Achieving a stealth state is an important capability of a plasma antenna that sets it apart from traditional antennas. When the RF source is off, a plasma antenna will revert to a dielectric tube with a small RCS, and EM scattering from the tube can be neglected. But when the RF source is active, a plasma antenna is much like a metal antenna—transmitting and receiving signals—and it will have an RCS that is visible to detection. Figure 1 shows an infinite unmagnetized cold plasma column with a dielectric tube for a plane incident wave in the direction of the positive x axis.

1. A plasma antenna can be modeled by surrounding the plasma with a dielectric tube and using FDTD analysis.

To demonstrate the use of the iterative formulas for FDTD based on the Z-transform, the RCS of a plasma column can be computed without the dielectric tube, when the frequency of the incident EM wave is much less than the operating frequency of the plasma. In such a case, the incident EM waves will interact with the plasma column, with the column behaving like a perfect conductor. Using a simplified two-dimensional (2D) model, the plasma column is simulated with an incident wave having a wavelength, λ, of 0.03 m, the side of the square column being equal to 2λ, the plasma density, ne, equal to 7.5 x 1018/m3 and the electron-neutral collision frequency, vc, equal to 5 x 109 Hz. Figure 2 shows that the RCS of a plasma column with high plasma density complies with the results for a perfecting conducting column.

2. These curves represent RCSs for a metal column and a plasma column without a dielectric tube.

Since plasma is a kind of ionized gas, a plasma antenna is typically molded with the help of a dielectric tube. Of course, the permittivity of a dielectric tube will have an impact and reduce the RCS of a plasma antenna. For a model of a plasma antenna with a dielectric tube, the key parameters are: the frequency of the incident wave, fin = 1 x 1010 Hz, the plasma density, ne = 1.3 x 1017/m3, the electron-neutral collision frequency, vc  = 5 x 109 Hz, the side of the square plasma column, a =1.5 cm, and the thickness of the dielectric tube, d = 0.15 cm. Since the dielectric tube is part of the plasma antenna, impedance matching between the air and plasma is further deteriorated.

3. These curves show RCSs for a plasma column without surrounding dielectric and for plasma columns with a number of different relative dielectric constants.

As Fig. 3 shows, when the dielectric tube is absent, the backscattering cross section of the plasma column decreases; the backscattering cross section for the plasma column with the dielectric tube present increases with decreasing permittivity. For a case where fin = 1 x 1010 Hz, ne = 1.3 x 1017/m3, a = 1.5 cm, d = 0.15 cm, and the relative dielectric constant of the dielectric tube is ε = 2.0, Fig. 4 shows that the collision frequency has only a minor impact on the backscattering cross section for a plasma column with dielectric tube.

4. The RCSs of a plasma column with a dielectric tube are shown for various plasma collision frequencies.

As Fig. 2 indicates, a plasma column acts as a perfect conductor if the frequency of an impacting EM wave (such as from an outside radar system) is much greater than the operating frequency of the plasma antenna, resulting in a large RCS from the antenna. If the plasma density is reduced by controlling the excitation source, the RCS of the plasma antenna will decrease significantly (as in Figs. 3 and 4). This is because the plasma antenna may be functional in a low-frequency range when the plasma density is smaller, and the plasma will act as a lossy medium, absorbing and scattering an EM wave from a radar transmitter, and the RCS of the plasma antenna will be reduced. Even with low density, a plasma antenna can normally transmit and receive signals. The characteristic plasma density is that required at a certain value to maintain a plasma column working as an antenna, To reduce the RCS, it is necessary to model the distribution function of the plasma density with reasonable accuracy. That density can be computed with the aid of Eq. 1612:

where:

ne = the electron density;

nr = 5 x 15 m-3 is the characteristic plasma density for the plasma antenna; and

nm = 1.3 x1018 m-3 to maximize the electron density of the plasma antenna.

5. The RCSs of a plasma column with inhomogeneous plasma in a dielectric tube are shown for different electron densities.

Assuming fin = 1 x 1010 Hz, vc = 5 x 109 Hz, ε = 2, a = 1.5 cm, and d = 0.15 cm, Fig. 5 shows the RCS of a plasma column with inhomogeneous plasma having different electron densities (n = 0.5, 1, 1.5, and 2). Fig. 5 indicates that as the value of n decreases, it has greater impact on reducing the antenna’s RCS. For a case where nc = 5 x 1015, nm = 1.12 x 1017 (a plasma frequency, fpe, of 3 GHz), vc = 5 x 109 Hz, ε = 2, a = 1.5 cm, and d = 0.15 cm. Figure 6 shows the RCS for a plasma column for different EM wave frequencies (fin = 3.0, 3.5, 4.0, and 5.0 GHz).

6. The RCSs of a plasma column with inhomogeneous plasma in a dielectric tube are shown for different incident EM frequencies.

As Fig. 6 shows, different EM wave frequencies have different capabilities of absorption for a distribution of inhomogeneous plasma density. When the frequency of the incident EM waves approaches the upper limit of the plasma frequency, attenuation of the incident EM waves increases due to the absorption of EM waves by the plasma resonance.

### Acknowledgments

This work was supported by the Natural Science Foundation Project of Chongqing (CSTC 2010BB2202 and cstcjjA40013) and the Foundation of Chongqing Educational Committee (KJ120532), People’s Republic of China.

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