**1. Introduction**

Plasma antennas use partially or fully ionized gas as the conducting medium instead of metal to create an antenna. The advantages of plasma antennas are that they are highly reconfigurable and can be turned on and off. Hence research to reduce the power required to ionize the gas at various plasma densities is important and this has been achieved by various techniques including pulsing techniques. The power requirements for plasma antenna operation continue to decrease.

The same geometric resonances apply to plasma antennas as metal antennas. Plasma antennas of the same shape, length, and frequency of corresponding metal antennas will have the same radiation patterns. Plasma antennas have the advantage of reconfigurability.

High frequency antennas can transmit and receive through lower frequency plasma antennas eliminating or reducing co-site interference. Because of this principle, higher frequency plasma antennas can be nested inside lower frequency plasma antennas and the higher frequency plasma antennas can transmit and receive through the lower frequency plasma antennas. Higher frequency plasma antenna arrays can transmit and receive through lower frequency plasma antenna arrays. Co-site interference occurs when larger frequency antennas block or partially block the radiation patterns of smaller higher frequency antennas. With plasma antennas, co-site interference can be reduced or eliminated. The interference among plasma antennas can be reduced or eliminated by turning all the plasma antennas off (extinguishing the plasma) except the plasma antennas that are

transmitting and/or receiving. This is not possible with metal antennas. A general rule is that when an incident electromagnetic wave upon a plasma antenna is such that the frequency of the incident electromagnetic wave is greater than the plasma frequency of the plasma, the incident electromagnetic wave passes through the plasma without attenuation. If the incident electromagnetic wave has a frequency much less than the plasma frequency, the plasma behaves similar to a metal. The frequency at which plasma behaves like a metal or a dielectric is reconfigurable. The plasma frequency is a natural frequency of the plasma and it is a measure of the amount of ionization in the plasma. It is defined and used throughout this book. Both plasma antennas and metal antennas increase in size as the frequencies they operate goes down to maintain geometric resonance and high efficiency. However as the frequency of operation of the plasma antenna decreases, the density of the plasma needed to operate the plasma antenna also goes down. A rule of thumb is that the plasma frequency should be about twice the operating frequency of the plasma antenna. Hence the plasma frequency goes down as the frequency of the plasma antenna goes down. As the plasma frequency decreases, the plasma antenna becomes transparent to a greater bandwidth of electromagnetic waves. In short as the plasma antenna increases in size, the RCS of the plasma antenna goes down whereas for the corresponding metal antenna, the RCS goes up as the metal antenna increases in size. This gives the plasma antenna some great advantages at low frequencies over the corresponding metal antenna. In addition plasma antennas do not receive electromagnetic noise greater than the plasma frequency since these frequencies pass through the plasma antenna.

Satellite plasma antennas benefit from the lower thermal noise at the frequencies

they operate. Ground based satellite antennas point at space where the thermal noise is about 5° K. A low thermal noise, high data rate satellite plasma antenna system is possible with low noise plasma feeds and a low noise receiver. Satellite plasma antennas can operate in the reflective or refractive mode. Satellite plasma antennas need not be parabolic but can be flat or conformal and effectively parabolic. The effective plasma parabolic dish antenna is part of the scope of future work. Electromagnetic waves reflecting off of a bank of plasma tubes get phase shifted as a function of the plasma density in the tube. This becomes an effective phase array except that the phase shifts are determined by the plasma density. If the plasma density in the tubes is computer controlled, the reflected beam can be steered or focused even when the bank of tubes is flat or conformal. In the refractive mode, the refraction of electromagnetic waves depends upon the density of the plasma. In the refractive mode, steering and focusing can be computer controlled even when the bank of tubes is flat. This eliminates the problem of the blind spot and feed losses caused by the feed horn and receiver in front of a metal satellite

Pulsing techniques instead of applying continuous energy were developed to increase the plasma density and decrease the amount of energy to maintain the

