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A novel, fast, and accurate analytical method based on input impedance is proposed for antenna directivity determination. The detailed theoretical expressions are examined and presented to determine the directivity of the wire-dipole and biconical antennas. The proposed analytical directivity results of the antennas are compared with conventional methods and measurements as well as a commercial electromagnetic (EM) simulator. A good correlation is achieved between the proposed method, the CST full-wave EM simulator, and the measurements. Additionally, the proposed method can be used to determine the antenna directivity in complicated environments, i.e., large array, for which the antenna characteristics may be deviated under coupling effects.

Wireless systems manufacturers desire a low-profile, wideband, circularly polarized (CP) antenna. Generally, a single-feed CP microstrip antenna (CPMA) has a simple configuration but is hindered by a narrow, 3-dB axial ratio (AR) bandwidth. The bandwidth of a multiple-feed CPMA can be enhanced but requires a complicated and larger feeding network. Many techniques have been employed to improve AR bandwidth, such as using dual-feed, network-based structures [1], stacked patches [2]?[3], array antennas with sequential feeding network [4], and multilayered structures [5]. However, these CP antennas are based on multiradiating patches or complicated feeding structures.

A wideband, single-feed, circularly polarized, stacked, square, slotted microstrip patch antenna is proposed for radio-frequency-identification-reader applications. The proposed antenna consists of an upper square asymmetric slotted patch, stacked over a lower square asymmetric slotted radiating patch and a microstrip stub, along with a coaxial feed. The wideband circularly polarized radiation of the antenna is achieved using a microstrip stub in the lower patch and a stacked slotted square patch. The proposed antenna is designed using a combination of low-cost FR4 substrate and foam. The measured 10 dB return-loss bandwidth of the antenna was around 21% (833 MHz to 1033 MHz), and the 3 dB axial-ratio bandwidth of the antenna was around 13.4% (835 MHz to 955 MHz). The gain was around 7.0 dBic over the 3 dB axial-ratio bandwidth. The overall antenna volume was 0.601?0 × 0.601?0 × 0.895?0 (where ?0 is the free-space wavelength at 900 MHz).

Circularly polarized diagonally symmetric slotted microstrip-patch antennas are proposed with compact size. The compact circularly polarized (CP) diagonally symmetric slotted microstrip-patch (DSSMP) antenna design rules are also summarized. Circularly polarized radiation can be achieved using any arbitrarily shaped slots in diagonal directions on the square microstrip-patch antenna. Different shapes for the slots are studied and compared, based on the fixed overall volume of the antenna for circularly polarized diagonally symmetric slotted microstrip-patch antennas. The cross-shaped-slot diagonally symmetric slotted microstrip-patch antenna is compact when compared with the circular-, square-, and circular-ring-shaped diagonally symmetric slotted microstrip-patch antennas. A measured 3 dB axial-ratio (AR) bandwidth of around 0.7% (6.0 MHz) with 2.0% (18.0 MHz) impedance bandwidth was achieved. The measured boresight gain was more than 3.3 dBic over the operating band, while the overall antenna size was 0.272λ0 × 0.272λ0 × 0.0138λ0 at 0.905 GHz.

Compact symmetric-slit square microstrip-patch antennas are proposed for circularly polarized (CP) radiation. A symmetric slit on a patch radiator can be used for circularly polarized radiation with a compact size. Various slit shapes embedded along the four diagonal directions on the patch radiators are realized for compact circularly polarized symmetric-slit microstrip antennas based on a single feed. Circularly polarized radiation is achieved by slightly varying the slit's circumference along one of the diagonal axes. The performance and sizes of the proposed antennas with various slit shapes are studied and compared. The proposed antennas were designed on a low-loss Rogers RO4003C substrate and on low-cost lossy FR4 substrate. The 3-dB axial ratio (AR) and 10-dB return-loss bandwidths of the antenna on the FR4 substrate were larger compared to those of the antenna on the RO4003C substrate. The gain of the antenna on the RO4003C substrate was higher compared to that of the antenna on the FR4 substrate with a fixed overall antenna size. The measured 10-dB return loss and 3-dB axial-ratio bandwidths were 3.85% and 1.5%, respectively, with a maximum gain of around 0.8 dBic, for the antenna on the FR4 substrate. For the antenna on the RO4003 substrate, the measured 10-dB return loss and 3-dB axial-ratio bandwidths were 1.5% and 0.5%, respectively, with a maximum gain of around 3.4 dBic.

We present two sets of simple, closed-form expressions for accurately computing the directivity of a circular microstrip antenna on thin and thick substrates of any relative permittivity. These expressions are useful for computation by a hand calculator and, therefore, useful to a practical antenna designer.