There are four main states of matter - solid, liquid, gas and plasma. Plasma is the fourth state of matter that consists of ions and electrons. An antenna that uses plasma instead of metal-conducting elements is called a plasma antenna i.e. plasma is used as the medium to transmit and receive electromagnetic waves.

There are two types of Plasma Antennas: 

1. Gas Plasma Antenna: In this antenna Gas filled tubes are used in place of metal elements. As the current is passed through the tube the gas gets ionized to plasma, becomes conductive and acts as a mirror, eventually receiving and transmitting signals. When the gas de-ionizes, the antenna becomes non-conductive.

2. Plasma Silicon Antenna (PSiAn): These antennas have a large number of diodes embedded into a silicon chip. When current is passed, these diodes get charged resulting in the creation of localized plasma, which is used to receive and transmit signals. PSiAn antennas have 'beam-steering' capabilities, as you can select specific diodes to focus a beam of signals in a specific direction.

Advantages of Plasma Antennas over Conventional Antennas

• Plasma antennas are more efficient than conventional antennas, as they have very low noise levels - this is due to the low collision rate between the charge carriers.
• They are lighter and smaller in size
• These antennas do not need to be aligned
• They can operate at very high frequencies
• Plasma antennas have no mechanical parts, unlike traditional antennas
• Plasma antennas are dynamically reconfigurable (Frequency, direction, bandwidth, and gain) while normal antennas are not.
• No ringing effect in plasma antennas.