In the history of antennas, it has been difficult to develop low frequency directional and electronically steerable antennas that fit on land vehicles and aircraft. Low frequency means the wavelength is on the order or larger than the vehicle. With plasma antennas this is possible with multipole expansions of clusters of plasma antennas that are all within a wavelength of each other. This depends on the ability of turning plasma antennas on or off (extinguishing the plasma) to create reconfigurable multipoles of plasma antennas that can be rotated in time creating directional and steerable antenna beams. This is not possible with metal antennas

Several groups have done work in using numerical techniques to plot plasma

antenna radiation patterns. Zhou et al. [1] used FDTD Method techniques. Bogachev et al. [2] predicted radiation patterns for plasma asymmetrical dipole antenna. Zhivko Kiss'ovski [3] calculated the radiation pattern of miniaturized plasma antennas. Golazari et al. [4] did measurements and simulations of a loop plasma antenna in UHF band Barro et al. [5] did simulations to get the radiation patterns of cylindrical plasma antennas. Kumar et al. [6] have done simulations of a plasma antenna array. Melazzi et al. [7] have developed a plasma antenna numerical code called ADAMANT. An overview of experimental and numerical research is Anderson et al. [8]. Mansutti et al. [9] have done numerical work on metal-plasma

The phase speed of electromagnetic waves in a plasma is given by:

antenna.

*Plasma Antennas*

*DOI: http://dx.doi.org/10.5772/intechopen.91944*

plasma.

L band antenna.

**73**

because they cannot be turned on and off.

Where the plasma frequency is given by:

Related to plasma antennas, plasma frequency selective surfaces, plasma waveguides, and plasma co-axial cables have been developed. Unlike metal frequency selective surfaces, plasma frequency selective surfaces have the properties of reconfigurable filtering of electromagnetic waves. This could have tremendous advantages to radome design. Plasma frequency selective surfaces can be reconfigured by varying the plasma density, varying the shape of the elements, or tuning any number of the plasma FSS elements on or off. Plasma wave guides and plasma co-axial cables can be stealth like plasma antennas, and they can operate at low frequencies, and be invisible at high frequencies. Plasma waveguides and coaxial cables can be feeds for plasma antennas. Plasma feeds as well as the plasma antennas have reconfigurable impedances. If the impedance of the plasma antenna is changed, the impedance of the plasma antenna feeds can be changed to maintain impedance matching.

Thermal noise in a plasma antenna is less than the thermal noise in a metal antenna at the higher frequencies. Higher frequencies mean that there is a point in the RF spectrum in which the thermal noise of plasma antennas is equal to the thermal noise of metal antennas. At higher frequencies than this point, the plasma antenna thermal noise decreases drastically compared to a metal antenna. Below this point the thermal noise of the plasma antenna is greater than a metal antenna. For a fluorescent tube which has been built as a plasma antenna, the point where the thermal noise of the plasma antenna is equal to the metal antenna is about 1 GHz. This point can be decreased in frequency by decreasing the plasma density and/or gas pressure. The plasma in the plasma antennas are inert gases that operate at energies and frequencies in which Ramsauer Townsend Effects apply.

Ramsauer Townsend Effects mean that the electrons in the plasma diffract around the ions and neutral atoms in the plasma. This means that the collision rate of the unbound electrons in the plasma with ions and neutral atoms is small and much smaller than in a metal. This phenomenon contributes to the lower thermal noise plasma antennas have over corresponding metal antennas.

