179

1 INTRODUCTION

Automatic Identification System (AIS) plays an

irreplaceable role in maritime communication

[3]

However, the research on antenna of AIS base station

still remains in primary stage. Nowadays,

omnidirectional metal antenna is adopted on AIS base

station in Chinese Ships ’ Routeing waters.

Although it is possible to implement bursts of AIS

information, there are also many problems.

1 Some Ships’ Routeing waters in China, for

example in Chengshanjiao Ships’ Routeing waters,

are more than 25 nautical miles offshore. However,

omnidirectional antenna lower the transmitting

rage of AIS information due to low gain. This issue

can be fixed by directional antenna instead of

omnidirectional type.

2 In AIS base station communication system, the

omnidirectional transmitting antenna power is

above 20W, which has strong electromagnetic

interference with shore facilities and users.

3 Due to the increased traffic density in the ships’

routeing waters, communication channels are

often occupied and time slot conflicts happened

occasionally which greatly increases the burden on

the VTS attendant. Therefore, the AIS shore station

can utilize VHF high-gain directional antennas to

enhance the communication effect of the AIS base

station and the VHF shore station.

We propose the idea of applying the electronically

controlled plasma Yagi antenna in the field of

maritime intelligent communication as the

transmitting antenna of the AIS base station in ships’

routeing waters to improve the problems above-

mentioned. A plasma antenna is an antenna that uses

a plasma instead of a metal antenna element as an

electromagnetic energy conducting medium

[4,5]

Compared with metal antennas, plasma antennas

have the characteristics of light weight, good stealth

performance and reconfigurability

[6,7]

. Using the

plasma to construct the Yagi antenna, when the

Research on Radiation Characteristics of Plasma Yagi

Antenna Based on AIS base station in

Ships’ Routeing

Waters

Y. Sun, Y. Chen, F. Kong, Y. Wei, F. Zhan &

J. Zhao

Merchant Marine College, Shanghai Maritime University, Shanghai, China

ABSTRACT: A Yagi plasma antenna model was established by HFSS according to the relationship between

plasma dielectric constant and electron density. The patterns were simulated by changing plasma parameters

and the number of director dipoles. Results show that when the passive vibrators were switched off, the

antenna is omnidirectional antenna. The directionality increases with the increase of the number of passive

dipole and the main lobe of which narrows down. Then the plasma Yagi antenna model is established by

plasma tube, the gain changed by changing the number of passive dipoles, so the plasma Yagi antenna has a

very good reconfigurability. Results prove that the feasibility of the plasma Yagi antenna can be used on AIS

base station of Ships’ Routeing waters. It can promote the communication and capability of maritime

supervision in Ships’ Routeing waters.

http://www.transnav.eu

the

International Journal

on Marine Navigat

ion

and Safety of Sea Transportation

Volume 14

Number 1

March 2020

DOI:

10.12716/1001.14.01.22

180

passive vibrators are all turned off, the antenna

system only has the active vibrator, which is an

omnidirectional antenna. When changing the plasma

parameters and increasing the number of directors,

the antenna directivity can be dynamically adjusted

and communicate in different region. Compared with

the metal Yagi antenna, the plasma Yagi antenna has

the advantages of reconfigurability and low mutual

coupling.

2 MODEL AND PROTOTYPE OF PLASMA

ANTENNA

The AIS base station plasma Yagi antenna model

includes: an active plasmon, a passive plasmon.

According to the plasma antenna theory, it is assumed

that the plasma has a uniform axial and radial

distribution, and its relative dielectric constant is

[8,9]

)

(

νωω

−=

(1)

where

is the collision frequency of electrons and

neutral particles in the plasma,

is the angular

frequency of the incident electromagnetic wave,

the plasma angular frequency. which can be written

as:

(2)

Assuming the plasma collision frequency is very

low, then the plasma relative dielectric constant can

be written as:

−=

(3)

When the signal frequency is less than the plasma

frequency,

. The electromagnetic wave can

propagate between the outer surface of the plasma

and the inner surface of the dielectric tube as a surface

wave and it cannot propagate in the plasma. And the

electromagnetic wave propagating in the radial

direction is rapidly attenuated. At this time, the

plasma can propagate electromagnetic waves as an

antenna.

The HFSS is used to construct the plasma-guided

antenna model. According to formulas (1) and (3), the

appropriate dielectric constant is selected according to

the plasma electron density and collision frequency.

