Design of an axial mode helical antenna with buffer layer for underwater applications

Recently, there is an increasing demand for high-speed wireless communication network for short-range underwater communication. From previous research, most underwater antennas produced omnidirectional radiation pattern which has lower antenna gain. There are a few considerations that need to be taken if the antenna is designed to operate in water environment. This paper discusses the electromagnetic properties which affect the underwater antenna design. Physical properties such as electrical permittivity and conductivity of water contribute significant effect to the size of the antenna as it influences the behavior of electromagnetic signal that propagates in water. In this study, an axial mode helical antenna with waterproof container is presented which operates at 433 MHz. The axial mode helical antenna has circular polarization and is suitable to support wireless application which is surrounded by some obstruction. The proposed antenna produces a bidirectional radiation pattern by placing it into a waterproof casing. Good agreement between the simulation and measurement results validates the concept. However, a little discrepancy between the simulated and measured results may be attributed to the noise originated from the equipment and the environment.


INTRODUCTION
Nowadays, wireless communication is preferred as the use of copper wires in the connections of the underwater sensor networks with radio signals or acoustic signals may result in higher costs and the difficulty of maintenance or installation [1]. There are three techniques that are commonly used in wireless underwater communication. The oldest technology is acoustic system which occupied sound signal as the medium to transfer the information. It is widely used as it allows the longest range of signal transmission [2]. However, it has some limitations in term of the low transmission rate, and it is also affected with the doppler effects [3]. The propagation of sound underwater is affected by temperature, salinity, hydrostatic pressure and other factors [4]. Apart from that, optical and radio frequency (RF) are also available in water communication. Nevertheless, these optical and RF technologies can provide higher throughput compared to acoustic but both technologies have limitation in term of the transmission distance [5]. For optical signal it normally requires line-of-sight between the transmitter and receiver that is hard to be achieved in water due to the higher particle inside [6]. Meanwhile, RF technology which relied on the antenna seems more suitable to be considered for short distance transmission with moderate data rate [7]. Performance of conventional RF communication schemes in water is limited by the relatively small RF skin depth [8]. The underwater antenna design is strongly dependent on the physical characteristic of water such as the permittivity and conductivity [9]. Therefore, the wavelength in the medium which depends on the physical characteristics of the environment where it is immersed, eventually influenced the size of antenna.
There are a few studies that focused on the underwater communication which utilized RF signal to transfer the information signal. Karagianni [10] explained about a bowtie microstrip antenna with arc-shaped circular slots that is designed to encounter the high path loss and to fulfil the bandwidth requirements in sea water. Research conducted by Ryecroft et al. [11] presents a real world results at 433 MHz operating frequency in raw water. This research proposed a bowtie antenna that achieved distances of 7 m at 1.2 Kbps and 5 m at 25 Kbps. Aboderin et al. [12] studied about three types of antenna, specifically, loop, dipole and J-pole for application in fresh and sea water environment. The research shows that J-pole antenna has significant advantages over the other antenna. Pasya et al. [13] proposed the utilization of a buffer layer with dielectric constant value calculated using geometric average among air and the transmission medium. Sinha et al. [14] created a micro-strip patch antenna that operates in the 402 to 405 MHz band and could possibly be implanted in a human body phantom model due to its flexibility and lower radiation properties.
A suitable antenna design is proposed to establish an underwater communication employing RF technology that can transmit higher data rate over a short distance. The axial mode helical structure that can exhibit higher gain is presented for the first time. The operating frequency of the proposed antenna is 433 MHz, and it can be tuned accordingly by changing the axial dimension and diameter. With 6 to 7 dB gain, the proposed antenna is expected to be reliable for underwater communication in a harsh environment.