#### *Plasma Antennas DOI: http://dx.doi.org/10.5772/intechopen.91944*

transmitting and/or receiving. This is not possible with metal antennas. A general rule is that when an incident electromagnetic wave upon a plasma antenna is such that the frequency of the incident electromagnetic wave is greater than the plasma frequency of the plasma, the incident electromagnetic wave passes through the plasma without attenuation. If the incident electromagnetic wave has a frequency much less than the plasma frequency, the plasma behaves similar to a metal. The frequency at which plasma behaves like a metal or a dielectric is reconfigurable. The plasma frequency is a natural frequency of the plasma and it is a measure of the amount of ionization in the plasma. It is defined and used throughout this book. Both plasma antennas and metal antennas increase in size as the frequencies they operate goes down to maintain geometric resonance and high efficiency. However as the frequency of operation of the plasma antenna decreases, the density of the plasma needed to operate the plasma antenna also goes down. A rule of thumb is that the plasma frequency should be about twice the operating frequency of the plasma antenna. Hence the plasma frequency goes down as the frequency of the plasma antenna goes down. As the plasma frequency decreases, the plasma antenna becomes transparent to a greater bandwidth of electromagnetic waves. In short as the plasma antenna increases in size, the RCS of the plasma antenna goes down whereas for the corresponding metal antenna, the RCS goes up as the metal antenna increases in size. This gives the plasma antenna some great advantages at low frequencies over the corresponding metal antenna. In addition plasma antennas do not receive electromagnetic noise greater than the plasma frequency since these

Related to plasma antennas, plasma frequency selective surfaces, plasma waveguides, and plasma co-axial cables have been developed. Unlike metal frequency selective surfaces, plasma frequency selective surfaces have the properties of reconfigurable filtering of electromagnetic waves. This could have tremendous advantages to radome design. Plasma frequency selective surfaces can be

reconfigured by varying the plasma density, varying the shape of the elements, or tuning any number of the plasma FSS elements on or off. Plasma wave guides and plasma co-axial cables can be stealth like plasma antennas, and they can operate at low frequencies, and be invisible at high frequencies. Plasma waveguides and coaxial cables can be feeds for plasma antennas. Plasma feeds as well as the plasma antennas have reconfigurable impedances. If the impedance of the plasma antenna is changed, the impedance of the plasma antenna feeds can be changed to maintain

Thermal noise in a plasma antenna is less than the thermal noise in a metal antenna at the higher frequencies. Higher frequencies mean that there is a point in the RF spectrum in which the thermal noise of plasma antennas is equal to the thermal noise of metal antennas. At higher frequencies than this point, the plasma antenna thermal noise decreases drastically compared to a metal antenna. Below this point the thermal noise of the plasma antenna is greater than a metal antenna. For a fluorescent tube which has been built as a plasma antenna, the point where the thermal noise of the plasma antenna is equal to the metal antenna is about 1 GHz. This point can be decreased in frequency by decreasing the plasma density and/or gas pressure. The plasma in the plasma antennas are inert gases that operate at

Ramsauer Townsend Effects mean that the electrons in the plasma diffract around the ions and neutral atoms in the plasma. This means that the collision rate of the unbound electrons in the plasma with ions and neutral atoms is small and much smaller than in a metal. This phenomenon contributes to the lower thermal

energies and frequencies in which Ramsauer Townsend Effects apply.

noise plasma antennas have over corresponding metal antennas.

frequencies pass through the plasma antenna.

*Selected Topics in Plasma Physics*

impedance matching.

**72**

Satellite plasma antennas benefit from the lower thermal noise at the frequencies they operate. Ground based satellite antennas point at space where the thermal noise is about 5° K. A low thermal noise, high data rate satellite plasma antenna system is possible with low noise plasma feeds and a low noise receiver. Satellite plasma antennas can operate in the reflective or refractive mode. Satellite plasma antennas need not be parabolic but can be flat or conformal and effectively parabolic. The effective plasma parabolic dish antenna is part of the scope of future work. Electromagnetic waves reflecting off of a bank of plasma tubes get phase shifted as a function of the plasma density in the tube. This becomes an effective phase array except that the phase shifts are determined by the plasma density. If the plasma density in the tubes is computer controlled, the reflected beam can be steered or focused even when the bank of tubes is flat or conformal. In the refractive mode, the refraction of electromagnetic waves depends upon the density of the plasma. In the refractive mode, steering and focusing can be computer controlled even when the bank of tubes is flat. This eliminates the problem of the blind spot and feed losses caused by the feed horn and receiver in front of a metal satellite antenna.