The antenna center frequency selects two

communication frequencies of AIS, 161.975 MHz and

162.025 MHz respectively. The antenna pattern is

simulated by changing the plasma parameters and the

number of directors. The plasma Yagi antenna model

is shown in Figure 1. The designed antenna consists of

an active vibrator, a passive reflector, and three

passive directors. Since there are more than five

passive oscillators, the gain of the antenna does not

change much. Here, in order to highlight the obvious

reconfigurable characteristics of the antenna, only

three passive directors are selected.

Figure 1. Plasma Yagi antenna simulation model

Figure 2 Schematic diagram of plasma capacitive coupling

feed mode. 1. plasma, 2. discharge tube, 3. coupling collar,

4. Dielectric layer, 5. Ground plane, 6. Coaxial line

Since the plasma has a certain potential inside, it

cannot be fed by direct connection coupling. The

feeding mode of the antenna is shown in Fig. 2. The

outside of the antenna is a tempered glass cavity, and

the active oscillator adopts a capacitive coupling

feeding mode. The metal collar is tightly wrapped

around the outside of the tempered glass cavity, and

the internal plasma forms a coupling capacitor, which

is simulated by changing the working state of the

passive oscillator. At the same time, the physical

model of the plasma antenna was built by using the

discharge tube. The plasmon electron density can be

adjusted by adjusting the operating state of the power

supply. Figure 3 is a schematic prototype of a plasma

Yagi antenna. The antenna prototype has a reflective

vibrator, an active vibrator, and three passive

directors, wherein the antenna active oscillator uses a

half-wave folded oscillator. All antenna elements

have an outer diameter of 12 mm and an inner

diameter of 10 mm, all excited by a 20 kHz high-

voltage excitation source, and the excitation power

can be adjusted. According to the gas discharge

Penning effect, the filling gas inside the discharge

tube is helium gas and a small amount of argon gas.

The antenna bracket and the movable collar are used

to fix the active and passive vibrators of the plasma

181

Yagi antenna, and the movable collar can move along

the axial direction of the bracket to adjust the distance

between the antenna vibrators. Antenna vibrators of

different lengths can be replaced. Therefore, the

antenna prototype can adjust the directivity and

impedance bandwidth of the antenna by adjusting

parameters such as discharge state, antenna vibrator

spacing, and the vibrator length.

Figure 3/ Plasma Yagi antenna principle prototype

3 RESULTS AND ANALYSIS

3.1 Impedance bandwidth characteristics

The largest electron density of glow plasma can reach

-3

, the electron density of plasma driven by 20

kHz power supply is from 10

-10

-3

measured by

Langmuir probe

[14]

. The axial and radial changes of

plasma electron density are very small for two ends

excitation. So, in the HFSS simulation, the axial and

radial electron density is considered to be uniformity.

Figure 4 and 5 show the results of simulating and

measuring impedance bandwidths when the electron

density reaches 10

and 10

-3

respectively. The

antenna impedance experimental field is a wide

playground, the transmitting antenna is a plasma

Yagi antenna, and the receiving antenna is a metal

antenna. At this time, all the antenna oscillators are

activated. The testing instrument is the vector

network analyzer (Agilent, E5071C), and the working

frequency is AIS1 (161.975 MHz). The length of

reflection oscillator is 0.52 wavelength, the effective

length of active oscillator is 0.48 wavelength, the three

passive leading oscillators are equal length and 0.45

wavelength, and the distance between adjacent

oscillators is equal and 0.2 wavelength. In simulation,

the external and internal diameters are 12mm and 10

mm, respectively. And the feeding mode is capacitive

coupling mode. The length of antenna oscillator can

be changed. The simulating mode of the plasma

antenna is shown in figure 1. The designed plasma

antenna consists of an active oscillator, a passive

reflecting oscillator and three passive directing

oscillators, because when the number of the active

oscillator is more than five, the gain variation of

antenna is not very large. In order to highlight the

obvious reconfigurable characteristics of the plasma

antenna, only three passive directing oscillators are

selected. According to formula (3), the relative

dielectric constant of plasma ranges can be adjusted

from - 31.7 to - 283.76. From figure 4, when the length

of each oscillator of Yagi antenna is fixed, the

measured and simulated values of return loss (S

11) are

close to each other. The simulation value is slightly

higher than the measured one, and the impedance

bandwidth obtained by simulation is smaller than the

measured value, it may be because that certain axial

and radial density gradient exists in the plasma in the

antenna oscillator. From figure 5, when the density is

close to 10

-3

, the return loss of the antenna is

obviously reduced and the impedance bandwidth is

widened. The measured return loss of the antenna is

not much different from that of the simulation, which

shows that the simulation results are more reliable

and the radiation performance of the plasma antenna

is better when the collision frequency is low and the

electron density is high.

