In the present article, we quantify the relationship among the dielectric characteristics of the media (relative dielectric constants and their losses) inside the enclosed volume, antenna sizes and their positions and their influence on overall communications losses. For the purpose of the experiment, a cylindrical metal vessel (barrel) with a height of H = 80 cm and radius, R = 30 cm is used, Fig. 1 (a). The resonator formed in this way is excited using monopole antennas/sensors (Tx and Rx antennas), Fig. 1 (a), and the barrel is filled with a high relative dielectric and high loss loss dielectric (tap water).

The main findings of the article are:

1. Losses of the medium are detrimental to the overall transmission loss; however, they also result in the reduction of optimal frequency of operation, inferring that smaller probes can be used to excite such a cavity, Fig. 1 (b).
2. Overall transmission losses decrease as the size of the excitation antennas are increased, Fig. 1 (c) however, that occurs only up to a certain frequency. Increasing antenna size beyond this frequency is detrimental to communications. For resonant communications, probe size should be kept at a minimum.
3. Positions of the transmitting and receiving probes are of utmost importance, since their position may or may not coincide with the location of electric field maxima and, hence, low losses.

Points 1-3 above indicate that in static systems, i.e. systems when the locations of the transmitting and receiving

probes are predefined, it is always possible to find the optimum frequency of operation, considering probe size, media losses and size constraints. However, in dynamic systems, or systems where the transmitting and receiving probes are moving, optimal frequency of operation will be highly dependent on the exact location of the probes. In this case, the frequency of operation should be an adjustable parameter and its exact value can be found by performing a scan in a predefined frequency range, from which the frequency exhibiting lowest losses are selected.

References:

[1] V. Kirillov, D. Kozlov, H. Claussen and S. Bulja, “Performance Estimation of In-Vessel Resonant Communications”, 18^th European Conference on Antennas and Propagation, (EuCAP), 2024, United Kingdom.

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As is known, optical communication links are reliable under line-of-sight conditions, however, they are adversely affected by the opacity and turbidity of liquids. Acoustic communications is a well-established approach for such scenarios but it is hampered by environmental factors such as temperature, pressure, and influence of external interference. Radio Frequency (RF) communications can be an attractive solution to overcome the above-mentioned limitations of optical and acoustic in-vessel communications. A standard RF link is established by the interaction of transmitting and receiving antennas, which are traditionally equal to half or a quarter of the wavelength. This means that the operational frequency needs to be relatively high (GHz range) to be able to use small antennas in the limited space of the vessel. However, at that frequency range, losses related to the propagation of electromagnetic (EM) waves through liquids are too high to establish a reliable communication link. Thus, a new approach for in-vessel communications is required to overcome these challenges.

Here we propose an alternative approach, which considers an enclosed volume as a low-frequency resonator with communication performed at its resonant frequencies. To this end, it is well known that any resonator has an infinite number of eigenmodes, which are characterized by their own eigenfrequency and a predefined EM field distribution. In this case, efficient data transmission between transmitting and receiving antennas is possible at eigenfrequencies. Due to the multiple reflections of EM waves from cavity walls, EM energy remains inside the enclosed volume, leading to reduced transmission loss between two antennas located inside the cavity in comparison with free-space propagation.

As a demonstration of the proposed principle, a cylindrical vessel, which has a height H and a radius R = H/2, as shown in Fig. 1 is used to demonstrate the practical feasibility of this approach. Simulation and experimental results are presented together with the analysis of the efficiency of the excitation of cavity resonators. Further, the influence of antenna sizes and dielectric properties of liquids on the transmission characteristics between two antennas is investigated. It is demonstrated that the determination of optimal antenna size providing communication links is one of the most important practical tasks for the development of an in-vessel communication system.

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