Pulsing techniques instead of applying continuous energy were developed to increase the plasma density and decrease the amount of energy to maintain the plasma.

In the history of antennas, it has been difficult to develop low frequency directional and electronically steerable antennas that fit on land vehicles and aircraft. Low frequency means the wavelength is on the order or larger than the vehicle. With plasma antennas this is possible with multipole expansions of clusters of plasma antennas that are all within a wavelength of each other. This depends on the ability of turning plasma antennas on or off (extinguishing the plasma) to create reconfigurable multipoles of plasma antennas that can be rotated in time creating directional and steerable antenna beams. This is not possible with metal antennas because they cannot be turned on and off.

Several groups have done work in using numerical techniques to plot plasma antenna radiation patterns. Zhou et al. [1] used FDTD Method techniques. Bogachev et al. [2] predicted radiation patterns for plasma asymmetrical dipole antenna. Zhivko Kiss'ovski [3] calculated the radiation pattern of miniaturized plasma antennas. Golazari et al. [4] did measurements and simulations of a loop plasma antenna in UHF band Barro et al. [5] did simulations to get the radiation patterns of cylindrical plasma antennas. Kumar et al. [6] have done simulations of a plasma antenna array. Melazzi et al. [7] have developed a plasma antenna numerical code called ADAMANT. An overview of experimental and numerical research is Anderson et al. [8]. Mansutti et al. [9] have done numerical work on metal-plasma L band antenna.

The phase speed of electromagnetic waves in a plasma is given by:

$$v\_{\mathfrak{p}} = \frac{c}{\sqrt{1 - \omega\_{\mathfrak{p}}^2/\omega^2}},$$

Where the plasma frequency is given by:

$$
\omega\_{\mathfrak{p}} = \sqrt{\frac{n\_{\mathfrak{e}} \, e^2}{\epsilon\_0 \, m\_{\mathfrak{e}}}}
$$

In this paper, we are experimenting in the region where the antenna frequency is greater than the plasma frequency:

a convex glass lens focuses a signal to a point while a convex plasma diverges the signal similar to a concave glass lens that diverges the signal while a concave plasma

We have built converging and diverging plasma lenses using plasma tubes as shown in **Figures 1** and **2**. A single plasma tube with the beam passing through its diameter acts as a diverging plasma lens (**Figures 2** and **3**) and two plasma tubes

We have built the "concave" set-up for beam shown in **Figure 1**, and **Figure 4** is

a 24 GHz, 5 mW Gunn diode is used as the microwave source with the signal

*Schematic for experimental setup of antenna beam focusing with tubes with plasma using two COTS*

*Schematic for experimental setup for antenna beam spreading with tubes with plasma in one COTS*

side-by-side form a converging (focusing) lens (**Figures 1** and **4**).

lens focuses to a point.

*DOI: http://dx.doi.org/10.5772/intechopen.91944*

*Plasma Antennas*

**Figure 2.**

**75**

**Figure 1.**

*(commercial off the shelf) tubes.*

*(commercial off the shelf) tubes.*

## *ω*>*ω<sup>p</sup>*

In this region refraction and not reflection takes place.

The phase speed of electromagnetic waves in a plasma is greater than in free space. The greater the density of the plasma the greater the phase speed. Since the plasma density can be reconfigured, the steering and focusing of antenna beams by the physics of refraction through a plasma is reconfigurable [10, 11]. The amount of refraction through a plasma depends on the path length through a plasma and the change in plasma density over that path length [12]. This physical process can also be considered as a plasma lens [13].

Refraction in a plasma depends on:

1.Plasma density

2.Path length

3.Gradient of plasma density

We have very good results at 24 GHz and above using COTS plasma tubes. We made custom plasma tubes with larger diameter and refraction that worked well at 10 GHz.

