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Microstrip antennas have long been used as reliable components with small profiles, large directional beams, and high gain.1 Unfortunately, they also exhibit narrow impedance bandwidths, with single-layer dielectric-resonator-type patch antennas providing only a few percent of center frequency.2

Fig. 1

Microstrip antenna impedance bandwidth can be increased by means of crossfeed logarithmic modes,3,4 where microstrip antennas have been achieved with several octaves of coverage. In addition, through a coplanar feed approach with quarter-wavelength (λ/4) microstrip lines, an impedance bandwidth from about 1.3 to 4.0 GHz has been reached.5

A number of reports6,7 have described the use of various feed approaches—including perforated feeding, aperture coupled feeding, and embedded coplanar feed logarithmic periodic microstrip antennas—for improved impedance bandwidths, with log-period microstrip antennas showing great promise. One problem with log-period microstrip antennas, however, has been the use of a double-layer circuit medium, in which said medium must be perfectly aligned for best performance. This can be difficult to achieve at the design and production stages. 

A log-periodic microstrip antenna is a form of traveling-wave antenna. Its terminal portion works with the impedance-matching load to absorb radiated energy and reduce reflected energy. The terminal section is important, since it can greatly improve the antenna’s radiation pattern in the working frequency band. But terminal matching load absorption can significantly reduce the overall efficiency of a microstrip antenna (especially across the upper frequency range), and efficiency loss can reach 10% or more.8

Fig. 2

Adding a terminal matching load to a log-periodic microstrip antenna will increase the complexity and cost, and eventually may cause it to fail. The bandwidth of a log-periodic microstrip antenna is determined by periodic factor k and microstrip patch unit number n, with the antenna bandwidth equal to kn - 1. If k is large, the number of required microstrip units is reduced.

Fig. 3(a)

Fig. 3(b)

For example, for a scale factor of 1.05 or less, at least 21 microstrip units are required for a working bandwidth of 2.6:1. For a scale factor of 1.1, only 11 microstrip units are needed to achieve the same bandwidth.

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