Ted Anderson

Experimental and theoretical results with plasma antennas

This report is a summary of an extensive research program on plasma antennas. We have found that plasma antennas are just as effective as metal antennas. In addition, they can transmit, receive, and reflect lower frequency signals while being transparent to higher frequency signals. When de-energized, they electrically disappear. Plasma noise does not appear to be a problem.

Recent results for plasma antennas

Plasma antennas are just as effective as metal antennas. They can transmit, receive, and reflect radio waves just as well as metal antennas. In addition, plasma generated noise does not appear to be a problem.

Advances in Plasma Antenna Design

Summary form only given. We have made significant progress in developing plasma antennas. Our antennas have been operating in the region 1 to 10 GHz. The basic advantages of plasma antennas over metal antennas are threefold. First, the plasma antennas are reconfigurable. When one plasma antenna is de-energized, the antenna reverts to a dielectric tube, and a second antenna can transmit through it. This allows us to use several large antennas stacked over each other instead of several small antennas placed next to each other. This results in better sensitivity and directivity. Second, the plasma antenna is stealthy. When de-energized, the plasma antenna does not reflect incident, probing RADAR signals. Third, the plasma antenna is resistant to electronic warfare. An operating plasma antenna can be at the same time transparent and immune to incident high frequency, high power electronic warfare pulses. The question is, how well do plasma antennas operate? Our tests at the Malibu Research Corporation in California have shown that an energized plasma reflector is essentially as effective as a metal reflector. However, when de-energized, the reflected signal drops by over 20 dB. Three remaining questions are, first, how to increase the operating plasma density without overloading the plasma discharge tubes? Second, how can we reduce the power required? And third, how can we reduce plasma noise caused by the ionizing power supply.

An Operating Intelligent Plasma Antenna

Summary form only given. We have developed and demonstrated a prototype for an intelligent plasma antenna. The object is to have an antenna observe a designated transmitter, while disregarding unwanted signals coming in from other azimuth angles. In this way, both unwanted background noise and multi-path reception clutter are reduced. The advantage of a plasma antenna over a scanning mechanical antenna is that the plasma antenna is electrically reconfigurable, can operate at high speeds and has no moving parts. The unit operates at about 2.5 GHz, near the operating frequencies of many cell phones. A ring of plasma tubes operating beyond microwave cut-off surrounds a metal transmitting antenna. A computer de-energizes a plasma tube, causing a lobe of microwave radiation to be emitted. Sequentially de-energizing the plasma tubes causes the radiation lobe to scan in azimuth. When a receiving antenna is detected, the computer ceases scanning, and locks onto the receiving antenna. When the receiving antenna is disconnected, the computer recommences scanning, looking for another receiving antenna.

Plasma antennas for lowering co-site interference among closely spaced antennas

Summary form only given. Plasma antennas were built and tested to operate at frequencies between 30 MHz and 1500 MHz. Vertical antennas were tested with RF coupling to each antennas by a capacitive sleeve located near the lower end of the glass discharge tube. At frequencies below 500 MHz, a DC current is used to ionize and maintain the plasma discharge. Above 500 MHz, the DC current required for operation as an antenna causes excessive heating of the glass envelope and electrodes. To solve this problem, current is pulsed to several Amperes for about 5 microseconds; with a repetition rate of 1500 Hz. Therefore the average current is greatly reduced to less than 20 mA. Ion density, however, decays slowly after the fast pulse ends, and plasma frequency is relatively constant and remains higher than the antenna operating frequency. Unlike a conventional metal antenna, the frequency of plasma antenna for optimum efficiency can be tuned by varying discharge current. As an example, our plasma antenna operating at 50 MHz is much less susceptible (by 40 dB) to co-site interference from a nearby antenna transmitting at 170 MHz. Another way to lower co-site interference is to place a plasma shield around an antenna to reflect interference from a nearby antenna transmitting at lower frequencies. Our plasma shield consists of an array of plasma discharge tubes placed side by side around the antenna. Plasma discharge current is set at an appropriate value to assure that desirable signals pass unimpeded through the shield, but lower frequency interfering signals are reflected away by the shield. We have built plasma shields, using pulsed discharge current, that reflect signals with frequencies up to 24 GHz.

