A new type of printed rectangular loop or 100-51 transmission line. The unidirectional antenna with capacitive loads is proposed for radiation pattem is achieved by introducing two -unidirectional radiation pattern production. The arrangement presented here is shown to have a current distribution with balanced magnitude and 90' phase difference on parallel radiating edges of the loop. As a result, the printed loop is equivalent to an .gaps into ;he loop in order to produce capacitive phasing elements. It is experimentally demonstrated that a front-to-back ratio of than 2o dB and a 4.5-dBi gain can be achieved.array of two dipoles with quarter-wavelength space separation and 90' phase excitation difference.
II. ANTENNA CONFIGURATIONSimulated and measured results for 1 GHz center-The geometry and optimised sizes of the frequency operation show that the new unidirectional unidirectional rectangular loop antenna are depicted printed loop antenna can achieve a front-to-hack can he easily matched. GHz. The loop is made of etched copper strips of width w=6 mm and thickness F17.5 pm printed on an RT/duroid 5880 dielectric substrate (thichess=0.254 mm, er =2.2). The antenna is centre-fed by a quafier-wavelength folded balun constructed from 5 0 0 coaxial cable.ratio of more than 20 dB, power gain of 4.5 dBi, and in l'ig. The antenna is designed to Operate at
A wideband probe-fed circularly polarised circular loop antenna is presented. The wideband performance is achieved by introducing a parasitic loop inside the original loop that is driven by a feed probe. It is found that the 2 dB axial ratio bandwidth can be increased from 6% for a single circular loop to 16% by the introduction of a parasitic loop.
A novel type of low-profile broadband and dual-frequency monopole antennas is developed for wireless applications. The broadband monopole consists of an S-strip and a folded T-strip which are separately printed on the two sides of a thin planar substrate. No via process is involved in the fabrication. The two-strip planar monopole can achieve a bandwidth (VSWR<2) of ~35% with a height of 0.056 0 . The dual-frequency operation of the broadband monopole antenna is realized by adding a short T-strip to the S-strip and a couple of inverted-L strips to the folded T-strip. The dual-frequency monopole can accomplish a bandwidth of ~40% at a lower-frequency band and ~10% at a higher-frequency with a total height of 0.08 0 .
New types of wideband circularly polarised (CP) square-and rectangular-loop antennas are developed and discussed. It is shown that the bandwidth for circular polarisation of a loop antenna can be significantly increased by adding a parasitic loop inside the original loop. The addition of the parasitic element can create one more minimum axial ratio (AR) point, which can appropriately combine with the original one, leading to a significant enhancement for the AR bandwidth. It is found that the AR ( r 2 dB) bandwidth of a square-loop antenna can be increased from 6.5% to 20% by adding a parasitic square loop. By replacing the square loop with a rectangular loop, the AR bandwidth can be further enhanced. For a rectangular loop with an aspect ratio of 1/2, a bandwidth of nearly 50% for AR r 2dB is obtained with a gain of 8.5-7.5 dBi. A broadband balun is introduced for impedance matching.
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