Design of a compact, fully‐autonomous 433 MHz tunable antenna for wearable wireless sensor applications

Abstract

The authors present the design of a tunable 433 MHz antenna that is tailored for wearable wireless sensor applications. This study first presents a detailed analysis of the measured impedance characteristics of a chosen antenna under test (AUT) in varying proximity to a human test subject. Instead of limiting the analysis to the head and hand only, this analysis measures the AUT impedance at varying distances from 11 different body positions. A novel antenna equivalent circuit model is then developed that enables both the free-space and total on-body AUT impedance variation to be rapidly computed using a circuit simulator instead of the requirement for computationally intensive finite-element methods for example. The design and characterisation of a tunable matching network that enables AUT impedance matching for 11 different positions on the human body is then outlined. Finally, a fully-autonomous 433 MHz tunable antenna is demonstrated. The antenna occupies a small printed circuit board area of 51 × 28 mm and is printed on standard FR-4 material with the tuner completely integrated into the antenna itself. Prototype measurements show an improvement of 3.9 dB in power delivery to the antenna for a load voltage standing wave ratio of 17:1, with a maximum matching loss of 0.84 dB and S 11 (−10 dB) ≥ 18 MHz for all load conditions.

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“…This in turn provides control of the impedance bandwidth. This topology also provides large impedance coverage, has good harmonic rejection capability, and has relatively small losses of a few tenths of a dB when high Q-factor microwave-grade components are selected [ 69 ]. The π-type matching network model is depicted in Figure 9 with the antenna feed denoted by SMA connector Port P 1 at Point A.…”

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“…In this section, an impedance-matching network to reduce the antenna reflection coefficient while also improving the impedance bandwidth of the antenna is presented. The matching circuit shown in Figure 10 ensures the maximum power transfer from the source (A) to the antenna and maintains the desired performance even under small detuning effects [66]. First, a vector network analyzer [67] was used to measure Z IN , which was later used to design an optimal matching network for the antenna.…”