At 24 GHz, two plasma tubes were used to get antenna beam focusing and one plasma tube was used to get antenna beam spreading. Antenna beam steering was achieved with one and two plasma tubes at 24 GHz. In another case our beam steering experiments from using the physics of refraction through a plasma were done at 44 GHz.

## **2. Focusing antenna beams with the physics of refraction through plasma**

In the following sections we show our work on antenna beam focusing, beam spreading, and beam steering using refraction of RF waves in a plasma. This is our first iteration of the plasma lens work and it can only improve. We found it was easier to show the lensing effects of plasma at 24 GHz since the size and shape of COTS plasma tubes are amenable to a 24 GHz. These effects all scale according to wavelength, but cylindrical annular rings of plasma are the best way to control the plasma density variations of plasma to optimize the engineering effects of plasma refraction to control beam focusing, beam spreading, and beam steering.

We have demonstrated the ability to use a plasma for manipulation of a microwave signals by focusing a wide beam into a more narrow beam and also by steering the beam.

**Figure 1** shows the experimental set-up for beam steering and lensing. A narrow-beam 24 GHz signal is directed into the side of two 1.5 inch diameter plasma tubes which focuses the antenna beam into higher directivity, gain, and range.

This change in velocity of the signal inside the plasma results in a lensing effect if the beam passes through varying lengths of plasma similar to light passing through glass of varying thickness to make a lens. But there is an important and interesting difference between an ordinary lens made of glass or plastic and a plasma lens: The glass lens slows-down the signal while a plasma lens speeds-up the signal. Therefore a convex glass lens focuses a signal to a point while a convex plasma diverges the signal similar to a concave glass lens that diverges the signal while a concave plasma lens focuses to a point.

We have built converging and diverging plasma lenses using plasma tubes as shown in **Figures 1** and **2**. A single plasma tube with the beam passing through its diameter acts as a diverging plasma lens (**Figures 2** and **3**) and two plasma tubes side-by-side form a converging (focusing) lens (**Figures 1** and **4**).

We have built the "concave" set-up for beam shown in **Figure 1**, and **Figure 4** is a 24 GHz, 5 mW Gunn diode is used as the microwave source with the signal

#### **Figure 1.**

In this paper, we are experimenting in the region where the antenna frequency is

*ω*>*ω<sup>p</sup>*

The phase speed of electromagnetic waves in a plasma is greater than in free space. The greater the density of the plasma the greater the phase speed. Since the plasma density can be reconfigured, the steering and focusing of antenna beams by the physics of refraction through a plasma is reconfigurable [10, 11]. The amount of refraction through a plasma depends on the path length through a plasma and the change in plasma density over that path length [12]. This physical process can also

We have very good results at 24 GHz and above using COTS plasma tubes. We made custom plasma tubes with larger diameter and refraction that worked well at

At 24 GHz, two plasma tubes were used to get antenna beam focusing and one plasma tube was used to get antenna beam spreading. Antenna beam steering was achieved with one and two plasma tubes at 24 GHz. In another case our beam steering experiments from using the physics of refraction through a plasma were

**2. Focusing antenna beams with the physics of refraction through**

refraction to control beam focusing, beam spreading, and beam steering.

In the following sections we show our work on antenna beam focusing, beam spreading, and beam steering using refraction of RF waves in a plasma. This is our first iteration of the plasma lens work and it can only improve. We found it was easier to show the lensing effects of plasma at 24 GHz since the size and shape of COTS plasma tubes are amenable to a 24 GHz. These effects all scale according to wavelength, but cylindrical annular rings of plasma are the best way to control the plasma density variations of plasma to optimize the engineering effects of plasma

We have demonstrated the ability to use a plasma for manipulation of a microwave signals by focusing a wide beam into a more narrow beam and also by steering

This change in velocity of the signal inside the plasma results in a lensing effect if the beam passes through varying lengths of plasma similar to light passing through glass of varying thickness to make a lens. But there is an important and interesting difference between an ordinary lens made of glass or plastic and a plasma lens: The glass lens slows-down the signal while a plasma lens speeds-up the signal. Therefore

**Figure 1** shows the experimental set-up for beam steering and lensing. A narrow-beam 24 GHz signal is directed into the side of two 1.5 inch diameter plasma tubes which focuses the antenna beam into higher directivity, gain, and range.

