Multiband linear dipole antennas are necessary for many strip- or bar-shaped gateway devices of the Internet of Things (IoT) for connectivity under various communication protocols. However, the conventional methodology to design multiband dipole antennas is generally empirically based. More frequency bands usually mean even more arms/slots, which results in increasingly bulky size. In this article, we introduce an algorithm to use the fewest arms to design any multiband linear dipole antenna. The algorithm is based on sharing arms after the effective ranges of mode excitation are determined by characteristic mode analysis (CMA). It also applies to multiple wideband or ultrawideband antennas. By this algorithm, an exemplified pentaband dipole antenna is designed, effectively covering 433, 868, 1176, 1575, and 2450 MHz bands for low-power wide-area network (LPWAN), global navigation satellite system (GNSS), and industrial, scientific and medical (ISM) applications. The antenna has only 2.5 pairs of arms. 50% of arms are reduced in comparison to traditional methods. This algorithm significantly simplifies the structure of a multiband dipole antenna, especially suitable for strip- or bar-shaped IoT devices.

Dipole antenna is the simplest type of antenna from the theoretical point of view. However, the majority of the research on it focuses on adding supplementary structures to produce more resonant modes to achieve more functions. Here, we focus on the inherent modes of the dipole itself. Dipole antenna has alternating odd and even modes. In classical antenna theory, only odd modes are excited. The even modes are difficult to be excited due to their very high input impedances. In this paper, both odd and even modes of the dipole antenna are excited simultaneously by a single port with a novel feeding structure. Moreover, in order to tune the resonant frequencies of such multiple modes, a method by combining antithetical structures of indentations and outdentations is proposed. Its principle is explained intuitively by eigencurrent distribution. By these new techniques, all the first six modes of a dipole are excited, among which five modes are utilized for IoT applications. The antenna can cover ISM bands of 868 MHz and 2450 MHz, GNSS band of 1575 MHz, and 5G bands of 3500 MHz and 4900 MHz. This research could broaden the application of this classic antenna.