Novel multiband circular microstrip antennas (C-MSAs) with multiple half-ring slots are presented in this paper. Two antenna-feeding systems, i.e. an embedded L-probe beneath the patch and a coplanar circular arc- shaped probe (T-probe), are used to feed the C-MSA with multiple half-ring slots. The embedded L-probe is used to excite the C-MSA with a double-layer dielectric substrate due to its tremendous performance to provide a wideband impedance matching. The coplanar T-probe is proposed to realize an excellent multiband C-MSA in a single-layer structure. The C-MSA with four half-ring slots fed by the embedded L-probe exhibits satisfactory radiation characteristics. Five frequencies with broadside radiation patterns and gains of at least 5.0 dBi are obtained. It is also confirmed by simulation that resonant frequency and gain can be easily controlled to meet the desired frequency requirements. Moreover, the C-MSA with three half-ring slots fed by the coplanar T-probe presents satisfactory performance over the four observed frequencies with good return losses (-10 dB reference). Broadside radiation patterns with acceptable cross-polarization level and gains in the range of 3.0-7.0 dBi are obtained. Couplings between the C-MSA and the coplanar T-probe have an important contribution to obtain good return losses. It can be controlled by adjusting the distance between them and the arc angle of the coplanar T-probe appropriately. Experiments of both types of antenna were conducted to verify the simulation results and good agreements are confirmed. Due to the performances, these two C-MSAs are considered to be an effective model as a multiband planar antenna.

This paper presents novel multi-band equilateral triangular microstrip antennas (ET-MSAs) fed by an L-shaped feeding probe. Two types of the ET-MSA are proposed in this paper; they are ET-MSA with closed folded slots and open folded slots with an embedded bridge. The antenna prototypes presented in this paper assume four folded slots in which a metal strip is inserted to realize a good multi-band performance. In addition, the open folded slots, in which the metal strip inside the folded slot is connected to the ET-MSA by the embedded metal bridge, leads to miniaturize the antenna. The proposed antennas are printed on the top layer of a PTFE substrate while the L-probe is printed on the bottom one. The L-shaped probe is utilized due to its tremendous performance in providing a wideband impedance matching. Five resonant frequencies of the ET-MSAs with closed folded slots and with open folded slots are predicted by the electromagnetic simulator (IE3D) based on the method of moment, respectively. Some parametric studies have been also investigated to meet an appropriate multi-band performance. A broadside radiation pattern and gain of 3.0-7.0 dBi have been confirmed by the measurement for the entire observed frequencies. Measured results agree well with the prediction. Thus, these antenna systems are demonstrated to be useful models for a multi-band planar antenna.