Characterization of ultrawideband antennas

Abstract

There are two different ways to communicate with wide bandwidth signals. One is to take a signal with a very narrow instantaneous bandwidth and use a frequency hopping technique as being currently used in a CDMA type of environment to generate a wideband signal. This is easy to implement and currently being done for transmission and reception of signals which have small instantaneous bandwidth. The other approach is to deal directly with transmitting a wide baseband signal. For wide baseband data transmission, like signals from computers, the instantaneous bandwidth is very large. The point of this paper is that antennas are highly dispersive devices and their impulse responses must be taken into account in the design of any system which plans to transmit a wide baseband signal. Without taking the antenna effects into account, the wide transmitted baseband signals are quite meaningless. Simple expressions are derived to illustrate that even a point source type of antenna which is virtually nonexistent produces a severe distortion to the signal. The objective of this paper is to bring out the importance of antennas in any wide baseband type of transmission. Also, it is important to note that the transmit waveform is not equal to the receive waveform so the reciprocity of the antenna patterns do not hold in the time domain! Experimental evidence will be presented to illustrate what principles are involved in the design of a century bandwidth antenna. Moreover, what is necessary to establish a completely dispersionless channel with 20 GHz bandwidth!