with our new IHI configuration, which is introduced. The impedance at the second resonance can be tuned over a range of 26-82 /spl Omega/.     

An impedance-matching technique for increasing the bandwidth ofmicrostrip antennas

The nature of the inherent narrow bandwidth of conventional microstrip patch antennas is considered. It is observed that, except for single-feed circularly polarized elements, their bandwidth is limited only by the resonant behavior of the input impedance and not by radiation pattern or gain variations, which usually are negligible over a moderate 10 to 20% bandwidth. Therefore, broadband impedance matching is proposed as a natural to increase the bandwidth. The maximum obtainable bandwidth is calculated using Fano's broadband matching theory. It is found that by using an optimally designed impedance-matching network, the bandwidth can be increased by a factor of at least 3.9, the exact value depending on the degree of matching required. A transmission-line prototype for a proper matching network is developed. The translation of this prototype network into a practical structure (e.g. a microstrip or stripline circuit) is considered. Practical design examples and experimental results which clearly show the validity of the technique are given     

An enhanced bandwidth design technique for electromagneticallycoupled microstrip antennas

This paper describes a method of enhancing the bandwidth of two different electromagnetically coupled microstrip antennas by utilization of a tuning stub. An approximate theory and equations are developed to demonstrate the potential bandwidth improvement and required stub impedance characteristics. A novel dual-stub design is presented that achieves better characteristics than a conventional quarter wavelength open-end stub. As examples, the bandwidth (VSWR<2) of a conventional proximity-coupled microstrip antenna is increased from 4.8 to 8.4% and the bandwidth of a stacked aperture-coupled microstrip antenna is increased from 27.5 to 34.5% using this technique     

Impedance matching improvement for a class of wideband antennas

A simple and efficient technique for improving the impedance matching of a class of wideband antennas is presented. These antennas have impedance characteristics that exhibit dual (or multiple) resonance. The results obtained are in good agreement with the experimental test.     

Bandwidth and Q of antennas radiating TE and TM modes

The time-domain Poynting theorem is used to develop a general expression for the complex Poynting vector applicable to any single-frequency electromagnetic radiation field. It is found that the traditional complex Poynting vector applies to TE or TM fields, which we call simple fields, but that it does not apply to TE and TM fields, which we call compound fields. Either TE or TM fields are generated by most antennas. We show that previously imposed theoretical minimum size-to-wavelength ratios for useful antenna operation apply to simple fields but not always to compound ones. We conclude that electrically small, efficient compound antennas may be possible. As an example, the general form of the Poynting vector is used to analyze a compound source consisting of four antenna elements; idealized, superimposed, properly phased and oriented, coherent, electric and magnetic, dipole and quadrupole radiators. When properly driven, the antenna supports zero reactance on a circumscribing virtual surface of radius a, even in the limit as the radius-to-wavelength ratio of that surface goes to zero. The directivity pattern has a fixed 9 dB gain; the radiative Q of the surface is less and the bandwidth more by a factor of (ka)², where k is the wave number, than for similarly sized radiators of simple fields     

柱面共形蝶形微带天线的阻抗特性和方向图研究

研究了小曲率半径有限长度圆柱体上共形蝶形微带天线的阻抗特性和方向图.给出了不同曲率半径下的柱面共形蝶形微带天线反射损失和方向图的仿真结果.实际制作和测试了柱面共形微带天线的单元和二元阵列天线,仿真结果与实测结果比较吻合.研究表明:当圆柱半径较小时,不但方向图改变,而且谐振频率下降、阻抗带宽明显减小;当圆柱长度缩短时,方向图的起伏增大.     

A bandwidth enhancement technique for mobile handset antennas using wavetraps

A novel technique for enhancing the impedance bandwidth of wireless terminal antennas is presented. By introducing resonant short circuit transmission lines to the long sides of the chassis edges, an effective electrical shortening of the terminal ground plane is achieved. This effect has been used to realize terminal ground planes with resonant lengths at high frequencies, such as GSM 1800/1900 MHz or UMTS 2 GHz, thereby extending the impedance bandwidth. The proposed technique has been validated by simulations and measurements. Three typical applications are presented where the introduction of wavetraps improves the bandwidth and/or in-band performance.     

Impedance and large signal excitation of satellite-borne antennas in the ionosphere

The input impedance of a spacecraft-borne electric antenna operating below approximately 30 kHz in the ionosphere is investigated with emphasis on sheath effects. The impedance concept is generalized to be applicable to nonlinear antenna-magnetoplasma systems. Explicit formulas are obtained for the generalized impedance of a near cylindrical antenna taking into account the effects of large signal excitation, induced electromotive forces due to antenna motion across the Earth's magnetic field, the influence of the ion collection surface of the space vehicle, and of dc bias between antenna and vehicle. Comparison is made of the pertinent part of the theory and LOFTI-IIA satellite data.