ANALYTICAL METHOD
Water is a material which has high permittivity (ε) and electrical conductivity (σ). Therefore, in the aqueous environment, electromagnetic signal propagates with different behavior as compared to the normal propagation through air [15]. With a relative permittivity of 78 to 81 [16], water has the highest permittivity among any material and has a significant impact on the angle of refraction from air to water and vice versa [17]. High conductivity leads to strong attenuation of electromagnetic signal in water. Relative permeability, μr is approximately equal to 1 as water and air, both are non-magnetic in nature [18]. Therefore, there is only little effect of relative permeability of water to the magnetic field component [10].
Compared to acoustic and optical signals, the RF signal can pass smoothly through the water/air boundary provide with moderate transmission rates up to Mbps [10]. Nevertheless, due to the strong attenuation of electromagnetic signal in water [19], it is restricted for short distance transmission only. The speed of electromagnetic wave is affected by relative permittivity and relative permeability [18] as in (1): where v(water) is the velocity of signal propagation in water, μr and εr are the relative permeability and permittivity of water, respectively. ε and µ are calculated as (2) and (3).
The high permittivity of water limits the velocity of signal propagation when it propagates in this medium. Propagation loss inside water is as in (4) [20]: where σ is the water conductivity, ω is the angular frequency of the RF signal, and μ is the vacuum permeability. Attenuation loss (in dB) can be computed by (5): where d is the distance which the signal is propagated. Increasing the frequency of the signal will cause the attenuation of the signal increases exponentially [21]. Therefore, selection of frequency is crucial to ensure RF signal can be transmitted successfully to the receiver point. Due to the attenuation issue, special arrangement of devices that are immersed in water should be properly designed to ensure there are no significant losses that will occur during the propagation.
An axial mode helical antenna at 433 MHz is considered in this study for underwater application as it provides circular polarization with higher gain value than normal mode. This characteristic is important to be considered when the antenna is installed in the environment which has many obstacles that leads to greater signal reflection. Furthermore, axial mode helical antennas have many advantages compared to other types such as microstrip patch antennas, especially in terms of the gain value and stability of the antenna performance. This antenna is also ideal to radiate signals in a directional pattern, as compared to the normal-mode helical antenna which radiates an omni-directional pattern.
Generally, in order to get a strong directivity along the axis with circular polarization, the pitch angle α of the helix is designed to be between 12° to 14° and the circumference to be about one wavelength [22]. A helical wire antenna is more suitable to be used in underwater environment, as it can go directly into the medium, without a requirement to protect the antennas from water, unlike printed antennas that require spraying of epoxy resin on either side, which will alter the design frequency [23]. Figure 1 shows the structure of the proposed helical antenna with a front view in Figure 1(a) and a top view in Figure 1(b). It is made of a conductor wound into a helical shape and mounted on a small plate which acts as a ground plane. This structure is simulated using CST Studio Suite software. Performance of the helical antenna is determined by D/λ, and different D/λ values correspond to different radiation patterns of the antenna [24]. where v is the speed of electromagnetic signal that propagates in water while f is the proposed frequency of 433 MHz. Using (6), the calculated λ is 78.39 mm as the obtained v in water from (1) is 33.9×10 6 ms -1 . For the axial mode helical antenna, the helix circumference, C is equal to one wavelength [25]. The axial mode helical antenna performs well if the circumference of helix is in the following range [26]: The proposed antenna is excited by a coaxial feed line through a circular ground. Each parameter of the helix such as its diameter, axial height, spacing between turns and wire thickness are shown in Figure 1 and Table 1. These parameters will affect the performance of helical antennas, as discussed in section 4. The proposed antenna was immersed in the water and placed in the polyerhylene terephthalate casing with 3.3 dielectric constant. The proposed antenna comprises of a helical shape as a base structure while the casing is to encapsulate and protect the antenna from water. The velocity of electromagnetic signal will be decreased when it propagates in water environment. Consequently, the wavelength of the signal also decreases. Smaller wavelength value represents smaller size of antenna as the circumference of axial mode helical antenna is approximately equal to the one wavelength, λ as shown in (3). Therefore, the antenna will show a reduction in size when the calculation is considered in water environment. This phenomenon is contributed by the reduction of electromagnetic signal's velocity which decreased when it propagates in water. The antenna was placed in the plastic casing with diameter of Dc with tc thickness. This design is simulated in water background using CST software for the performance analysis. Radiation pattern, S11 and voltage standing wave ratio (VSWR) value have been observed and an optimization process has been carried out to ensure good performance of underwater antenna.

RESULTS AND DISCUSSION
During the design optimizing process, physical parameter that was obtained from the initial calculation has been varied to examine the effect on the antenna performance. The number of turns, axial height, axial diameter, wire thickness and distance of wire wound from the ground plane has been varied, then the performance of antenna in terms of S11 and gain value has been observed. In response to changes in these variables, S11 and gain value both changed, either increasing or decreasing. The following subsection will explain a detail explanation of the result.

