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

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

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

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

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

*Plasma focusing experimental setup from a different angle. Gunn diode 24 GHz transmitter with fluorescent*

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

resulting in a gain of 2 (3 dB). This can be seen in **Figure 5**.

**Figure 3.**

*Selected Topics in Plasma Physics*

**Figure 4.**

**76**

*tubes used for plasma beam focus.*

receiver about 16 inches from the plasma tube. This corresponds to 60 wavelengths between the steering plasma tube and the detector and clearly qualifies as far field. In order to generate a plasma with a density high enough to interact with RF signals in microwave frequencies, we use short current pulses (1 μs) that quickly ionize the plasma; then rely on the plasma ions rather slowly migrating to the wall of the plasma tube. Using this technique (developed by Dr. Theodore Anderson and the late Professor Igor Alexeff) we generate a much higher average density plasma

Our high current pulses have so far had a period of 1 ms. With this time separation between pulses, the plasma density decays by about a factor of two

*Fluorescent plasma tube is located in front of the output waveguide on the HP signal generator. Aluminum shield on left of tube prevents stray RF from bypassing the plasma. High voltage pulser is on the right.*

*Beam steering (44 GHz) for two different plasma ionizing currents. Blue line: 5 A peak. Red line: 8 A peak. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings from the*

with a low average current and power.

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

*Plasma Antennas*

**Figure 8.**

**Figure 9.**

**79**

*crystal detector.*

**Figure 6.**

*Schematic for antenna beam steering using one COTS tube with plasma.*

#### **Figure 7.**

*Plasma beam steering experiment. Antenna rotator with holder (green) and receiver horn/detector is in the foreground. The oscilloscope used to monitor the signal waveform is on top of the HP signal generator. The solid state pulsing circuit is to the right of the signal generator.*

through the plasma and is not bypassed to the side of the tube. A 33–50 GHz microwave horn with HP crystal detector is placed on a rotating arm that is scanned in an arc around the plasma tube by an antenna rotator. The arm places the horn

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

receiver about 16 inches from the plasma tube. This corresponds to 60 wavelengths between the steering plasma tube and the detector and clearly qualifies as far field.

In order to generate a plasma with a density high enough to interact with RF signals in microwave frequencies, we use short current pulses (1 μs) that quickly ionize the plasma; then rely on the plasma ions rather slowly migrating to the wall of the plasma tube. Using this technique (developed by Dr. Theodore Anderson and the late Professor Igor Alexeff) we generate a much higher average density plasma with a low average current and power.

Our high current pulses have so far had a period of 1 ms. With this time separation between pulses, the plasma density decays by about a factor of two

#### **Figure 8.**

*Fluorescent plasma tube is located in front of the output waveguide on the HP signal generator. Aluminum shield on left of tube prevents stray RF from bypassing the plasma. High voltage pulser is on the right.*

#### **Figure 9.**

*Beam steering (44 GHz) for two different plasma ionizing currents. Blue line: 5 A peak. Red line: 8 A peak. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings from the crystal detector.*

through the plasma and is not bypassed to the side of the tube. A 33–50 GHz microwave horn with HP crystal detector is placed on a rotating arm that is scanned in an arc around the plasma tube by an antenna rotator. The arm places the horn

*state pulsing circuit is to the right of the signal generator.*

*Plasma beam steering experiment. Antenna rotator with holder (green) and receiver horn/detector is in the foreground. The oscilloscope used to monitor the signal waveform is on top of the HP signal generator. The solid*

**Figure 6.**

*Selected Topics in Plasma Physics*

**Figure 7.**

**78**

*Schematic for antenna beam steering using one COTS tube with plasma.*

#### *Selected Topics in Plasma Physics*

before the next pulse comes to refresh the plasma back to peak density. This has not caused a problem with the smart antenna because we are using the plasma to totally block the RF signal, like a shutter. But lensing and beam steering require a plasma of a specific density to get a consistent beam deflection angle.

**Figure 9** demonstrates that we can vary the angle of deflection by changing plasma ionizing current; but the most striking result shown in **Figure 9** is the very narrow beam-width of the deflected signals. This is quite surprising. **Figure 10** shows the 50 deflected beam (red line) along-side the un-deflected beam with no plasma. The incident un-deflected beam has a much wider beam-width compared

**Figure 11** shows the beam deflection (blue line) with a lower peak plasma current of 3 A with a beam deflection of 15°. For comparison the 50° beam deflec-

A plasma antenna operating in the microwave frequency range requires higher operating currents (Greater than 1A) and consequently can overheat when used continuously. Our plasma antennas are able to work CW at high frequencies (>1 GHz) because of a concept invented by Igor Alexeff and Theodore Anderson

The plasma initiates quickly in less than a microsecond, but when plasma ioniz-

ing current is turned off, the ions take about a millisecond to recombine with electrons. Therefore plasma density stays high for almost a millisecond even though the ionizing current is no longer on. We use an even short pulse width (1 μs), and

