IEEE Antennas and Propagation Society Distinguished Lecturer 講演会

An Overview of Ultrawideband Antennas
Professor Tapan K. Sarkar(Department of Electrical Engineering and Computer Science, Syracuse University, USA)

Date and Time : November 1st, 2014 13:00-14:00
Venue: Seminar Room (FIT Hall 2F), Fukuoka Institute of Technology

Abstract
Conventionally, the design of antennas is narrowband and little attention is paid to the phase responses ofthe devices as functions of frequency. Even the use of the term broadband is misleading as one essentiallytakes a narrow band signal and sweeps it across the band of interest. In fact, it is not necessary to pay too
much attention to the phase for narrowband signals, as the role played by the frequency factor is that of ascalar multiplier. However, if one now wants to use multiple frequencies and attempts to relate the dataobtained at each frequency, then this frequency term can no longer be ignored. Depending on theapplication, this scale factor can actually have significant variations, which also depend on the size and theshape of the bandwidth over which the performance of the system is observed. In the time domain, theeffect of this frequency term creates havoc as it provides a highly nonlinear operation and hence must be
studied carefully. By broadband we mean temporal signals with good signal integrity. When it comes towaveform diversity, which implicitly assumes time-dependent phenomena, it is not possible to do any
meaningful system design unless the effects of the antennas are taken into account. These effects will beillustrated in terms of the responses of the antennas and on the applicability of the current popularmethodology of time reversal for the vector electromagnetic problem.

To provide a background, notions of bandwidth will be discussed, especially in light of recent interest in
ultrawideband (UWB) systems ? the last decade has witnessed significantly greater interest in UWB radar.
In that time, more than 15 nonmilitary UWB radars have been designed and fielded, which include
applications to forestry, detecting underground utilities, and humanitarian demining. Since an antenna is anintegral part of sensing systems, selected highlights of UWB antenna development will be very briefly
summarized. It will also be illustrated how to design a discrete finite time domain pulse under theconstraint of the Federal Communications Commission (FCC) ultra-wideband (UWB) spectral mask. Thispulse also enjoys the advantage of having a linear phase over the frequency band of interest and isorthogonal to its shifted version of one or more baud time. The finite time pulse is designed by an
optimization method and concentrates its energy in the allowed bands specified by the FCC. Finally, anexample is presented to illustrate how these types of wideband pulses can be transmitted and received withlittle distortion.

Some fundamental problems in studying concepts involving the responses of antennas in the time domain
are related to our subconscious definition of reciprocity. In the frequency domain, reciprocity is related
simply to the fact that the spatial response of the sensor in the transmit mode is EQUAL to the spatial
response of the sensor in the receive mode at any frequency of interest. In the time domain, the spatial
response of the sensor will be time dependent. Hence, both the transmit and the receive impulse responsesof the sensor will be a function of azimuth and elevation angles. However, for a fixed spatial angle, thetransmit impulse response is NOT EQUAL to the receive impulse response of ANY sensor. In fact,
mathematically one can argue that the transmit impulse response is the time derivative of the receiveimpulse response for any sensor. One may then conclude that somehow reciprocity is violated through thisprinciple. The important fact is that the product in the frequency domain results in a convolution in the time
domain and that the reciprocity relationship is no longer a simple one. Even though the transmit impulse
response is the time derivative of the receive impulse response, reciprocity still holds! The above principle
now helps us in characterizing different sensors for different applications as their temporal responses arequite different.

As a first example, it will be shown that an electrically large wide-angle biconical antenna on transmit does
not distort the waveform, whereas on receive it does an integration of the waveform for certain conditions.
In contrast, a TEM horn antenna on transmit differentiates the input waveform, whereas on receive it does
not distort the waveform. Use of such transmit/receive antennas can actually produce channels withpractically no dispersion. Experimental results covering GHz bandwidth signals will be provided toillustrate these methodologies. Examples regarding the impulse responses of other types of antennas, likethe century bandwidth antenna, impulse radiating antenna, and the like will also be presented.