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In this article, the effect of actual antenna elements on rectangular chassis modes is investigated. The study is performed on three different antenna types, that is, printed inverted-F antenna (PIFA), printed monopole, and printed loop antennas. For the first time, we show that the presence of the antenna generates a completely new mode called the "antenna mode" (compared with chassis mode modifications as in other existing works). The new "antenna mode" only depends on the shape or geometry of the antenna and is independent of its location on the chassis (unlike chassis modes). In the presence of the antenna, the current maxima lies on the antenna element for all the modified chassis modes and the newly generated "antenna mode." In such scenarios, pure excitation of single characteristic mode (CM) is not possible with known existing techniques. K E Y W O R D S loop antenna, monopole, PIFA, TCM

In this article, the use of the mobile chassis for frequencies greater than 2 GHz is investigated. The analysis is conducted on 120 × 60 mm2 chassis size with 2 antenna types; monopole and planar inverted‐F antenna (PIFA). The use of the mobile chassis as the main radiating element for frequencies greater than 2 GHz is not practical or realistic. For such frequencies, the antenna is main radiating source that is why in an antenna and chassis scenario a 90% reduction in the ground plane for 2 different antenna types at 2.0 GHz did not affect the response (impedance bandwidth) of the system. At frequencies around 1 GHz, a 44% reduction in the ground plane did not affect the response either but one can observe that at lower frequencies the effect is more severe. The study shows that the antenna type and its localized currents are responsible for radiation. The effect of practical antenna elements on the chassis current modes is investigated. Different antenna types affect the chassis modes differently and the characteristic mode current maxima concentrate around the antenna element. Under such scenarios the excitation of a single mode is very difficult.

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In this article, with the help of the theory of characteristic modes (TCM), it is found that the introduction of circular slots in a chassis modifies the chassis modes. Based on the location of the slot, different chassis modes will be affected. For a fixed location, the slot to chassis size ratio plays an important role in its effect on the radiating bandwidth (BW). For a small size ratio, a modified chassis mode will be created without affecting the radiating BW. The current concentration across the slot depends on the shape of the slot (circular, rectangular, triangular, or square) and its location on the chassis. Moreover, the effect of multiple circular slots on the chassis modes is investigated. For slots introduced in a symmetrical fashion, that is, four elements, the modes are not affected and are similar to the original chassis ones. We also investigate the principle behind frequency reconfigurability using TCM analysis. The varactors used for frequency reconfigurability do not alter the radiating BW (Modal Significance plots) but only help in the input impedance matching at different frequency bands of the same mode. If a mode is excited purely using a proper feeding arrangement, the same frequency reconfigurable antenna will behave as a wide band one. Based on the detailed analysis presented in this work, a compact multiple‐input‐multiple‐output annular slot frequency reconfigurable antenna operating between 1.8 and 2.45 GHz is presented. Frequency tuning is achieved using varactor diodes.

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