Wideband MIMO Rooftop Antenna for Vehicular Communication Systems, A Literature Survey and Design Gap Analysis

This review compared published work on shark-fin MIMO antennas, rooftop-cavity alternatives, and mutual-coupling reduction against one design brief, like four elements on a 175 × 75 mm ground plane, a height below 20 mm, and a built-in GNSS notch, covering 617 MHz to 6 GHz except that notch, at least in principle. None of the reviewed designs is meeting all of these; the only reviewed design under 20 mm is the four-port module of [1], which accepts about 10 dB of low-band isolation and efficiency below 30%, and doesn't state full-band coverage or a notch, on the whole. The designs with better isolation are 28 to 30 mm tall and port [4], [5], [11]; and thus, the gap is therefore not height alone but isolation, efficiency, and a characterised notch together, at a height below 20 mm. Two takeaways are following. Spacing alone gives about 11 dB at 0.25 λ in the low band, and the four-element module gives about 4 dB in its worst case without a network, so any decoupling structure has to beat about 10 dB for the explanation of its place, at least empirically. And neutralisation lines are confirmed on narrow bands and small boards, not yet on a 617 MHz - 6 GHz vehicular antenna, in real-world scenarios. The design work after this review is a four-element MIMO antenna modelled on [1], decoupled by neutralisation lines on its closest element pairs and aimed at better low-band isolation and efficiency below 20 mm, with the benefit of the lines shown against a no-line baseline; indeed, the shortcomings of Sec. 5.4 apply to every conclusion above. The next step is to run the verification plan of Sec. 5.3, and the module of [1] is rebuilt first, the height sweep is following. And before the neutralisation line is added, the spacing-only baseline is copied; and thus, the findings, including any in which the line doesn't help, will be reported against the figures collected in this review, so that the comparison in Table 3 can be generalized with measured data from the same brief, at least empirically.

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