Analyze the Operating Mode of the Spiral Antenna

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Many antennas deployed in basic communication systems are linearly polarized, which means that polarization is limited to a single plane in terms of electric field direction. The antenna that can generate circularly polarized electromagnetic waves can give us more choices, because the polarization of the wave will change in the process of propagation, for example, the spiral antenna can generate circularly polarized waves in the axial operating mode. RF simulation can be used to optimize spiral antenna design.

Sprial:more and more spiral antenna applications

The spiral antenna is named after its spiral geometry. It consists of one or more wires wound into a spiral. Because of its special shape, the spiral antenna can transmit circularly polarized fields. The spiral antenna is simple in design, but powerful in function and rich in applications. For example, smart implants and other RFID devices are used for extremely small antennas.

Large spiral antennas are commonly used in radio, GPS and ballistic missile systems, as well as in outer space communications with satellites orbiting the earth and the moon and space probes.

Helical antenna

Helical antenna in satellite communication system. The copyright of the pictures belongs to the public domain of the United States and is shared through Wikimedia Commons.

Helical antenna: two working modes

1.Normal direction

2.Axial

Similar to monopole antennas, normal mode helical antennas are also linearly polarized, but because the latter are helical, they are relatively shorter and more compact. If the circumference of the helix is obviously smaller than the wavelength and the pitch is obviously less than a quarter of the wavelength, we classify it as a normal mode spiral antenna. In normal or vertical radiation mode, the far-field radiation pattern of the antenna is similar to the circular pattern of the classical dipole antenna.

In the axial or end radiation mode, the antenna will radiate circularly polarized waves. One of the advantages of circularly polarized waves is that they are not susceptible to multipath fading, and their polarization dependence is smaller than that of linearly polarized waves. If the spiral perimeter is close to the working wavelength, the antenna belongs to axial mode spiral antenna. Compared with the normal mode, in the axial mode, the working frequency band of the spiral antenna is much higher. Similar to the end fired array along the spiral axis, it will produce a highly directional radiation pattern.

Impedance matching ability of folded dipole antenna

The design advantage of spiral antenna with double arms is that it has stronger matching impedance. When a single spiral antenna resonates in normal mode, its impedance is much lower. If a second short circuited antenna is added, it is equivalent to adding a foldable antenna whose input resistance is three times larger than that of a dipole antenna of the same size. In this way, impedance matching can be achieved with low impedance components close to the impedance of the coaxial cable.

double arm spiral antenna

The model of double arm spiral antenna and its axial mode radiation pattern. The model displays only half of the radiation pattern

At COMSOL Multiphysics ® Simulation of double arm spiral antenna in software

This double arm spiral antenna example uses COMSOL Multiphysics ® The software add-on "RF module" is modeled.

As shown below, the geometric model includes a double arm spiral radiator, a circular bottom plate (blue), a tuning stub, a coaxial cable, and a perfectly matched layer (PML) surrounding the air domain. In addition, the two spiral structures circle along the Z-axis and meet at the top.

 geometric model

In this example, all metal parts are modeled as ideal electrical conductors (PEC), and the space between the inner and outer conductors of the coaxial cable is filled with polytetrafluoroethylene (PTFE). We use coaxial lumped ports to excite the antenna. In addition, all domains (except PML) are meshed with tetrahedral grids. Each wavelength corresponds to about 5 cells. The physical field is used to control the grid to automatically sweep PML in the absorption direction.

In addition to adding a second antenna to adjust the impedance, you can also add a stub in the center of the ground plate to adjust the impedance of the axis mode. Please note that the ground plate, PML spherical shell and the maximum grid size will be automatically adjusted according to the wavelength of each operating mode.

Research simulation results

We calculated the S parameter and far-field mode under two operating modes: the logarithmic electric field intensity under normal mode is 0.385 GHz, and the logarithmic electric field intensity under axial mode is 4.77 GHz. Please refer to the following result plot. You can observe the difference of field strength around the antenna in normal mode (above) and axial mode (below). 

antenna in normal mode

axial mode

The logarithmic electric field intensity around the antenna is 0.385 GHz (normal mode, left) and 4.77 GHz (axial mode, right).

Next, let's take a look at the polar diagram of the two-dimensional radiation pattern of the yz plane. The figure shows two operating modes. As expected, you can see the classical E-plane pattern of dipole antenna in normal mode (blue) and the directional radiation pattern in axial mode (green).

dipole antenna

In the polar diagram of the far field mode in the yz plane, the normal mode is displayed in blue, and the axial mode is displayed in green.

In addition, you can use 3D far-field mapping to visualize each mode. The S parameter of both modes is less than - 10 dB. By observing the three-dimensional far-field pattern, it can be found that these results once again prove that the dipole antenna shape in normal mode and the end fired array shape in axial mode are reasonable.

dipole antenna

 spiral antenna

The three-dimensional far-field pattern of spiral antenna in normal mode (left) is similar to that of dipole antenna. The three-dimensional far-field pattern in axial mode (right) is similar to the end fired array antenna supported by the grounding plane along the z-axis.

The axial ratio diagram below shows the degree of circular polarization of the antenna. When the antenna is characterized as ideal circular polarization, the axial ratio is 1 or 0dB. When it is less than 3 dB (within the red circle), we usually think that the antenna is circularly polarized. In the following figure, the axis ratio at the antenna line of sight is less than 3 dB. The LOS of the antenna is the main propagation direction of the axial mode, which is parallel to the spiral torsional axis.

axial ratio

Axis ratio in dB (blue), with the red line representing 3 dB.

By modeling the double arm spiral antenna, you can effectively analyze the normal and axial operating modes, which will help to improve the antenna design in Earth and outer space applications.

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