In this region refraction and not reflection takes place.

greater than the plasma frequency:

*Selected Topics in Plasma Physics*

be considered as a plasma lens [13]. Refraction in a plasma depends on:

3.Gradient of plasma density

1.Plasma density

2.Path length

10 GHz.

done at 44 GHz.

**plasma**

the beam.

**74**

*Schematic for experimental setup of antenna beam focusing with tubes with plasma using two COTS (commercial off the shelf) tubes.*

#### **Figure 2.**

*Schematic for experimental setup for antenna beam spreading with tubes with plasma in one COTS (commercial off the shelf) tubes.*

#### **Figure 3.**

*Schematic showing antenna beam spreading with one 3-inch diameter custom made plasma tube at 10 GHz.*

We are the only group utilizing a pulsed high voltage power supply to give a much higher average plasma density and much lower average power to ionize the gas into plasma. This ability to tune the focusing of a RF beam is very useful because the same lensing structure can be used with different frequencies and to vary focal

*24 GHz beam focus with plasma. Red line is with no plasma. Blue line is with plasma. Note that the plasma focus increases beam amplitude by a factor of two compared to no plasma (3 dB gain). A crystal waveguide detector was used as a receiver. Amplitude numbers are relative voltage readings from the crystal detector.*

**3. Steering antenna beams with the physics of refraction through**

this and has shown significantly narrower beam steering and less signal loss.

dimensions of the lens, or in our case the plasma tube.

We have done microwave beam steering using a cylindrical plasma tube. Those tests were done using fluorescents lamps with a diameter of 1.5 in (3.8 cm); and with a 24 GHz microwave beam. A shortcoming of the set-up was that the 24 GHz signal has a wavelength of 1.25 cm, which is 1/3 the diameter of the tube. For a properly working lens, the wavelength should be small compared to the physical

This work involved using a much higher frequency (44 GHz, 0.68 cm) and therefore shorter wavelength. This wavelength is a factor of 5.6 smaller than the 3.8 cm tube diameter which should result in a more ideal lensing action. Our testing has confirmed

Of course all the dimensions scales with wavelengths used so going down to lower frequencies requires larger diameter tubes or other shaped plasma containers. This is why it is important to make custom made plasma tubes or other geometries. **Figure 6** is a schematic for the experimental setup, and **Figures 7** and **8** are photos of the setup. A 33–50 GHz HP Microwave Signal Generator is used to generate the incident microwave beam. The plasma tube (fluorescent lamp) is placed 1 inch in front of the open-ended waveguide, and an aluminum shield is placed against the side the tube to ensure that most of the microwave signal goes

length as needed.

**Figure 5.**

*Plasma Antennas*

*DOI: http://dx.doi.org/10.5772/intechopen.91944*

**plasma**

**77**

#### **Figure 4.**

*Plasma focusing experimental setup from a different angle. Gunn diode 24 GHz transmitter with fluorescent tubes used for plasma beam focus.*

radiating from the open waveguide, which gives a directional but unfocused microwave output. This setup allows us to focus the beam in the forward direction resulting in a gain of 2 (3 dB). This can be seen in **Figure 5**.

#### **Figure 5.**

*24 GHz beam focus with plasma. Red line is with no plasma. Blue line is with plasma. Note that the plasma focus increases beam amplitude by a factor of two compared to no plasma (3 dB gain). A crystal waveguide detector was used as a receiver. Amplitude numbers are relative voltage readings from the crystal detector.*

We are the only group utilizing a pulsed high voltage power supply to give a much higher average plasma density and much lower average power to ionize the gas into plasma. This ability to tune the focusing of a RF beam is very useful because the same lensing structure can be used with different frequencies and to vary focal length as needed.