Figure 4. Antenna impedance bandwidth measurement and

simulation results when the electron density is on the order

of 10

-3

Figure 5. Antenna impedance bandwidth measurement and

simulation results when the electron density is on the order

of 10

-3

3.2 Radiation characteristics

3.2.1 Effect of electron density on antenna radiation

characteristics

The radiation characteristics of the antenna mainly

depend on the directivity and gain. The radiation

characteristics and reconstruction characteristics of

the plasma Yagi antenna are studied by changing the

182

working state of each oscillator, the distance between

the oscillators and the length of the oscillators. In

antenna measurements, data are measured every 10

degrees. Because any antenna measurements have

uncertainty, the number of measurements for the

pattern under each condition is 6 times, and then the

average value is taken. Measurements and simulation

results of E-plane normalized direction map of

plasma Yagi antenna at 10

-3

electron density are

shown in Fig. 6. When all the oscillators are fully

open, the antenna array has good directivity, and the

measured value is not much different from the

simulated value. The backscatter of the measured

pattern is slightly larger than the simulated value

because of the reflection of the site itself.

0.0

0.2

0.4

0.6

0.8

1.0

120

150

180

210

240

270

300

330

0.0

0.2

0.4

0.6

0.8

1.0

Simulation

Experimentation

Figure 6. Plasma Yagi antenna pattern measurement and

simulation results

Table 1 shows the measurement results of the

decrease of half power beam width (HPBW), gain and

directivity coefficient of plasma Yagi antenna under

different electron densities. When the electron density

is in the order of 10

-3

, the antenna has a high gain,

approaching 7.5 dBi, and the communication

coverage angle of the antenna is very small. We use

HFSS software to simulate the metal Yagi antenna of

the same size and size as the plasma Yagi antenna,

and the gain is about 9 dBi. Hence, the gain

performance of high-density plasma antenna is close

to that of metal antenna. When the electron density

decreases, the communication area becomes wider.

When the density is 10

-3

, the half-power angle of

the antenna approaches 60 degrees, the maximum

radiation direction gains decreases to about 6 dBi, and

the half-power angle approaches 70 degrees and the

maximum direction gain is about 5 dBi when the

electron density is about 10

-3

. So, the plasma Yagi

antenna has very good pattern reconfigurability and

can quickly construct the communication coverage

area of AIS base station.

Table 1. Half-power angle and gain of the vertical plane of

the antenna at different electron densities

_______________________________________________

density (m

-3

) gain (dBi) HPBW (°)

_______________________________________________

5.07 67.5

6.24 59.4

7.41 51.7

_______________________________________________

3.2.2 Working state of the passive oscillator

When the passive oscillator is all closed and only

the active oscillator is working, the antenna is a

monopole antenna. At this time, the horizontal

direction is omni-directional, which can realize omni-

directional broadcasting of information. When the

reflection oscillator is opened and the number of

leading oscillators is changed, the directionality

changes quickly. Figure 7 shows the simulation

results of three-dimensional pattern of the plasma

Yagi antenna when the passive oscillator is all closed

and all opened at the signal frequency of AIS2

(162.025 MHz). When all passive dipoles are closed,

the antenna is a half-wave symmetrical dipole

antenna, and the horizontal direction is

omnidirectional. When all passive oscillators (one

reflecting oscillator and three leading oscillators) are

turned on, the directivity of the antenna changes

rapidly. Table 2 shows the effect of the number of

leading oscillators on the main lobe HPBW of the

vertical plane pattern of the plasma Yagi antenna

when the electron density of the plasma is 10

-3

under the condition of opening a reflecting oscillator.

When all the leading dipoles are closed, the half

power angle of the antenna is very large. When the

number of the leading dipoles is increased, the

directivity of the antenna increases, the

communication range narrows and the HPBW

decreases. When the number of leading dipoles

increases to 3, the HPBW decreases to about 65

degrees. When the number of leading dipoles

increases to 5, the HPBW decreases to nearly 50

degrees. Therefore, when the plasma electron density

is a constant, by changing the number of passive

leading oscillators, the antenna directivity can be

rapidly adjusted, and the number of leading

oscillators can be increased. The front-to-back ratio in

the antenna pattern increases, the directivity

increases, and the half power angle narrows. Table 3

shows the experimental and simulation results of the

effect of the number of leading oscillators on the

maximum radiation directional gain. The electron

density is 10

-3

, and the gain measurement method

of plasma antenna is comparative method. The

measured and simulated gains have little difference

and the variation trend is the same. As the number of

oscillators increases, the antenna gain increases. The

reconfigurable range of antenna can be enlarged by

synthetically changing the electronic density and the

working state of antenna oscillator

Figure 7. Three-dimensional pattern of the passive reflection

of the plasma Yagi antenna and the full opening and full

closing of the guiding oscillator

183

Table 2. Influence of the number of directors on the half

power angle of the vertical plane pattern of the antenna

_______________________________________________

Number of directing oscillator HPBW (°)

