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Since the discovery of radio frequency ("RF") transmission, antenna design has been an integral part of virtually every communication and radar application. In its most common form, an antenna represents a conducting metal surface that is sized to emit radiation at one or more selected frequencies. Antennas must be efficient so the maximum amount of signal strength is expended in the propogated wave and not wasted in antenna reflection. The modern requirements to antenna include compactness and conformality, rapid reconfigurability for directionality and frequency agility and should also allow low absolute or out-of-band radar cross-section and facilitate low probability of intercept communications. The need for an antenna that is "invisible" (thus not detectable while not in operation) has, already in the 1980's, sparked work on the feasibility of using an atmospheric discharge plasma1 as an RF antenna. Moreover, data communications can be made more secure if the antenna only "exists" during the transmission of each data packet. Such antennas use plasma formations as the receiver or transmitter elements. The characteristics of the plasma formations are determined by purpose of the specific antenna. Plasmas have two important properties that are relevant for interaction with electromagnetic waves: • For frequencies above the plasma frequency, a semi-infinite plasma transmits EM waves with a wavelength, l /e r where l is the free space wavelength. Thus plasmas can in principle be used for electronic tuning or control of a radiation pattern by varying the plasma density. For the densities typical of discharge tubes, this phenomenon appears especially useful at microwave frequencies. • For frequencies below the plasma frequency, however, the dielectric constant is e r < 0, the plasma behaves as a metal, free space EM waves cannot penetrate, and are reflected. Radio communications via the ionosphere rely on this effect. Plasma technology can be utilized to create secure WiFi data transmission capability for use in different applications up to 100 GHz [33]. WiFi has enabled a wide array of inexpensive communication devices that are utilized in desk-top computing, networking, PDA’s etc. Its

Explosive Pulsed Plasma Antennas for Information Protection