Main Effects Ensured by Symmetric Circular Slots Etched on the Radiating Patch of a Compact Monopole Antenna on the Impedance Bandwidth and Radiation Patterns

Three compact monopole ultrawideband (UWB) antennas with half-wavelength symmetric circular slots etched on the radiating patch and operating in the UWB spectrum are proposed. Parametric studies followed by experimental results of the return loss coefficient S-11 versus the circular slot length prove dual ensured functions. In fact, a switching from an impedance mismatch to an impedance matching appears when we replace a semicircular etched slot by a small circular slot while maintaining its inner radius constant. Simulated and measured reflection coefficient S-11 of a fabricated prototype with a stack of two small circular slots concord well and prove an impedance bandwidth enhancement that covers 3.9-14 GHz frequencies band (S-11 < -10 dB) compared to the impedance bandwidth 3.55-4.77 and 7.65-11.08 GHz (S-11 < -10 dB) of the baseline antenna without slots. In addition, measured radiation patterns in the H-plane of the monopole with two etched symmetric slots, are stably omnidirectional at 5.8 and 7.5 GHz. It is noted that due to some strong transverse currents around the small circular symmetric embedded slots, the cross-polarization in the same plane and at the same frequencies are not as low as single monopole. We noticed that there appears to be a close correspondence between the minimum of E-Phi and the maximum of E-theta electrical components. That is to say, the signals of the co-polarizations are irrelevant to those of the cross-polarizations. Due to its impedance matching and its stable omnidirectional radiation patterns, reliability and the reduced occupation volume, the proposed printed monopole with two etched small symmetric circular slots is suitable for the emerging applications of the wireless technology.