Parametric analysis
For axial mode helical antenna, there are a few parameters that contribute to the antenna performance. Certain parameters will influence the antenna performance significantly while some of them just show a little effect. For instance, generally the beam-width decreases as the number of turns increases [27]. Figure 2 shows the effect of physical parameter of antenna to the operating frequency and S11 with the effect of the number of turns, N in Figure 2(a), axial height in Figure 2(b), axial diameter in Figure 2(c), wire thickness in Figure 2(d) and distance of antenna from the ground plane in Figure 2 consequently proved that the operating frequency gives a significant effect on the size of antenna. Small changes in diameter values cause the operating frequency to be shifted to higher or lower value. Hence the correct value of the diameter must be properly considered to ensure that the antenna will operate at the desired frequency as a little change has a great effect on the antenna output. The result shows the diameter of a helical antenna has a significant impact on antenna performance. Figure 2(d) shows the effect of wire thickness, dc on the antenna performance. This parameter has no significant effect on the performance of the antenna as there are a little difference between the S11 values at different thicknesses.
A ground plane is placed at the bottom of the proposed antenna with a certain distance, Hg from the coils. Figure 2(e) shows the effect of this parameter on the antenna performance. From the result, it is observed that the antenna distance causes a small effect as there was a little difference to S11 value for each distance. As mentioned previously in Table 1, the distance is calculated up to 0.14 λ.  Figure 3(e). From these results, it can be observed that when we increase the number of turns, the gain value will also increase. Similar to the axial length, as it is clear that increasing the height of antenna increases the gain value. The result complies with the concept of the longer the axial length, the greater the forward gain of the helix [28]. the relationship between antenna diameter and the gain level. The reading indicates that the gain value is reduced when the axial diameter is increased. From these results, it can be observed that there is no specific relationship between the wire thickness and gain value, but obviously variations of the thickness of the wire will affect the gain value. Although there is a small difference, the wire thickness still contributes to the antenna performance. Similar with other parameter, the gain value remains at 6 to 7 dB for each distance of helical from ground plane.

Simulation and measurement result
The main purpose of this research is to design a high gain of underwater antenna with directional radiation pattern. Simulation of the proposed antenna has been carried out using CST software. The proposed design successfully obtained the directional pattern when it was simulated in water without a casing as shown in Figure 4(a). Further optimization process has been made by changing the number of coils to N=5 and a few other parameters as shown in Figure 4(b). Figure 5 shows the radiation pattern when the antenna was placed in a plastic casing. The pattern was remained in a unidirectional pattern, but there was little improvement in term of back lobe as it shown that the back lobes has been reduced. Further analysis on the buffer layer characteristic will be presented in the upcoming report.

Simulation analysis
The proposed antenna has a VSWR value approximately 1.5:1 at 433 MHz while S11 value is -15.73 dB as revealed in Figure 6. The antenna yielded S11 below -10 dB at 433 MHz resonant frequency. At this point, the gain obtained was 6.99 dB. This indicates that the proposed antenna has good radiation characteristics within the targeted frequency. From the simulation result, the proposed antenna has been fabricated according to the dimension that was previously obtained from the simulation process.

Measurement analysis
The antenna was fabricated according to the values that were obtained from the optimization process. Figure 7 shows the experimental setup of the proposed antenna. The antenna was immersed in the water to measure S11 output after it was sealed in a waterproof polyethylene terephthalate casing. There is a small difference due to some measurement error such as noise that originated from equipment or environment. From the simulation result, we observed that the S11 is the best at 433 MHz with 13 dB return loss. There was a 25% difference between simulation and the measurement result. The measurement shows that the best return loss is at 426 MHz with S11 is equal to -17 dB while 433 MHz at 13 dB for the simulation.

CONCLUSION
This paper studies the effect of physical parameter of water on the helical antenna performance when the antenna operates in water environment. The proposed antenna has been designed and placed into polyethylene terephthalate casing. Result shows that the size of the proposed antenna was reduced if the antenna is designed to operate in water because its size is closely related to the wavelength value. This result agrees with the theory which states that the signal wavelength will be reduced when it propagates in higher density medium. From the simulation result, the proposed antenna is working at 433 MHz with S11 and gain equal to -13 dB and 6.99 dB respectively. The radiation pattern is changed from unidirectional to bidirectional behavior after the antenna is placed into polyethylene terephthalate casing. For the future, another similar antenna will be fabricated to ensure the measurement of radiation pattern can be carried out.

Mohd Khairil Adzhar Mahmood
is the Deputy Research Officer at Microwave Research Institute (MRI), UiTM Shah Alam. Prior to his current position at MRI, he used to serve as R&D Electrical Engineer in Motorola. His research interest is in the Microwave Engineering field where he works extensively in the process of circuit design, fabrication process and the measurement process involved in any particular project. He is also doing research in the nanotechnology-based materials for microwave applications. He can be contacted at email: khairil833@uitm.edu.my.