Theory of plasma antenna windowing

Summary form only given. This paper sets forth a detailed numerical analysis of the performance of a reconfigurable antenna comprised of a linear omni-directional antenna surrounded by a cylindrical shell of conducting plasma. The plasma shield consists of a series of tubes containing a gas, which upon electrification, forms a plasma (in practice fluorescent light bulbs are used). The plasma is highly conducting and acts as a reflector for radiation for frequencies below the plasma frequency. Thus when all of the tubes surrounding the antenna are electrified, the radiation is trapped inside. By leaving one or more of the tubes in a non-electrified state, apertures are formed in the plasma shield which allow radiation to escape. This is the essence of the plasma window based reconfigurable antenna. The apertures can be closed or opened rapidly (on micro-second time scales) simply by applying voltages. The goal of the theoretical analysis is the prediction of the far-field radiation pattern of the plasma window antenna (PWA) for a given configuration. In order to simplify the analysis we make the approximation that the length of the antenna and surrounding plasma tubes are irrelevant to the analysis. Physically we assume that the tubes are sufficiently long so that end effects and be ignored. In so doing, the problem becomes two-dimensional and as such allows for an exact solution. The problem is therefore posed as follows:(1) assume a wire (the antenna) is located at the origin and carries a sinusoidal current of some specified frequency and amplitude.(2) Next assume that the wire is surrounded by a collection of cylindrical conductors each of the same radius and distance from the origin.(3) Solve for the field distribution everywhere in space and thus obtain the radiation pattern.

Theory, measurements, and prototypes of plasma antennas

Plasma antennas have more degrees of freedom then metal antennas making their applications have enormous possibilities. 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. Alexeff and Anderson and [1]-[2] Anderson and Alexeff [3] have done theory, experiments, and have built prototype plasma antennas. Anderson [4] wrote a comprehensive book on plasma antennas. Plasmas are a type of metamaterial. [5].

Plasma antenna VSWR and co-site and parasitic interference reduction or elimination

Plasma antennas use partially or fully ionized gas as a conductor instead of metal. Plasma antennas can perform as metal antennas do but with reconfiguration, lower thermal noise at the higher frequencies, and lower side lobes in some experiments. At the higher frequencies, plasma antennas have lower thermal noise than metal antennas and the thermal noise of plasma antennas decreases with the operating frequency of the plasma antenna making them ideal for satellite antennas Plasma reflector antennas, plasma FM/AM radio antennas, various plasma transmitting antennas, high power plasma antennas, plasma frequency selective surfaces, plasma waveguides, plasma co-axial cables, and smart plasma antennas have been built. Alexeff and Anderson and [1]-[2] Anderson and Alexeff [3] have done theory, experiments, and have built prototype plasma antennas. Anderson [4] wrote a comprehensive book on plasma antennas. S. Sakai [5] et al have shown that a plasma antenna is a type of metamaterial.

Experimental results of plasma antennas

Summary form only given. Considerable progress has been made on plasma antennas of which the major advances are: operation at higher plasma densities in the steady state, considerable reduction of power consumption and reduction of noise from the electrical current, which generates the plasma. We have performed experiments concerning transmission and reception, stealth, reconfigurability, shielding, protection from electronic warfare, mechanical robustness, mechanical reconfigurability, plasma waveguides and noise reduction of plasma antennas. In the past, our plasma tubes were ionized by steady state DC current. If the tubes are ionized by extremely short bursts of DC current, we find that the plasma is produced in an extremely short time of about 2 microseconds. However, the plasma persists for a much longer time of about 1/100 second. This is the reason why fluorescent lamps can operate on 60 or 50 Hz electric power. In the new mode of operation, we observe that the plasma density produced by the pulsed power technique is considerably higher than the plasma density produced by the same power supplied in the steady state, which produces two beneficial results: we can operate at much higher plasma densities and at several giga Hertz. In addition, we can operate during the long, non-current carrying phase, which should not have noise generated by current-driven instabilities. We have also operated our plasma antennas at several megawatts using a spark-gap-driven separate RF power supply. We find that even at very high power levels, the plasma antenna operates as efficiently as a metal antenna. We also find that with the proper operating mode, the plasma antenna will not ignite even in the presence of a megawatt RF field. In conclusion, our recent inclusion of a pulsed power supply for our plasma tubes provides reduced noise, higher steady state DC plasma density and reduced power consumption. There are possibly minor problems because of a slight plasma density fluctuation during the pulsing cycle, which will be addressed in the future work

Generation of dense plasmas at low average power input by power pulsing

One of the remarkable plasma effects experimentally discovered is the large increase in plasma density at the same average power input provided by pulsing the power input1. In our experiments, a density increase of over 100 has been observed. Although various experimenters have observed similar effects using different power input techniques, to my knowledge no one has provided a theoretical explanation as yet.