_______________________________________________

0 117.4

1 85.7

2 72.2

3 64.6

4 56.7

5 51.7

_______________________________________________

Table 3. Influence of the number of directors on the

maximum gain

_______________________________________________

Number of measure simulation

directing oscillator dBi dBi

_______________________________________________

0 2.78 3.02

1 3.57 3.82

2 4.93 5.23

3 5.34 5.58

4 5.92 6.27

5 6.23 6.56

_______________________________________________

4 FASIBILITY ANALYSIS OF ELECTRONICALLY

CONTROLLED PLASMA YAGI ANTENNA FOR

AIS BASE STATION TRANSMITTING

ANTENNA IN SHIPS’ ROUTEING WATERS

The results of experiments combined with the

simulation show that by adjusting the plasma

electrical parameters, the plasma antenna system can

be adjusted to match the transmission line. At the

same time, by adjusting the antenna oscillator

electrical parameters, the distance between the

vibrators and the vibrator electrical length, the

directivity and gain of the antenna can be

dynamically adjusted, and also the coverage area of

the antenna can be adjusted too. When only the active

vibrator of the antenna system is turned on and all

passive vibrators are turned off, it can be used as an

omnidirectional antenna to realize AIS information

broadcasting. While in order to achieve different

operating frequencies, metal antennas must change

the size or shape of the antenna. In the VHF

communication band, only dual or triple frequency

communication can be realized. In the design of the

reconfigurable antenna, the plasma antenna is more

convenient.

The experimental results of the absorption and

reflection of electromagnetic wave by plasmons are

given in the literature

[13]

. When the plasma frequency

is large enough, the electromagnetic wave reflection

performance is enhanced. When the density of the

plasma electron is increased, the reflection oscillator

enhances the reflection of electromagnetic waves. On

the basis of increasing the number of directors, the

orientation of the antenna is enhanced and the

backscattering is reduced, thereby reducing the

backward electromagnetic interference of the antenna.

When the antenna is not required to work, the

antenna oscillator is turned off to achieve zero

interference.

In terms of energy consumption, the literature

[13]

gives the power required for the plasma antenna

excitation source to maintain a-1m-long antenna. In

the excitation mode of the kHz-level AC power

supply, the discharge power is generally less than

2W.

In addition, in the plasma antenna system, if the

plasma parameter is properly adjusted, the size of the

plasma discharge tube can be reduced, which can

miniaturize the plasma Yagi antenna and also reduce

the weight of the antenna. In this respect, there is a

certain advantage over metal antennas.

Of course, the plasma antenna has a certain gap in

terms of antenna gain and electromagnetic

compatibility. At present, both the MHz-level AC and

kHz-level AC plasma excitation sources can generate

higher-density plasmas, and the advanced filtering

technology and shielding technology can greatly

reduce electromagnetic interference. Therefore, in the

near future, the plasma antenna can be applied to the

field of maritime communication, and in particular, it

can significantly improve the communication effect

and maritime supervision capability of the ship's

routeing waters.

5 CONCLUSION

Through the simulation and experimental study of the

plasma Yagi antenna model and the feasibility

analysis of the plasma antenna used in AIS base

station transmitting antenna within ships’ routeing

waters above-mentioned, it is concluded that:

1 By changing the plasma electron density, the

number of reflected vibrators, the number of

directors and the working state, the antenna

parameters such as directivity, gain and input

impedance of the antenna can be dynamically

adjusted to achieve fast and dynamic adjustment

of the plasma antenna. Thus, the plasma Yagi

antenna can be used as a smart antenna.

2 The plasma Yagi antenna can achieve impedance

matching by adjusting the plasma state without

applying any matching network at the AIS

communication band.

3 Plasma Yagi antenna can be improved in terms of

power consumption, electromagnetic

compatibility, gain, reconfigurability, etc., and

meet the communication requirements of AIS base

station transmitting antennas. As a consequence, It

can be used as the transmitting antenna of AIS

base station in ships’ routeing waters.

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