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.