We developed a pulsed voltage doubler circuit, allowing us to use a lower voltage DC power supply for the input power to the pulsing circuit using a modified Marx Generator. A Marx Generator is a pulsed voltage multiplier. A series of capacitors was charged in parallel and then discharged in series through spark gaps. **Figure 12** shows a two stage voltage doubler circuit. We have built and tested a modified Marx Generator that replaces the spark gaps with an IGBT electronic

Keeping the second spark gap in the circuit results in a faster rise time than in our previous pulsing circuits. The voltage doubler allows the use of a lower voltage

A simple non-voltage multiplying IGBT pulsing driver circuit is shown in **Figure 13**. A CMOS timer IC is used to generate short 1 μs pulses with a repetition time of 750 μs. An IXYS brand 2500 V IGBT is the high voltage switch. This pulsing arrangement allows us to use a factor of 750 less DC current and power from the DC

We have been driving our plasma antenna tube with the fast, high-current pulses described here to allow operation at higher frequencies than is possible with CW current. Our pulsing circuit had required that the both electrodes of the plasma tube operate at high voltage; with the positive electrode at constant maximum DC voltage (2–3 kV). We have modified the circuit to allow the negative electrode to remain grounded while a positive 1 μs pulse is applied to the anode electrode. This

Improved electrical safety because one electrode remains grounded and the other electrode is supplied with short pulses. Less capacitive loss in the current leads allowing the use of faster pulses; since one electrode is grounded and can be

Eliminating the need for a negative lead wire means less high voltage wiring.

Grounding one side of the plasma tube requires that the high voltage switcher (IGBT) be able to float up and down in voltage with the tube's anode. To do this we

to the deflected beam with plasma turned on.

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

tion with 8 A peak is shown in red.

*Plasma Antennas*

switch.

power supply.

**81**

change offers several advantages:

attached to a ground plane.

**4. Pulsed plasma antenna circuitry**

that uses fast high-current pulses; instead of DC current.

therefore less power is required to run the antenna.

DC power supply than would otherwise be required.

Lower EMI caused by high current pulses in wiring.

We are avoiding this problem by recording the received microwave signal immediately after the current pulse; therefore measurements are taken at a constant plasma density, which is also the maximum density before significant decay of density occurs.

Our recent experimental results with 44 GHz show dramatic improvement in beam deflection characteristics compared to previous testing at 24 GHz. The shorter wavelength compared to tube dimensions has clearly resulted in cleaner and more consistent beam steering.

**Figure 9** shows beam deflection at two peak ionization currents. The blue curve shows about 26 deflection with a current of 5 A peak. The red curve shows about 50 deflection with a current of 8 A peak.

#### **Figure 10.**

*Plasma beam steering. Beam is steered 45° clockwise. Blue line: No plasma. Red line: 8 A peak ionizing current. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings from the crystal detector.*

#### **Figure 11.**

*Beam steering for two different plasma ionizing currents. Blue line: 3 A peak. Red line: 8 A peak. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings from the crystal detector.*

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

before the next pulse comes to refresh the plasma back to peak density. This has not caused a problem with the smart antenna because we are using the plasma to totally block the RF signal, like a shutter. But lensing and beam steering require a plasma of

We are avoiding this problem by recording the received microwave signal immediately after the current pulse; therefore measurements are taken at a constant plasma density, which is also the maximum density before significant decay of

Our recent experimental results with 44 GHz show dramatic improvement in beam deflection characteristics compared to previous testing at 24 GHz. The shorter wavelength compared to tube dimensions has clearly resulted in cleaner and more

**Figure 9** shows beam deflection at two peak ionization currents. The blue curve shows about 26 deflection with a current of 5 A peak. The red curve shows about

*Plasma beam steering. Beam is steered 45° clockwise. Blue line: No plasma. Red line: 8 A peak ionizing current. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings*

*Beam steering for two different plasma ionizing currents. Blue line: 3 A peak. Red line: 8 A peak. A crystal waveguide detector is used as a receiver. Amplitude numbers are relative voltage readings from the crystal*

a specific density to get a consistent beam deflection angle.

density occurs.

**Figure 10.**

**Figure 11.**

*detector.*

**80**

*from the crystal detector.*

consistent beam steering.

*Selected Topics in Plasma Physics*

50 deflection with a current of 8 A peak.

**Figure 9** demonstrates that we can vary the angle of deflection by changing plasma ionizing current; but the most striking result shown in **Figure 9** is the very narrow beam-width of the deflected signals. This is quite surprising. **Figure 10** shows the 50 deflected beam (red line) along-side the un-deflected beam with no plasma. The incident un-deflected beam has a much wider beam-width compared to the deflected beam with plasma turned on.

**Figure 11** shows the beam deflection (blue line) with a lower peak plasma current of 3 A with a beam deflection of 15°. For comparison the 50° beam deflection with 8 A peak is shown in red.
