A High-Voltage UWB Coupled-Line Directional Coupler

Ultra-Wideband (UWB) radar systems will be useful for remote target identification, especially when looking through foliage, rain, or soil. The current state of the art, however, requires two separate antennas, one each for transmit and receive. Becausethe space available for antennas is limited, it would be highly convenient to use a single antenna. We investigate here a number of technologies that, when combined, allow a single antenna to be used. The most immediate requirement is for a broadband,high-voltage directional coupler. A new class of directional coupler is proposed that is fabricated from two coupled parallel transmission lines. These couplers operate over two decades of bandwidth, and they can be designed to withstand very highvoltages. Because directional couplers have a finite directivity, there is some signal leakage from the high-voltage source directly into the digitizer. This leakage can easily burn out the digitizer, so we propose using a combination of signalconditioning and limiters to reduce the spurious signal. Signal conditioning involves using a portion of the source signal to cancel out the spurious signal. Limiters may be based on either spark gaps or solid state devices. We will investigate theusefulness of both signal conditioners and limiters. This research will lead to a new high-voltage Ultra-Wideband directional coupler that will allow a single antenna to be used for both transmit and receive. A prototype design will be built and testedduring Phase I. This research will also develop methods for reducing or limiting the spurious response in the receive channel using signal conditioning or limiters.