Vertically Polarized, High-Performance, Electrically Small Monopole Filtennas

Vertically polarized, omnidirectional- and endfire-radiating, bandwidth-enhanced, high efficiency, electrically small monopole filtennas are developed. Both designs are attained by organically combining a coax-fed top-hat loaded monopole with two hybrid quarter-wavelength open-end stubs and a shorting post. The hybrid stubs serve two functions. They produce two radiation nulls at both the lower and upper band edges to sharpen the roll-off rate. Moreover, they facilitate the appearance of an additional resonance without affecting the original resonant mode. The shorting post is introduced to engineer the overlap of two resonant modes to yield an enhanced bandwidth. Prototypes of both filtennas were fabricated, assembled, and tested. The measured results are in good agreement with their simulated values, validating their efficacy. The omnidirectional filtenna has an electrically small size (
ka
= 0.73), a fractional bandwidth (FBW) of 11.3%, and a maximum realized gain of 3.01 dBi. The endfire-radiating filtenna has an electrically small size (
ka
= 0.93), an operating FBW of 12.8%, and a peak realized gain of 6.09 dBi. Both filtennas exhibit a flat realized gain response and high radiation efficiencies over their entire passbands, as well as very good selectivity with high out-of-band suppression levels.