Compact broadband omnidirectional antennas are in high demand for a wide range of applications in the wireless communication and defense system industries. Discone and biconical antennas are conventional broadband omnidirectional antennas designed to operate over multi-octave frequency ranges [1, 2]. However, the conical sections of these antennas have dimensions of the order of quarter wavelength at the lowest frequency. Thus, the dimensions of these antennas become large when implemented in VHF/UHF bands. Researchers in the field have been active in studying design techniques for realizing omnidirectional antennas with broad bandwidth and compact size with several notable findings reported in the literature. Sleeve monopole antennas using open sleeve configurations have been reported for broad bandwidths [3, 4]. Such antennas require very large ground planes. Dual sleeve antennas with reduced size ground planes have also been reported in [5]. A sleeve antenna mounted on a cylindrical ground plane having 4:1 bandwidth ratio is presented by Zhou et al. [6]. Yang et al. [7] designed a novel wideband sleeve dipole array operating between the frequency range 707–3,300 MHz. A sleeve monopole antenna with top-loading and shorting pins have been implemented by Ravipati and Reddy [8], achieving a bandwidth ratio of 3.7:1 and a bandwidth ratio of 4.28:1 by Ghafari and Aloi [9]. Bandwidth enhancement using matching networks are reported by several studies [10–13]. Matching networks enhance bandwidth at the expense of antenna efficiency. Dual sleeve antenna with load is demonstrated in [14] operating between the frequency range 57–103 MHz.

Several other dual sleeve antenna designs have also been reported. These antenna designs operated in different frequency ranges as follows: 400–900 MHz by Wang et al. [15], 500–2,100 MHz by Thomas et al. [5] and 120–520 MHz by Zhang et al.[16]. Sleeve dipole antenna with 2.9:1 bandwidth is reported in [17]. Sleeve dipole antenna with three sleeves was reported in [18]. An Antenna with conical shaped sleeve operating between the frequency range 446–732 MHz was demonstrated by Zhang et al. [19]. A low profile antenna with conical shaped radiator, cylindrical sleeve, top-loading and shorting pins is designed to operate in the frequency range 750–2,660 MHz [20]. A compact broadband omnidirectional antenna using canonical structures and single sleeve configuration was designed by Sai-Ram et al. [21] to operate in the frequency range 500–3,000 MHz. Half disc dipole antenna has been noted to exhibit broadband omnidirectional characteristics [22]. Finally, Gao and Lu [23] designed a printed wideband dipole antenna operating in the range of 650–1,150 MHz.

In this paper, a canonical triple sleeve antenna having compact broadband omnidirectional characteristics in 370–5,000 MHz is presented overcoming several limitations of earlier designs mentioned above. Novel concepts using canonical shaped radiating elements with triple sleeves are employed for achieving much wider bandwidth with compact form factor, compared to other sleeve antennas reported in the literature.

A conventional cylindrical dipole antenna is a narrowband resonant antenna. The bandwidth of this dipole antenna can be increased by increasing the diameter of the dipole. More bandwidth is achieved by increasing the resistance and reducing the reactance, compared to a thin dipole antenna.

The concept of sleeves can also be used to increase the bandwidth of this dipole antenna. A sleeve modifies the current distribution of the antenna and enhances its bandwidth. The design of the canonical triple sleeve antenna begins with a basic canonical antenna structure, resulting from a combination of hemispherical dipoles with conical and cylindrical extensions, fed at the center, as shown in Fig. 1.

The antenna is designed for the center frequency of f_o = 2,685 MHz, and the corresponding wavelength is λ_o = 111.73 mm. Antenna design is carried out using genetic algorithm optimization in CST Microwave Studio for the operating frequency band of 370–5,000 MHz.

The design parameters of the antenna are varied around their nominal design values as follows:

This antenna structure is further augmented by a single sleeve placed symmetrically with respect to the feed point resulting in canonical single sleeve antenna as shown in Fig. 1(b). The details of this augmentation are as follows:

The simulation models of these antennas are shown in Fig. 1.

The diameter of the hemisphere is obtained as 0.13λ_o. The diameter of the conical section at its one end is selected to match to the hemispherical section, which is 0.13λ_o, and the diameter of the other side of the cone is obtained as 0.77λ_o. The diameter and length of the first (central) sleeve are obtained as 1.05λ_o and 0.22λ_o, respectively. The diameter and length of the second and third sleeves are obtained as 0.81λ_o and 0.12λ_o, respectively. The parameters of the canonical triple sleeve antenna are given in Table 1.

The variations in resistance and reactance of the input impedance of the proposed antenna configurations are shown in Fig. 2(a) and 2(b).

As inferred from Fig. 2(a) that the resistance of input impedance has lower values for canonical antenna and canonical single sleeve antennas compared to canonical dual sleeve and canonical triple sleeve antennas. There is a substantial increase in resistance of input impedance for canonical dual sleeve and canonical triple sleeve antennas. The overall variation in resistance of input impedance is comparatively lower for the canonical triple sleeve antenna than canonical dual sleeve antenna over the frequency band 370–5,000 MHz. Variation in the reactance of input impedance is observed to be more for canonical antenna and canonical single sleeve antenna, and less for canonical dual sleeve and canonical triple sleeve antennas. The comparison of simulated voltage standing wave ratio (VSWR) of these antenna configurations is shown in Fig. 3.

It can be observed from Figs. 2 and 3 that the canonical triple sleeve antenna has the best impedance matching and hence VSWR performance in the broadband scenario from 370–5,000 MHz.

The VSWR measurement of canonical triple sleeve antenna is performed using a Vector Network Analyzer. The comparison of simulated and measured VSWR is given in Fig. 8.

The antenna has VSWR ≤ 1.9:1 in the frequency bands 415–530 MHz, 1,630–2,500 MHz and 4,560–5,000 MHz and VSWR ≤ 3:1 over the operating frequency range 370–5,000 MHz.

The measured and simulated E-plane and H-plane patterns of canonical triple sleeve antenna are shown in Fig. 9. It can be seen from Fig. 9 that the canonical triple sleeve antenna exhibits good omnidirectional characteristics. The measured omni-deviation is less than ±3 dB. The measured E-plane 3 dB beamwidth of the antenna varies from 15° to 120° over the frequency range 370–5,000 MHz. The measured radiation patterns are in good agreement with the simulated patterns.

The comparison of simulated and measured gain of the antenna is shown in Fig. 10. The measured gain of the antenna varies from −0.6 dBi to 4.5 dBi.

A comparison of the performance of the canonical triple sleeve antenna with existing published literature is shown in Table 2. It can be inferred that the proposed antenna, canonical triple sleeve antenna, has wideband characteristics and overcomes the limitations of the earlier designs with small form factor.

A compact broadband canonical triple sleeve antenna was designed and realized, to operate in the frequency range 370–5,000 MHz. The antenna has a bandwidth ratio of 13.51:1, catering to the needs of several wireless communication systems, spectrum monitoring, law enforcement and defense systems. The antenna configuration consists of a basic antenna comprised of canonical shaped hemispherical dipoles with conical, cylindrical extensions and three coaxial sleeves. The novel concepts of canonical antenna structure and triple sleeves has resulted in broadband antenna with compact form factor.